Rocker Geometryad

7/29/2019 Rocker Geometryad

1/1020 APR-JUN 2010engine professional

ROCKER ARM GEOMETRY seems to

raise its head every now and then, andwhen it does, I rarely ever see it statedaccurately. Too often a sound bite of onlya small piece of information is taken outof context and then used as the Gospel,totally ignoring the other dynamicsthat revolve around it. In some cases,something totally erroneous is stated thatis not only wrong, but makes no sensefor anyone who just stops and thinksabout it objectively.

When lecturing at trade shows,schools, engine shops or just gettingpinned down on the phone by a

knowledgeable engine builder goingdeeper than most on a technicalissue, I have found that I spend abouthalf my time trying to undo variousmisconceptions about rocker geometrybefore I ever begin explaining the facts.There has been so much info put outthere by reputable companies (and bymy reckoning, incorrect), that people arereluctant, by nature, to see somethingdifferent from the prejudice of whatthey already know or think they know.If people are used to doing something acertain way, they see everything from thatperspective. Usually, my getting through

to them involves discrediting what I think

is wrong with what they were doing and

then begin to explain what they neededto change. At that point I could breakdown the simple rules for what geometryreally is, and why.

Background of Rocker GeometryRocker geometry (or the lack of it),goes way back to many fathers on bothsides of the ocean, to when the WrightBrothers were still studying the theoriesof lift in an airfoil. But for our purposeshere, and to avoid boring the curiouswhove managed to get this far on thisstory, Ill come to the point about rocker

arms and explain as needed how themistakes got to where they did.In the old days, rocker arms were

all pretty much what we term a shoedesign; meaning the contact pad with thevalve had a large radius scruff surfacethat depressed upon the valve tip asthe rocker moved through its rotation.The term of course comes from theappearance to a shoes sole, but also tothe mechanical motion much like a footand boot would do, as it pushes off.This pushing off motion, as many willalready know, has the effect of the rockerarm stretching itself as it moves through

the depressing (lift) cycle. It is actually

lengthening itself as it moves across the

valve tip, and you see this by the widefoot print (we call a witness mark)atop the valve tip.

The use of rocker arms goes back tomany things predating engines, but theprinciples were never required to be sospecific on axis point heights and theirconsequences, as it is for helicopter bellcranks, and racing engines! There wasno rocket science to designing theseparts a hundred years ago, which endedup on our prehistoric cars and earlyairplanes. Engineers simply made designsthat tried to minimize the degree of how

much scuffing was imposed on the valvetip; got it close, and moved on to moreimportant questions. Somewhere alongthe line, there became a principle to getthis in a general ballpark, that someonelater coined as the 1/3-2/3 theory (oreither of the two). This placed the pivotpoint of the rocker arm so that it was2/3 of the way below the valve tip, or thevalve tip was 1/3 of the way above therocker shaft, depending on your point ofview. But the answer was the same. Thisthinking was originally derived from theintention that a near 90 degree arc couldbe realized when the valve reached its

intended full open position. Bear in mind

Rocker Geometry)@JIM MILLER


2/10engine professionalAPR-JUN 2010 21

that valve lifts back then were usuallyin the quarter inch or so range, on littletwo and four cylinder engines. So beingoff a little really had no measurabledifference in performance of the engine,and wear and tear was the real yardstickengineering back at the turn of thecentury was aimed at. Also, the abilityto accurately measure wear and tear,horsepower, thermal loss and manyother cool things we take for grantedon todays computers, wasnt even apossibility back then.

The advent of more valve lift,and thus pushing a budding internalcombustion engine technology higher toproduce more power was really inspiredfor leaps and bounds by the advent ofaviation, not Henry Ford. Not to takeanything away from the automotivecrowds contributions, but only aviationimposed the second requirement thatdefined efficiency and that waslight weight. Making Goliath enginesthat had more power was a lot easier

than making more power from lightweight engines that would be flying oversomebodys head, somewhere. So thewhole thinking process for efficiencyin engine technology really found itsimpetus in aviation, because racing backin the early 1900s was still done on theback (or behind) of one-horsepowerwhose exhaust was more easily steppedin than emitted from a pipe. And as faras my 30 year old memory serves meon the research, aviation was also thefirst use of a roller tip rocker arm, onradial engines as far back as the 1930s,

and perhaps before. In fact, to thisday, I never cease to be amazed at theforesight and creativity of both aviationand automotive engineers in the 1920s,and 30s, and 40s. Four valve Pent roofcombustion chambers, roller cams, fuelinjection, nitrous oxide, water injection,two stage superchargers, turbo chargers,and many other cool things we assumewere concepts of the last 20 or 30 years,were actually done and done quite well,seventy and eighty years ago.

Aside from the roller tip rockersof aviation long ago, the fundamentalrules of rocker arm design were basedaround the shoe tip, contact pad designstill used today. Many of you may knowthat you cant use a roller tappet on aflat tappet cam, and of course vice versenot only because of hardness of materialdifference, but because of geometry.The principles of trailing motion anddynamics between something that ismaking direct LINEAR contact uponanother object that is imposing orreceiving a RADIAL (circular) is entirelydifferent than if that contact is occurringwith a roller tip making or following thecontact. This isnt rocket science either,

and you can see how this happens by

drawing a roller tappet in various stagesof lift as the cam lobe goes around topush upon it, and see a straight line fromits axis to the cam lobe is constantlyshifting around as the tappet goes fromthe close (base circle) position, up alongthe acceleration ramps, then over thenose. When finally, as it crosses deadcenter at full LOBE lift, this straight linebetween its axis and the cam centerlineis also in alignment with the tappetbore itself. At all other times, the tappetis actually receiving some level of sidethrust in its bore (engine block) from thepushing out forces that the lobe imposesas it chases it up.

Even when engineers were chasingefficiency with aircraft engine and powerdevelopment, the threshold for seeingmeasurable loss of engine life, like valvestem or valve guide wear, was not easyunless things were really out of whack.They werent worried about loss of camevents through small changes in rockergeometry, even though they knew the

variables existed. So if they kept to this1/3-2/3 rule, everything looked goodon the valve tip, and the leverage of therocker arm upon the spring or moreaccurately stated the leverage of thespring on the rocker arm was at nearperpendicular relationship with the valvewhen rates were at their highest. Eventhough by todays standards for valvesprings, spring rates back in the 1920sand 30s were negligible. This attitudecontinued on throughout the decadesafterwards. More importantly, andunfortunately, it bled over into the soon

to come roller rocker arm market, thatgot its impetus in the 1950s. The firstperson who made a working aluminumroller tip rocker arm for automotiveapplication belongs to my dear old anddeparted friend, the late Harland. Someother garage efforts might have beengetting tinkered with out in Californiaabout the same time, but it is pretty wellundisputed that 1958 is the beginning ofwhat we know today as the aluminumroller rocker. Keep in mind that aviationroller rockers existed twenty or moreyears before, but they were steel, theywere radial engines, and they didntcomport to the automotive needlebearing aluminum body that Harlandintroduced.

Just like the flat tappet cam andthe roller tappet having entirelydifferent geometry because of where themeasurement for motion is made, thesame rules apply to the shoe tip rockerversus the roller tip rocker arm. But whenHarland made his silhouette, he didntallow for this, and inadvertently movedthe axis of the roller roughly .300 ofan inch higher than what it should havebeen. The axis of the roller should have

been in the same place as the contact

pad. So when his rocker was placed onthe engine, and the roller was positionedfor a good eye-ball track on the valve,now the push-rod cup was too high.The result, was that it went way up andin toward the stud. In actuality though,most engine builders in the sixtiescontinued to keep using standard lengthpushrods, and the excessive motion fromthis mistake was occurring on the valvetip, which was deemed normal becausethe roller rolled! Believe it or not, evento this day, people think that the rollertip is for rolling on the valve. It is NOT.The roller tip is for one reason only, andthat is to convert the shifting length ofthe rockers arc (that moves across thevalve on a shoe design), to a fixed lengththat moves far less in its effect, becauseit is always point down in line with thevalves motion, just as a roller tappet of acam is always aiming its contact tangentline with the axis of the camshaft.

This error stayed, and was copied bymany manufacturers and eventually by

everyone in some measure or another.It would take several decades beforeenough trial and error, and even a patentwould be studied to make manufacturersrethink this, slowly improving theirdesigns. Ironically, some of the mostwell known names continue to promotedesigns they never changed, and evenpromote the less accurate means of usinglittle tools, that tell an engine builderwhat pushrod they need, while nevereven taking into account the valve liftthat will be used. Make no mistake;you cannot set rocker geometry without

knowing exactly how much the rockerarm is going to move.It seems logical that since a roller

cam can provide all the acceleration anyof todays heads need, for any rockergeometry scenario, then why not set therocker geometry to ONE STANDARDthat has the least amount of wastedmotion, and will always duplicate thesame percentage of cam information,regardless of what cam you use? Forunderstanding this, understanding a littlehistory is always best. This ends a lot ofrhetoric.

Whenever pushrods leave their in-line paths to now have their end followaround the rocker shaft by any amount,this is LOST CAM INFORMATION.The cam literally has to turn moredegrees to affect the same LIFT at a laterpoint on the crank. Velocity, too, is lost.So youve lost duration, throughout theentire lift cycle (not just overall), andyouve lost RATE of acceleration, byslowing the rocker down.

To put this in perspective, lets takea simple even value of cam lift, like.400 to make a point. Rocker geometryis usually thought about as only what

is happening at the valve. In our .400


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and so forth have been elevated. The

principles for cam technology and

specifically rocker arm geometry that

would soon come along in 1980, but

spawned in 1973, have not changed to

this day.

DefinitionWhat is rocker geometry? Rocker

geometry is angles of motion. It is not

some linear reference point on the tip of

the valve, that trying to adjust the wear

pattern will guarantee it being correct.

What is correct? Correct, is efficiency.

It is having the least amount of wasted

motion being used to do the greatest

amount of work (that is designed to

be done by the cam). This last point

is important, because the rocker arm

can be used to add to the cam, besides

what it usually does by error, which is

take away from the cam. But I will get

into each of these below. I just wanted a

simple mission statement that defines

what geometry is and is not, so that the

following hopefully makes sense.

ImportanceWHY is rocker geometry so important?When you change the pivot point ofwhere the rocker arm is, in relation tothe valve tip, it CHANGES THE CAM.It doesnt matter whose rockers youuse, it doesnt matter what style rockeryou have, it doesnt even matter whatapplication your engine is; whenever youchange location of the rocker pivot pointin relation to the tip of the valve, youare changing cams. You are changing allthree parameters simultaneously: LIFT,DURATION and VELOCITY (rate ofacceleration).

The degree you change these dependson how much you move the pivot point.And one or two of these three parametersmay be affected more than the other.But if you dont LOCK your geometry

in to the SAME THING all the time,which has the least amount of wastedmotion, then you are aiming at a movingtarget with every cam change. Whateverresults you get from one cam to anotheris tainted by the diluted effects of wastedmotion in the rocker arm.

Rocker arms are a radial devicebeing ordered to do a linear thing.They rotate on an axis, moving in acircle. But what they have to impart isa straight line command. They get theirorder from the camshaft, in the formof IN-LINE information that they then

have to ROTATE around an axis andthen MULTIPLY it by some ratio, andfinally TRANSFER this result back toanother IN-LINE component of greatermovement. This movement has THREEvalues: LIFT, DURATION (of lift), andVELOCITY (acceleration of lift). If therocker arm does ANYTHING ELSEbesides this, then it is NOT efficient, andSOME of this information is being lost.

Let me make a point about somethingon this. Your camshaft is ground toten-thousandths of an inch precision. Itis computer designed to millionths of an

inch, you (or your cam manufacturer)selected it for a division of durationvalues where you considered two or threedegrees important; any more was too big,and any less was too small. Hopefully bynow, you realize that moving this rockerpivot point will change this at the valve.



ROCKER GEOMETRY)@JIM MILLER

You just dont know how much. Well, itis MORE than the two or three degreesyou think is important. In some casesit can exceed TEN degrees, and is oftenfive or six degrees. As if this isnt enoughreason, consider this: It is that value ofloss throughout most of the lift cycle, not

just total where youre only inclinedto measure it from, and where your camcard is limited to. That is whats missing.Engine builders only check at FULLLIFT. They check rocker ratio and totalvalve lift, and thats that. But when yourrocker geometry is off, youve lost thosedegrees of duration throughout mostof the entire cam profile. Which meansrate of acceleration is lost, but you mayonly see a small change in lost valve lift,thinking the difference is just flex in therocker ratio.

Two GeometriesRocker Geometry is the correct DESIGNand INSTALLATION of the rocker armso that its relationship to both sides ofup-and-down motion is fully realized byBOTH. This is of course, the pushrod,and the valve (respectively).

The rocker arm is a RADIAL tool,asked to do a LINEAR job. It pivotsaround an axis in a reciprocatingradial (circular) motion, and has all thedynamics that anything revolving aroundan axis will have. But on either end of therocker arms connecting points are twoother instruments each live and breathe

by the laws of linear (in-line) motion.Now all this may sound like rocker

arms 101 and we may all know this,but few people I have found over 35years, seem to understand how sensitive,precise and important this observationis. I think this true because most treatboth the design of the rocker arm and itsinstallation with casual regard.

To have the most efficient designand use of the rocker arm requires TWOthings: (1) The rocker arm must bedesigned to mirror the inherent angles ofeach engines pushrod to valve geometry.

(2) Rocker geometry must have anaccurate location of its rotational axiswith the valve tips height. The firstof these is called DESIGN geometry;and the second is called INSTALLEDgeometry.

Every engine has an inherent acuteangle (I call the attack angle) whichthe pushrod leans either into or awayfrom the valve centerline. The smallblock Chevy for instance is 19 degreespositive (leans into). This comes fromthe engine block having a 4 degreeangle of its tappet bores with the pistoncylinder centerline. The head is already

a 23 degree valve angle (actually it is a

67 degree, because it references fromthe deck), so you simply subtract the4 degrees that the tappet and pushrodaim toward an already inclined 23degree valve, and you end up with simplemath: 19 degrees. Every engine is unique.The SB Ford is 20 degrees for this same

value. This is what rocker geometryneeds to be designed to, otherwise yourefforts of installing the rocker armaccurately will be limited to just one sideof the rocker or the other.

What Rocker Geometry IS NOTRocker geometry is NOT where theroller (or contact point) is at on thetop of the valve. Forget that. Rockergeometry, especially is NOT the ideathat you want to place the roller or wearpattern (shoe tip rocker) in the middleof the valve. Forget that, too.

The valve tip is everything. It isground zero. This is where all leverage,and the full stroke of valve lift begins,this is our reference point. There aremany ways to measure rocker geometry,but there is ONLY ONE way to SET IT.Now, you can set rocker geometry inthe closed position, or you can set it inthe MID-LIFT position (half open), oryou can do like most people have done,and simply roll the engine over a coupledozen times watching the valve openand close to see your witness mark (footprint) atop the valve tip and play hitand miss with chasing a minimal wear

pattern.The problem with this latter point is

that this is a symptom, it is not geometry.Granted, when you have the rockergeometry set properly, you will have theleast amount of wear pattern, but totry to set geometry through moving therocker and trying to see how small youcan get it, is better than nothing notquite good enough. You can easily be offby .005 to .010 (or a great deal more)on even seeing this actual width, let alonemeasuring it. And being off .010 on thehorizontal plane of what you are really

trying to measure, which is the verticalplane (valve lift), will multiply out quiteeasily to .030 or .050 or .080 ormore in your error of where the trunnionis to the valve tip! Those kinds of errorswill cost you several degrees of crankrotation to open the valve a like amount.

What about using a tool, or dialindicator designed to measure this inand out motion, resting on the springretainer? Well, this is a better way ofthe same thing, but it is still measuringa horizontal plane for a vertical planeresult. Error can be off several timesmore than measuring directly in the

vertical plane, or parallel to the valve

motion itself. When using these tools,just like a dial indicator on the top ofa piston, as soon as the piston reachesperfect TDC, you will have two or threedegrees of crank movement before yousee the dial indicator move. There is afloat time there, and so too is the effect

by using a tool on the roller tip of arocker to measure in and out motion. Itfloats enough to allow the valve lift tobe off by .005 to .010 or more. But ifyou can set it dead nuts within .002 to.003 without having to buy such a tool,why wouldnt you do it in a more preciseway?

Alternative GeometriesBefore getting too deep into philosophies,history and facts, let me restate the keypoint of what rocker geometry is, then Iwill mention the comparative arguments

people (and companies) have madeagainst this. MID-LIFT geometry isrocker geometry that has the ultimateefficiency, in that it is doing thegreatest amount of work with the leastamount of effort. It has the least amountof wasted motion in the pushrod andvalve, commonly referred to as the in-and-out motion. It affects the maximumresponse through linkage of whatever thecams instructions are. If you dont havegeometry set precisely, your consequencesrange from simply losing a little caminformation at the valve, to excessive sideloads in various directions, on various

parts that will at the very least rob youof power. In more extreme cases, wearingout parts or outright catastrophic partfailure may occur.

There are arguments to usingdifferent geometry than mid-lift. I simplydont agree with them because theyviolate the principles of efficiency. Oneof these theories is to adjust the rockerarms height so it reaches a 90 degreerelationship when the valve is about 2/3open, not half open. The logic being, thatthe spring loads on the rocker body areless. Another reason Ive heard is that it

accelerates the valve in the mid-rangebetter, thus making more power. Othervariations of this approach shift therocker arms pivot point higher on thevalve tip to create this 90 degree effectsooner in accelerating the opening ofthe valve at a lower point of lift, thusincreasing what is termed area-under-the-curve.

Both of these are a way to adddifferent cam information to the valveby using the rocker arms geometry. Theonly reason you would use the rockerarm for creating a second dynamicof valve acceleration, is if the cam was

unable to give you the acceleration




you needed. Now, in some cases thislimitation exists. They would be flattappet cams, of mechanical or hydraulicoperation, and an engine where cylinderheads had the flow potential, and/orcubic inches had the demand whichrequired a crazy acceleration to mid-

lift flow values off the seat. In otherwords, the engine was so big, and theheads were so big but for rules or someother illogical reason, the cam theyHAD TO USE was a flat tappet profilethat had limited rate of accelerationby its limited tappet diameter and basecircle constraints. I wont get into camtechnology and limits, but that is the firstreason I can think of for using the rockerarm as its own cam tuner. In some Stockclasses where the original cam must beused, a creative (and well funded) enginebuilder can play games with rocker

geometry to change valve lift rates, butthese are very limited differences, usuallynot worth the trouble, and most of all, inALL these examples, there is going to bedetriments that outweigh benefits.

In the first place, for those whohave an engine of large flowing heads,and big cubic inches, or heads for veryhigh rpms, they will have the benefitof using a roller cam. So the issue ofhow fast you can open the valve is noteven a consideration, because by natureof roller tappet geometry, any value ofacceleration up to and through suicidalparts destruction can be implemented

on the cam profile. And for those classeswhere a flat tappet cam is required, thecubic inches and head limitations of mostrules Ive seen over the years, fall withinairflow and rpm limits that a flat tappetcam fit just fine. Too many times, camcompanies talk customers into rollerprofiles that are not needed, and in factdont make as much power as a wellchosen flat tappet would, because it takesmore power to operate the roller. Usingrocker geometry as a second cam shaftis not a good idea. The velocity of therocker arm increases where its motion

line reaches a 90 degree angle, and tryingto pick a particular segment of the valvelift that you want to impose that thinkingover what the cam manufacturer hasdone, is bad news. But theres anotherpoint to consider on this issue.

The rocker arm is a symmetricaldevice to whatever geometry it is set at.In other words, whatever acceleration itexhibits on the opening side of the valvegets reversed on the closing side. Simply,if you set geometry with a HIGH pivotpoint, so it increases its velocity quicklyoff the valve seat, then slows to full lift...Guess what? Its going to accelerate

back to the close position when it leaves

full lift. Because it will always mirrorwhatever its settings are.

To see the difference all you haveto do is take an old fashioned needlepointer torque wrench and turn theengine over without spark plugs andmeasure the drag. Then set the geometry

to MID-LIFT, and see the difference.Ive had people do this and fall out oftheir seats. They see as much as 45 footpounds or more and LESS torque toturn the engine over. Usually it is 15 or20 foot pounds but it depends on howmuch spring pressure you have, howcomplex the rocker geometry is (Hemiversus SB Chevy, for instance), and howwrong their geometry really was. Eitherway, it quickly shows you the definitionof efficiency. If they MAP their valveacceleration, throughout the entire liftcurve, then I need say no more. The best

way is to DIVIDE the arcs EQUALLYand standardize this for all cam andcylinder head testing. Change your camas you need.

Setting GeometryAs mentioned, the rocker geometry hasTWO considerations: The accuracy ofhow it is installed, which I have alwaysreferred to as the installed geometry.And second, how accurately it isdesigned, which I have always referredto as the design geometry. This seconditem relates to where the adjusting screwor cup is placed in the body, and at what

angle.As an engine builder, you can move

the rocker arm up and down to the valvetip in setting that side of the rockerseffect, but you cant do too much aboutthe pushrod side, and that is where theinformation comes from. That is up tothe rocker arm manufacturer.

Of the two sides, however, the lesserof evils to shoot for is setting the VALVEside (or installed geometry) as closelyas possible, because (a) this is wherethe motion is constrained by the valveguide, (b) this is the side that has the

greatest motion, (c) this is the side thathas the valve spring, where harmonicsare generated and amplified, (d) and thisis the side where all of the foregoingmultiply into a measurable resistancevalue that generates heat, robs power andcreates additional friction. The pushrodis, by comparison, free floating with therocker bodys movement in and out aswell as up and down, and it is movingless (the cams lift). But make no mistakeabout it, when the pushrod side is notperforming to mid-lift geometry, it islosing information. The upside though,is that whatever is left, is getting through

to the valve. When the geometry is off on

BOTH sides of the rocker, because youdidnt install it accurately, you lose twice!Only 90% may go into the rocker fromthe cam, and of that, only 90% comesout, or 81% goes to the valve. Im ofcourse rounding things off for example,but the principle is the same.

I should add one other point here.Everything is net. So if you havethose cute little checker springs layingaround, find some other use for them,because outside of holding a valve in ahead for display where someone can usetheir finger pressure to push the valveopen, they have little use. You need theREAL running springs for any geometrysetup. The same goes for checking flex orpiston to valve clearance or anything elsecritical. Checking springs ADD about.040 (or more) NET valve lift to yourengine. Or, another way to say it is you

will LOSE .040 or more NET valve liftwhen you put the heads together with therunning springs, compared to whateveryou measured using the checking springs.This is true across the board, flat tappetcams, roller cams, aluminum rockers,steel rockers it makes no appreciabledifference.

Later in this article, I will offerinstallation and assembly tips but fornow, here is the easiest of accurate waysto set INSTALLED rocker geometry:

The closed valve position is the easiestand the best. The cam must be in theclosed position, on its base circle. Heads

are assembled to the engine, with nopushrods in place. You must know whatyour net valve lift is supposed to be(we can get nitty-gritty later). You willsubtract any valve lash so you have anaccurate net lift.

For stud rockers, put it in place withan adjustable pushrod. You dont needthe poly locks; just let it set loosely onthe stud. Knowing your net valve lift,DIVIDE it in half, and write it downon a piece of paper. Then, lengthen orshorten the adjustable pushrod to raiseand lower the back of the rocker until

you get the center of the trunnion exactlyHALF of your net valve lift BELOWthe center of the ROLLER TIP. If forexample, you have .600 net valve lift,then this would be .300. Keep in mindthat I refer to center of the trunnionand roller pin. It is their axis that is whatyou are measuring. Some are easy to seeand some are not. For those with flatmachined surfaces, take a scribe, measureand mark these centers as best you can.But the main trick is that you want to besure you measure this from a precise 90degree reference to the valve centerline.To accomplish this, you are best served

to use the top of the valve spring retainer.



Simply lay a short machinist ruler (orsomething comparable) atop the valvespring retainer, and pass it along the sideof the rocker arm.

When you have the height installedaccurately, the trunnion will be exactlyHALF of your net valve lift, below the

roller tip centerline when the valve isCLOSED. As it opens, and moves toexactly mid-lift, the axis of the rollertip will have dropped down to bestraight across from the trunnion and animaginary line that runs between them (Icall the motion line) will form a preciseright angle with the valve centerline. Theroller is at its farthest point across thevalve when this happens. When the valvecontinues to open the second half of itslift, to full lift, the roller will have movedexactly an equal amount BELOW thetrunnion as it was above the trunnion

when it was closed. And the roller willbe at its closest inside point on top of thevalve. You will also have the ultimateleast amount of roll across the valve.

For shaft mount rockers, its a littledifferent, but the principle is the same.With shaft rockers you must use shims,

or have a stand that has a surplus ofmetal that you are machining exactlywhat you need away. But you can take ameasurement of the stand height withoutusing a rocker arm. Just bolt the standdown to the head with a couple of bolts,lay the shaft in the stands bed-way, and

use a machinist square along the sideof the valve (or spring) and shoot thelong end along the top of the shaft, sothere is a gap beneath it and the top ofthe valve. Everything is about findingthe centerlines, and being creative aboutdoing this, while being accurate at thesame time and measuring at an accurateright angle with the valve.

It makes no difference where thewear pattern is at on the top of the valve,when you have correct mid-lift geometry,and providing the pattern is on top ofthe valve. (Running off the edge of the

valve is not good.)

Graphing the Cam at the ValveTo see what is really happening at thevalve, you need to check your rockersmotion by measuring it at the valve. Thebest way is to essentially treat this like

youre degreeing your cam, but youremeasuring motion at the valve. Onlyinstead of just picking up points of liftto compare to the crank, as you wouldwith the .050 tappet lift measurementson a cam card, you will be creating agraph all the way through the entire

cycle of valve lift, opening and closing.If you are fortunate to have a CAMPRO or CAM DOCTOR, or somethingsimilar, life is good. If not, you can dothe old fashioned way. You will needgraph paper that can be found at artstores, engineering supplies and manyscience or school supply providers. Youneed a dial indicator that youll mountdirectly above the valve spring retainer,nearly fully compressed so you followthe valves stroke fully and you wantto be sure the indicator stem is lined upparallel with the valve. As with setting

your cam, you need to have a degreewheel in place on the crank and zeroedaccurately to TDC of the piston.

With the above in place and ready,you have TWO choices to how youmeasure this; which are merely oppositeperspectives. You can choose an even




number of crank degrees you movethrough to measure valve lift, or youcan choose an even number of valve liftto measure degrees of crank rotation.It doesnt really matter which you use,because it is the comparison againstother like tests that is important, and

both need to be the same. You dont haveto be too crazy about fine incrementshere, just choose valve lift jumps ofmaybe .020 and write down the crankmovement; or choose 5 or 10 degreecrank measurements and write down thevalve lift.

Personally, I like the second methodof using a fixed crank stepping, and thennoting the valve lift. This goes directlyto the points I make about area-under-the-curve that you are looking for.For those new to this term, area-under-the-curve refers to the VALVE LIFTCURVE as charted across a graph oftime (meaning crank degrees), and whatmost engine builders agree, is that liftingthe valve as quickly as possible and assoon as possible, while setting it downquickly after it has hung open for asbroad a period of time as needed, butwithout being too fast to damage thevalve train from excessive bounce iswhat everyone wants. So, when you wantto see the gains and losses in this areafrom inaccurate rocker geometry, yourereally looking for wasted time when thecrank is moving more than it needs tolift the valve the same amount. So, if

you standardize your testing to the samevalve lift measurements, the gains andlosses in the crank are readily seen.

Once youve charted one rockergeometry setup, perhaps the one youvebeen using, now make the changes withpushrod length and/or stand height (forshaft systems) to meet with what I hopeI have informed about earlier in thisarticle. You will often see that PEAKvalve lift is very close to the same, butmuch less at other points in the curve.That is the lost information. The degreeof this will depend on many factors

that take another story to itemize. Butthe bottom line is; you will appreciatehow important it is keeping the samegeometry, for making sure your camchanges show results that are directlyaccredited to them and only them.Otherwise, your information is tainted.

Shoe Tip Rocker GeometryAs with aluminum rocker arms, there aredifferent design geometry shoe tip rockerarms, but the priority for adjusting thevalve tip side is still there for the samereasons. However, you dont easily havethe same accuracy as you do with picking

up precise center points on the roller

and trunnion of a needle bearing rollertip rocker arm. Even finding the axis isnot easy. Setting it by the same rules isbest simply for standardizing one cam toanother. Only your reference is the actualcontact point of the pad itself. When itis at mid-lift, you will have a 90 degree

relationship between the TIP of the valveand the center of rotation for the rockerarm. But finding that center is tricky,because these are usually ball fulcrumrockers, and they are surrounded by thestamped metal that has no clear axis toit. One solution is to put a stud upsidedown in a vice and rotate it carefullywhile observing the fixed point on its sidethat most closely represents where thecenter is, then making a little felt tip penmark. This would then be set exactly halfof your net valve lift below the valve tipin the closed position. Its not as accurateas fixed points to set calipers against,but it will get you very close if you havepatience and a sharp eye; and with shoetip rockers, the amount of error youmight be off will have no measurableeffect in cam efficiency as it would withneedle bearing roller tip rockers.

Twisted RockersUnfortunately, engine builders are ledinto a false security by stud mountrockers sold for heads that shouldnt usethem. These are aftermarket heads withpushrod offsets that require an offsetpushrod cup, or adjusting screw. Shaft

systems usually have this adequatelyfixed, but when heads are sold with studsin them, that clearly need to be removedfor a shaft system, this is the false senseof security one gets. The two rockersshown here is exactly what you DONTwant to have (Figure 4).

When you have this much rotation,the roller tip does NOT lay flat on thevalve tip, and as it opens the valve itsenergy is making a cross sword slashacross the valve tip that rounds off thetop of the valve tips, side loads the guideon the X axis (length of head), and shifts

side loads to the bearings in a way thatoften push or break one prematurely. Itsbad news, costs horsepower and breaksparts.

Ratio & GeometryIf the rocker geometry is off, then sotoo is the ratio. Theres some good newsthough: Dont worry about it, becausevery few of the rocker arm makers did.The history of rocker design didnt havemuch accuracy involved. There was nostandard, because there was no need forsuch accuracy in the old days. Rockersalways were, and to a great degree,

still are designed in the closed valve

position. But as long as you stay focusedon what you need to do, you wont tryjuggling things around outside of the realpriorities, based on a false idea of whatYOUR rocker ratio ought to be. It canbe all over the place from .05 less, up tomaybe close to what it is supposed to

be, simply because many manufacturersbegan in the closed position, and thenstarted moving specs around from a hitand miss until it got close. Once it did,they left it there. Over the years, moreconsistent manufacturers with the leastamount of broken parts have been themodel other newer companies wouldcopy. But the mistakes get copied, too.You have to always check things foryourself, forget the advertising. If youdo, then you cant blame anyone butyourself.

Installation, Measurement andAdjustment Tips

Figure 1Heres the typical stud mounted needlebearing roller tip rocker shown in itsCLOSED position. Dimensions are notshown, because they are relative to yourinstallation, which is relative to yourNET valve lift (after lash, if used). Theimportant dimension you want to findand set, to be HALF your net valvelift, is the ROCKER HEIGHT, shown

to the upper right, and illustrating thecumulative value between the ROLLERaxis and TRUNNION axis. The otherreferences are shown with regard to theirheights above and below (respectively)the valve spring RETAINER. Allmeasurements for where the trunnionis sitting, is referenced to the top of theretainer, marked here by TRUNNIONREFERENCE. Raising and lowering yourrockers tail by adjusting the Pushrod asneeded, will set this. Although shownin this illustration, the adjuster doesnteven need to be here, as it will only get in

the way. Let the rocker sit loosely on thestudy, with its adjustable pushrod and setthis trunnion reference as needed to getthe trunnion exactly HALF of your NETvalve LIFT.

If using a HYDRAULIC TAPPET,be sure that it is fully extended duringthis check. You can prime the motorto do this. After getting your pushrodlength, ADD .020 to allow for hydraulictappet compression during actual engineoperation. Order EXACTLY what youneed for pushrod length, rounding off tothe nearest ten-hundredth of an inch (two

decimals).



Figure 2This is the references to pay attentionto for setting a SHOE tip rocker arm

for your NET valve lift. As mentioned,finding the centerline of the fulcrumsrotation is required first, since thiscutaway only shows where the axiswould be. This is not easily seen on theoutside of a stamped or cast rocker bodyso, you need to simulate this rotation andmake a mark to reference to.

Figure 3As crude as this may look, this isan actual recreation of the drawingillustrated to author by the late HarlandSharp, explaining his original layout of arocker silhouette on paper before scribing

where the roller would be, and using an

actual roller on his outline to calculatethis. Setting the rollers diameter in directposition where the scuffing surface was

at, instead of the rollers centerline iswhat is wrong here. It was the beginningof a duplicated error that would last overa quarter century.

Figure 4As explained in text, this is the realexample of what you dont want to do.But it is very typical, and the problem ispropagated more by the head companiesluring engine builders into a false sense ofacceptability to such an installation, byselling their heads with studs in them thathave this stretched over placement, whentheyve had to move the pushrod for

wider ports. The better alternative would

be to leave the STUDS on the same

centerline as the VALVES, and force stud

rocker manufacturers to put their offset

for the pushrod solely in the rocker arm,with an offset cup. This is what Ford

did on their N-Head, and it is the best

way. Otherwise, you need a stand (shaft)

system. Heres the bottom line: You can

never have an inline valve array cylinder

head, like a SB Chevy, Ford, Mopar, etc.,

and NOT have the rotating axis of the

rockers trunnion be IN LINE with the

CAM. Any twist at all, is COMPOUND

geometry, and will make it impossible

for the roller to lay flat on the valve, or

follow the correct straight down path on

the Y-axis.

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Figure 5This represents the STAND (SHAFT)rocker system, with only the mostimportant things to be considered;namely, the STAND, the SHAFT and theROLLER TIP.

The valve is shown for angle and

location, and is shown here in theMID-LIFT point of motion, as notedto the side. THIS IS THE GOAL. Theroller and shaft are to be horizontallyin line with each other as measured to aperpendicular right angle with the valve.

Figure 6This shows a SQUARE (crosshatched)being used to lie against the valve andatop the shaft of your stand system.The stand is bolted to the head, and ashaft is laid in position to now makethis check while the valve is in the

CLOSED position. The cool thing aboutthis is it can be done on a work bench.No springs, no anything, just the partsshown. If you are doing this with anassembled head, you can run the squarealong the side of the valve springs,providing theyre uniform diameter.Otherwise, you may have to use the valvespring retainer technique from our studmount example.

Figure 7Heres the stand (shaft) mount systemshown in the close position, and ourexample here has a NET valve lift of

.650, or a MID-LIFT value of .325.

You MUST know this for your engine.It is impossible to set correct mid-liftrocker geometry without knowingyour net valve lift. The ROLLERS areshown here in their two critical states,the dashed version representing wherethe roller will be when it has opened the

valve to FULL lift. But the valve is onlyshown in the closed position, as is thesolid roller atop it.

First, take HALF of your ROLLERdiameter; and HALF of your SHAFTdiameter, and ADD them together,you will come up with a standard.In this example, that standard wouldbe .521. This comes from a rollerdiameter of .480 and a shaft diameterof .562. Why half? Because this findsour CENTERLINE for each. It is alwaysthe centerline that you are setting withrocker geometry.

Second, write down the height of theSHAFTS TOP above the valve tip.

Third, you need to write down yourMID-LIFT value (half net valve lift).

Heres the TRICK:

With these three things written down:

1. Subtract HALF net valve lift fromyour STANDARD (.325 from .521in this case, for a sum of .196).

2. Subtract the sum of the above (.196in our example) from your rockersheight (for our example this wouldbe .350 minus .196 = .154. This

is how much the SHAFT needs to be

lowered to bring the centerline of the

shaft, half of the net valve lift below

the center of the roller). Usually, with

shimming, you might have figuresthat make you ADD shims to get the

correct value.

As you might notice, we are using theoutside diameters of the shaft and roller

to calculate this standard from, because

these are easily measured with common

tools. But it is their centerlines that are so

important.Q

Jim Miller has been involved in the racing industry for

more than 30 years. Jims roots stem from amateur

racing, starting at 16 but by age 17, he was the

youngest Ford trained line mechanic authorized to do

warranty work on all of Fords factory muscle cars.

At the age of 21 Jim was invited to take over Chief

Mechanic duties for Dyno Don Nicholsons Pro Stock

Maverick, which Jim respectfully would decline

passing the opportunity on to Jon Kaase. Jim holds

a number of patents for valve train design and is the

proprietor of MID-LIFT Precision Geometry. Jim can be

reached at 1777 Blount Rd., 501 Pompano Beach, FL

33069. Phone 954-978-7001.

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