What is Proper Rocker Arm Geometry? I used to believe you could buy a set of rocker arms, throw pushrods in and call it a day. This would be incorrect, or so it now seems... I am curious to know the following.

1.) What do you guys consider to be proper rocker arm geometry and why?

2.) What brand and/or style Rocker Arm do you recommend be utilized to achieve proper rocker geometry and why?

What direction are you heading Trent? BTW: Are your Mom and Dad doing any of the Cruising Speedway thing on Saturday Night?

i'll throw in my 2 cents. I look for a narrow sweep, not a centered sweep if you are doing the sharpie on the valve stem and turning the engine over. the wider the sweep, the more waste of lift, and from what I understand, side loading of the guides. I made a tool to measure the sweep across the valve and its really eye opening to see what a pretty insignificant change in pushrod length does to the width of the sweep. that being said, I don't have lot of experience with shaft rockers, primarily sbc and sbf stuff is what I have done. but im working on my first shaft motor so I am looking forward to seeing how what I have learned applies to these engines.

I'll take a simple stab at this.

The center line of the trunion (sp)and roller tip perpendicular to the center line of valve stem at mid lift. Anybody?

my question is why even worry about it? I have never checked nor really give a flip if they are off a little. unless you are doing something very specific or searching for every bit of HP you can get what does it matter? there are thousands of motors built and last for a long period of time with no problems.

I wrote a full chapter on valvetrains in my big block Mopar book. I'd suggest starting there. I included a bunch of pictures showing how different brands of rocker arms are different lengths and therefore fit differently on the various Mopar cylinder heads.

I have seen many sources state the following theories: Mid lift 90 degree theory is optimal and minimal sweep is desired. Both of these do provide benefits. However, there are other aspects that are overlooked and must be considered. Just because you have mid lift 90 degree geometry, a tight sweep pattern and a well positioned roller on the stem DOES NOT mean you have optimal geometry. I have since seen that there are other factors that MUST be considered before you can establish proper valvetrain geometry for your application.

Why does proper geometry matter? Because it is without a doubt relative to performance. To what extent though... It appears as though Mid lift theory will only get you so far. Perhaps, so far is stability which it does provide. However, I have by definition established this previously stated "Optimal Geometry" in my application and as such I have now sacrificed performance to a degree...

Thoughts???

You asked for it! I gathered info from here and there (mostly from Racer Brown)and put it together into a document that everyone can read and learn from. This document was created in September of 1999 but everything in it is still relevant today, Enjoy!

ROCKER ARM (Valve train)GEOMETRY

Rocker arm geometry is an expression that is frequently heard but seldom understood in the area of high-performance engines, or anywhere else, for that matter. It's a fair guess that only about one person in a thousand or more does anything about it, even superficially. Nevertheless, it is of the utmost importance in any high-performance engine, and this importance increases as valve lifts continue to increase.

Rocker arm geometry is defined as the relationship of the rocker arm tip to the valve stem tip through the valve opening and closing cycle. This is seen as a point of tangency where the radiused rocker arm tip contacts the flat surface of the valve stem tip. This sounds simple enough, and it is, but unless one has a grasp of the fundamentals, correcting the rocker arm geometry can be downright frustrating and difficult. It is time-consuming at best, but there's free horsepower here, and that's the best kind, as well as making life very much easier on the entire valve train, plus a few extra revs that normally cannot be found.

Ideally, rocker arm geometry is correct when the center-point of the rocker arm tip radius coincides with the centerline of the valve stem at EXACTLY one-half valve lift. Again, this sounds simple enough, but nearly everything that is touched in the build-up process of an engine affects rocker arm geometry, usually for the worse. The base radius of the cam lobes (one half the base circle), effective lifter length, pushrod length, cylinder block and/or cylinder head decking operations, the depth of the valve seats in the cylinder heads, the dimension from the valve stem tip to the valve seat in the cylinder heads, plus the "fixed" hardware of the rocker arm stands and shafts, and total valve lift, all combine, for better or worse, to affect rocker arm geometry.

Rocker arm geometry needs help when one or more of the following symptoms appear in an operational engine: Increased oil consumption and/or vacuum leaks due to eggshaped valve guide bores; scuffing of either or both rocker arm tips or valve stem tips when there is adequate lubrication; scuffing of pushrods or rocker arms at the pushrod ends of the rockers when there is adequate lubrication; scuffing of rocker arm balls and/or sockets in the case of engines with ball-type rocker arms; premature wear of rocker arm bushings in the case of engines with fixed-pivot shaft-type rocker arms; in extreme cases, bent valves, bent pushrods, broken rocker arms. Any or all of these symptoms can be caused by excessive forces required to open and close the valves due to the higher friction generated by incorrect rocker arm geometry.

The first area of correction is the dimension from the valve stem tips to the valve seats in the cylinder heads. This point is simplified during engine build-up by being certain that the depth of all valve seats in the cylinder heads is exactly the same, as measured with a depth micrometer from the cylinder head gasket face to the valve seats. There will be a difference between the depths of the intake valve seats and the exhaust valve seats because of the differences in valve seat diameter and the angles involved. However, the depth of all intake valve seats must be the same and the depth of all exhaust valve seats must be the same, but not necessarily the same as each other. Install an inner valve spring, valve locks and spring retainer on each valve so the valves will remain seated. Using a small precision square and a fairly long steel straightedge placed on the valve stem tips, determine which valve along the line of valves in the cylinder head is the shortest. With a depth micrometer, measure from the top of the straightedge to the top of each valve stem tip. These readings will give directly and accurately the differences in valve stem length.

Each valve and valve seat must be numbered before disassembly so that each valve will go back into the same valve guide bore. Log the amount that each valve stem is longer than the shortest one, then remove the valves from the cylinder heads.

If the intake valves are in a different plane from the exhaust valves, as in a hemispherical combustion chamber, the measurement and logging of valve stem lengths must be repeated. In this case, all intake valve stem lengths must be equalized, as must all exhaust valve stem lengths, but again, the intake valve stem length does not have to be the same as the exhaust valve stem length.

Next, the valve stem tips must be ground off squarely and accurately to match the shortest valve or valves. This must be done in a good valve refacing machine, with a freshly-dressed grinding wheel and with the valve stem located so the finished valve stem tip is exactly perpendicular to the valve stem. The finished valve stem tip must be as smooth as possible and dead flat.

Reassemble the cylinder heads with the valves, inner valve springs, valve locks and spring retainers, with each valve in its original valve guide bore. With the straightedge and depth micrometer, make a repeat measurement from the top of the straightedge to the valve stem tips. Ideally, they should be exactly the same as the shortest valve stem. Things of this nature are rarely ideal, so an acceptable target is if all valve stem lengths are within plus-or-minus .005 inch of each other. This is the time to correct any and all out-of-tolerance valve stem length discrepancies.

Install one cylinder head on the cylinder block with a used but clean cylinder head gasket. The cylinder head must be torqued down properly, as in final assembly, but we're not there yet. Install the camshaft, lifters, pushrods, rocker arms and related hardware for number one cylinder. At this point, the outer valve springs are not yet required and should be left off to make rotation of the camshaft easier. Place the spindle of a one-inch stroke dial indicator on the spring retainer (NOT on the rocker arm) of number one intake valve. Preload the dial indicator to something more than total valve lift and make certain that the spindle of the indicator is parallel to the valve stem in both longitudinal and transverse planes. Rotate the camshaft so that the lifter for number one intake valve is on the center of the heel of the cam lobe (directly opposite maximum valve lift). Watching the indicator, tighten the valve adjusting screw so the valve is off the seat by .002 to .003 inch, then zero the indicator dial. Rotate the camshaft so that maximum valve lift shows on the indicator and log, this number. Rotate the camshaft again until the indicator shows the valve is at exactly one-half maximurn valve lift.
Now very carefully observe the relationship of the rocker arm tip to the valve stem tip. Good light is essential, and a piece of white paper held behind the valve will help. Ideally, at exactly one-half valve lift, the rocker arm tip must contact the valve stem tip at the exact center of the valve stem.

If; at one-half valve lift; the rocker arm tip contacts the valve stem tip outboard (toward the outside of the engine, the pushrod is too long or the rocker shaft stands are too high. Conversely, if the rocker arm tip contacts the valve stem tip inboard (toward the center) of the engine, the pushrod is too short or the rocker shaft stands are too low. Unfortunately, for the most part, this must be an "eyeball" test, but good lighting, very careful observation and effort applied toward honesty of the evaluation rather than expediency will be long steps in the right direction.

If' the rocker arm tip appears" to contact the valve stem tip, very close to the center of the valve stem at one-half valve lift, there is one simple check that will tell if it's close enough. Rotate the camshaft back to zero or minimum lift and loosen the valve adjusting screw. Slip a piece of tissue paper between the rocker arm tip and the valve stem tip, then tighten the valve adjusting screw until there is about .002 to .003 inch lash. Rotate the camshaft through the full valve opening and closing cycle and remove the tissue paper. If the imprint of the rocker arm tip in the tissue paper is consistent and even, lock it up and be glad that corrections are not necessary. On the other hand, if the tissue paper is torn or is smashed to near-transparency at one point, this very strongly suggests that corrective actions are required. This process must be repeated on number one exhaust valve.

Theoretically, the rocker arm tip should roll across the valve stem tip as the valve opens and closes with no side loads being imposed upon the valve stem by the rocker arm. The rocker arm tip radius is therefore very critical, being a function of the lever arm length as the rocker arm swings through an arc during the valve opening and closing cycle. The lever arm length constantly varies as the rocker arm is in motion. The lever arm length is measured from the pivot point of the rocker arm to the contact point on the valve stem tip in a plane that is perpendicular to the centerline of the valve stem. Therefore, the lever arm length is one dimension at zero lift and another longer dimension at maximum lift, and at exactly one-half lift, the lever arm length will be somewhere between the minimum and maximum lever arm lengths.

As a point of clarification, the rocker arm tip radius is defined as that found on stock-type rocker arms. However, everything still applies to the so-called "roller tip" rocker arms made by specialty manufacturers. In these cases and in spite of contrary thoughts and words, the rocker arm geometry problem is made worse instead of better for two distinct reasons. First, the radius of the roller tip that is inserted in the end of the rocker arm is wrong for proper rocker arm geometry because these rollers are always too small in diameter. Second, high-speed motion picture studies have shown beyond any doubt that the roller rarely, if ever, actually rolls across the valve stem tip. This occurs because of too-small roller tip diameter, there being an insufficient mechanical couple to make the roller tip roll smoothly. If some rolling occurs, it is jerky and inconsistent.

The roller tip rocker arm is thought to be a "cure-all" in the area of rocker arm geometry, and not only does it fail in this respect but, as pointed out, it makes matters worse. If this type of rocker arm is to be used, it is absolutely essential that the rocker arm geometry be as near to perfect as possible in order to minimize the effects of a roller tip that is too small.

Correcting rocker arm geometry, in the case of engines with fixed-pivot shaft-type rocker arms, will involve either raising or lowering the rocker shaft stands. If the rocker arm tip contacts the valve stem tip more toward the outboard side of the engine, the rocker shaft stands must be lowered. If contact is made more toward the inboard side of the engine, the rocker shaft stands must be raised, as previously stated.

The amount the rocker shaft stands must be raised or lowered must be the result of a judicious "guesstimate." Raising the stands presents no particular problem because the stands can be shimmed with washers or shim stock for a trial fitting. Usually, from .030 to .045 inch is a good starting point. With the rocker shaft stands blocked up and torqued, take another long look at the rocker arm tip contact in relation to the valve stem tip. When the final spacer thickness has been determined, permanent spacers must be made up to the same size and shape as the bottoms of the rocker shaft stands where the stands bolt to the cylinder heads. Starrett ground flat stock is excellent for this purpose because of the wide variety of widths, lengths and thicknesses available, plus this material is surface ground on both sides to assure flatness and parallelism. All hold-down bolt holes, dowel pin holes, lubrication holes, etc., must be duplicated in size and location on the permanent rocker shaft stand spacers.

On the other hand, if the rocker shaft stands must be lowered to correct the rocker arm geometry, then metal must be removed from the bottoms of the stands. This is best done in a surface grinder or a milling machine, properly located and clamped so that all rocker shaft stands can be done in one set-up. Any metal that is removed cannot be easily added later, so it is essential that the finish cut be parallel in both planes to the original bottom surfaces of the rocker shaft stands. Initially, a cut of from .030 to .040 inch is a good starting point for a trial fit. The finished surfaces must be smooth, flat and free of burrs.

If the rocker arm geometry requires only a slight correction, this can usually be accommodated easily with a special-length pushrod, and raising or lowering the rocker shaft stands can be forgotten. If it appears that the rocker shaft stands should be raised, a longer pushrod is required; and, conversely, if it appears that the rocker shaft stands should be lowered, a shorter pushrod is required. This condition can be verified by the position of the rocker arm adjusting screw in the rocker arm. The threaded segment of the rocker arm adjusting screw should be centered in the rocker arm with the valves adjusted to the proper valve lash. If a locknut is used on the rocker arm adjusting screw, it must be removed for this observation. If the rocker arm adjusting screw must be backed out so that there are more threads visible on the top of the rocker arm than there are below it, the pushrod is too long. If the opposite is true, or if there aren't enough threads on the top of the rocker arm to give full-length contact for the rocker arm adjusting screw locknut, then the pushrod is too short. In any case, the pushrod must contact only the rocker arm adjusting screw, and not the rocker arm itself, throughout the full valve opening and closing cycle.

Again, changing the pushrod length in a fixed-pivot shaft-type rocker arm engine will not make a major change in rocker arm geometry, but it will definitely help. Any major changes must be made by raising or lowering the rocker shaft stands.

In engines with ball-type rocker arms, the ONLY way the rocker arm geometry can be corrected is with special length pushrods, assuming that the valve stem lengths have been equalized as previously described.

How much longer or shorter should the pushrod be in order to correct the rocker arm geometry? A good question deserving a good answer. The easy way is to have an adjustable pushrod made so that the effect of lengthening or shortening the pushrod can be very carefully observed in relation to rocker arm geometry. Then, when the rocker arm geometry is just exactly right, the overall length of the adjustable pushrod must be very carefully measured so non-adjustable pushrods can be made up to exactly duplicate this length. If the intake and exhaust pushrods are different lengths, then two adjustable pushrods are required and the same process repeated. The adjustable pushrod trick is valid for fixed-pivot shaft-type rocker arm engines as well as for engines with ball-type rocker arms.

For the most part, the rocker arm tip radius on stock-type rockers is best left alone. About all that can be done is to be certain the rocker tip radius is smooth and polished. Unless the proper equipment is available, this must be a "freehand" operation and is most easily done with Craytex abrasive sticks. Care must be taken not to "flatspot" the radius and also to be certain that in the plane perpendicular to the radius, the rocker arm tip is flat and not "cocked" off at an angle.

Valve stem tips must usually be rehardened if more than about .030 inch is removed in the process of correcting valve stem tip length. This is quickly and easily done, assuming the valves are of conventional materials and not stainless steel or other austenitic (non-heattreatable) alloys. Simply heat the valve stem tip with a welding torch until the tip color is a fairly bright red, then quench the stem in water. The trick is to use a fairly large welding torch tip and point the flame directly onto the valve stem tip. This will heat the tip quickly, and it must be quenched quickly because the tip surface only is to be hardened. If the red color extends more than about 1/8 inch from the valve stem tip, the valve can be considered junk because heat penetration will cause hardening of the valve stem in the area of the valve lock groove, which must be avoided at all costs to prevent valve breakage at this point. After hardening, the valve stem and the tip must be cleaned and polished with crocus cloth to remove any scale and also to improve the surface finish of the valve stem tip.

There are two alternatives to hardening the valve stem tips. One, the valve stem tips can be hard-faced with Stellite, a very hard and tough alloy, but this is best left to someone experienced in hard-facing techniques. Two, valve lash caps can be used between the valve stem tips and the rocker arm tips. These are inverted cups that fit over the valve stem tips and are very hard and tough. The valve lash caps must usually be quite shallow in order to avoid contact with the valve locks and the spring retainers.

If either alternative is used, the thickness of the finished hard facing or the thickness of the valve lash caps must be taken into consideration from the outset so that the rocker arm geometry will be correct when all pieces are assembled.

The foregoing description suggests that when rocker arm geometry is being observed, the valve lash should be zero or very close to it. The reason, particularly with ball type rocker arms, is that zero valve lash will prevent the rocker arm from "cocking" to the side as the valve is opened and closed, which in turn will prevent false observations. This method will result in a pushrod length that is very slightly shorter than that actually required. However, if it seems necessary to make an error in pushrod length, it is far better to make the error in favor of a shorter pushrod by a lot than a longer pushrod by a little.

The only engines that are exempt from rocker geometry problems are those in which rocker arms are not used. Therefore, overhead camshaft engines using rocker arms are subject to the same rocker arm geometry problems as those previously discussed, even though pushrods are not present in such engines.

In some of these engines in which the rocker arm pivot point is between the cam lobe and the valve stem, as in the single overhead camshaft 427 Ford V8, rocker arm geometry discrepancies are corrected by valve stem length and/or rocker stand height changes. Inasmuch as this has been outlined and the same corrective measures apply, it can be dismissed without further comment.

In overhead camshaft engines where the cam lobe contacts the rocker arm between the rocker arm pivot point and the valve stem, as exemplified by current Datsun single overhead camshaft four and six cylinder engines, rocker arm geometry can and does become almost unbelievably critical. The first rule of equalizing valve stem length must be followed in such cases, but the allowable valve stem length variations should be tightened to almost zero. Finished valve stem length is the important item in these engines because it has the most direct influence upon rocker arm geometry.

Most engines of this type use a rocker arm with a radiused pad that contacts the cam lobe, and the manner in which the cam lobe contacts the radiused pad is very nearly as important as rocker arm geometry itself, and the two factors are inseparably related. In fact, when the cam lobe-rocker pad contact condition is right, the rocker arm geometry will also be right. Problems occur if the cam lobe base circle is changed, larger or smaller than stock, or if the valve lift has been changed in either direction from stock with a cam lobe base circle that is the same or larger or smaller than stock. Therefore, any deviation from a camshaft of stock dimensions will not only change the rocker arm geometry but will also change the cam lobe-rocker pad contact condition.

This condition can be observed and corrected by the following methods: Assuming all valve stem lengths have been equalized, place a light inner valve spring or fuel pump spring on a valve with the retainer and valve locks. Make sure that both the cam lobe and the rocker arm pad are clean and dry, then spread a very light coat of Prussian blue paste on the nose of the cam lobe and down each flank of the lobe. With the nose of the cam lobe turned away from the rocker arm pad, install the rocker arm and set the valve lash to the proper figure. Rotate the camshaft slowly by hand one full revolution. Very carefully observe the traces of Prussian blue paste that have been transferred from the cam lobe to the rocker arm pad. Ideally, the cam lobe contact patch should be exactly centered on the rocker arm pad. If the contact patch is closer to one end or the other of the rocker arm pad, then corrections are absolutely essential. At no time can the cam lobe extend beyond either or both ends of the rocker arm pad. This will chew off the cam lobe in less time than it takes to tell the tale.

If the cam lobe contact patch on the rocker arm pad is closer to the point of the rocker arm than it is to the valve stem end of the rocker, the valve stems must be lengthened. If the opposite is true, that is, if the cam lobe contact patch is closer to the valve stem end of the rocker, the valve stems must be shortened. However, the effective length of the valve stems must normally be made longer.

Depending upon layout, this can be accomplished by either adding a hard-facing material of the correct finished thickness to the valve stem tips or by the use of valve lash caps, as previously described. The Datsun engines require a special valve lash pad that is slotted to accept and guide the rocker arm tip, and the outside diameter of the pad must fit inside the valve spring retainer properly.

As a starting point, the amount the effective valve stem length must be increased is the difference between the stock cam lobe base radius and the cam lobe base radius of the camshaft being installed, multiplied by the rocker arm ratio. As an example, assume the base circle of the stock cam lobe is 1.300 inches. Half this dimension is the base radius, .650 inch. If the base circle of the cam lobe is reduced to 1.100 inches, then the base radius is reduced to .550 inch, which is .100 inch smaller. If the rocker arm ratio is 1.5 to 1, the .100 inch difference in base radius must be multiplied by 1.5, which is equal to .150 inch, and is the amount the effective valve stem length must be increased. This method will usually be quite close, but final determination of effective valve stem length must be made by a very close visual examination of the cam lobe contact patch on the rocker arm pad. If there is a direction to lean with the thought of improving matters over and above stock, then the proper direction is toward a very slightly longer valve stem length than that actually required, and then by no more than about .010 inch.

With this type of overhead camshaft layout, any and all changes in the valve train become a function of rocker arm ratio. Consequently, dimensional tolerances and changes become much more critical, affecting not only rocker arm geometry and cam lobe patch location, but actual, measured rocker arm ratio as well.

This lengthy and detailed discussion will hopefully serve to emphasize the extreme importance of correct rocker arm geometry in any engine of any performance level. The results obtained are certainly worth all the time and effort involved to make it right. Fortunately, when the job is done correctly, all valve train components will usually last the life of the rest of the engine assembly, and more. Nevertheless, if special valve lengths or special pushrod lengths are required, it's a good plan to have a few spares on hand in the event of other unforeseen disasters.

Yes, any change to the valve train geometry will result in some associated change to the effective valve lift curve. Nothing new here, but it takes some seriously anal types like myself who will admit to having actually measured the entire lift curve as the result of making some changes to the geometry.

I've done the same thing to see what the net change to the lift curve is from different rocker arm brands & ratios, as well as lash changes for solid cams.

You still haven't provided the track test conditions (temp, humidity, barometric pressure, etc.) for the before & after comparisons, along w/ the actual incremental differences in ET & MPH.

Trent,

A few weeks back I was at the track and it was slow. The water grains were high I was told. Care to post the difference on the time slip. If your car fell off everywhere the DA and Humidity could have knocked your #'s down.

You can check the DA here.

http://www.dragtimes.com/da-density-altitude-calculator.php

Mark