OK...I'm building a 386 small block (4.155" bore/3.56" stroke) for our '70 Camaro. The cam is a Lunati solid roller with .646" lift with the 1.6 COMP "Pro Magnums". The block was decked .010", and the Dart head deck is stock. The pushrods are stock length COMP "Hi-Tech"s. The lifters are Lunati "horizontal bars".
I'm a little concerned about my rocker geometry at 0 lift. At half lift, the roller tip is dead centered on the valve tip, but at 0 lift it pulls it quite a bit toward the intake manifold side of the valve.
Is this an indication of the pushrods being too short? I just ordered a pushrod length checker, but I wanted to throw this question out here to see if anyone had any ideas.

Frank

 

http://www.compcams.com/Technical/Products/Pushrods/

It sounds like yours are fine. Unless the heads have been milled a lot or you have overlength valves, stock length pushrods will usually work fine. It's always a good thing to check the length every time anyway.

 

Due to the arc, that the rocker tip goes through, do not expect the rocker tip to be centered on stem at half lift. The tip contact pattern should be centered on the stem tip, after going through a full sweep of zero to full lift.

 

In a perfect world the rocker tip would start just to the intake side of the stem and be the same amount to the exhaust side at mid lift then end up at the same point as it started on the intake side of the stem centerline at full lift while never sweeping more than a few thousandths total.

This is not a perfect world.

You need to look at the total sweep of the rocker tip on the stem tip. At around .650 lift and below at the valve you should be able to get your sweep to less than .040 total. It may or may not be centered but the only way to really effect that is to change the tip length of your rocker arm. For the most part when doing a race engine I could care less where on the tip the pattern is located as long as the rocker isn't going off the tip and plucking the stem. Try using just the inner spring or a pretty stiff checking spring and varying pushrod length while rolling the engine over through its entire lift cycle. Black sharpie on the stem tip can help you see the wear pattern. Adjust the pushrod length until you get your sweep under .050. Preferably under .040 but .050 is still better than most people ever end up with. Even if the pattern isn't centered the small sweep area will still mean very little side loading of the stem and guide wear will be improved.

 

Dan, I'm confident you agree that is the reason for this thread. This was the purpose for the original design of roller tipped rocker arms. I believe, it is understood, the sbc rocker geometry is not perfect.

In case anyone misunderstood the wording of my previous post,
I stated "The tip contact PATTERN should be centered on the stem tip." I did not state contact POINT. If using the word SWEEP would have been less confusing, then that was my oversight.

For the most part when doing a race engine I could care less where on the tip the pattern is located as long as the rocker isn't going off the tip and plucking the stem.

Well, I do. To me, this is the main focus/reason, for correcting the contact pattern, in the first place, because......

At around .650 lift and below at the valve you should be able to get your sweep to less than .040 total. It may or may not be centered......Adjust the pushrod length until you get your sweep under .050. Preferably under .040 but .050 is still better than most people ever end up with. Even if the pattern isn't centered the small sweep area will still mean very little side loading of the stem and guide wear will be improved.

I have found, up to about .750" lift, centered pattern and achieving a .040-050" sweep, are very closely related.

To others reading this, this info pertains strictly to/with the use of roller rockers. Do not expect this cooperation with the oem rocker arms, even if you elongated the slot enuf, to allow such lift.

 

Its great if you can get a minimal sweep and have it dead center on the valve stem though somewhat unlikely unless you are varying rocker length, stud location, stand location, or valve stem tip height. I just don't want people to misunderstand this post and think that what they are looking for above all is a centered sweep pattern. In reality the location of the sweep pattern can be somewhat manipulated if you are in a position to modify some of the things I listed above but most people simply end up with whatever they have. Meaning that they bought a set of heads with some +.1 or .2 longer than stock valves or maybe more, and they bought some readily available rockers and pushrods and when checking their pattern thought they were alright with a centered pattern and over .080 sweep. With proper parts selection the sweep will almost always be in an acceptable location on the stem tip, though centered or not minimum sweep is always the main consideration of myself and pretty much all the engine builders I know who all have record holding engines in various racing classes from circle track to heads up drag racing.
I recently fixed a set of AFR heads that had been set up by a "professional" shop and that were run on a very fast nitrous car from this board. These heads had T&D rockers and Lash caps on valves with hardened stem tips and though the sweep pattern was perfectly centered it was nearly .110 wide! The guides had a tremendous amount of wear considering the number of passes on this engine. By removing the lash caps and nearly half the shims under the stands I reduced the sweep to .048 which I can guarantee will result in lower friction, heat, and guide wear even though now the wear pattern is slightly toward the intake manifold side.