is there a formula to calculate...

[jrp]

just when 7.350 PR's are needed or how many/what size shims to use for the rockers? heads milled at .010, .020, .030?

[Constrictor]

Stock LS1 cams have a base circle of ~1.550". Base circle is the diameter of a cam lobe with 0.000" lift.

Most aftermarket cams have a base circle between 1.525" - 1.500". Comp likes to use a base circle of ~1.520". Sadly, most cam cards do not tell you what your base circle is, but you can be almost assured that it wont have a stock base circle. The only way to know FOR SURE is to spec your cam yourself.

So what you need to do is think of a sliding scale, with 7.35" on the left end, and 7.45" on the right end.

A smaller base circle moves the scale to the right, since a smaller base circle requires the pushrod to be longer.

Milling your heads moves the scale to the left, since decreasing head deck thickness moves the whole valve assembly closer to the cam, thus requiring shorter pushrods.

A thicker compressed head gasket moves the head away from the cam, thus requiring a longer pushrod, and a thinner compressed head gasket moves the head closer to the cam, thus requiring a shorter pushrod.

Since most people keep stock head gaskets, we'll just use the base circle and head mill in our calculation.

The stock LS1 pushrods are really closer to 7.380".

If you have unmilled heads, stock thickness gaskets, and an aftermarket cam with a smaller base circle, you need a longer pushrod, but how much longer really depends. If we assume you have a .030" smaller base circle, then the exact length pushrod you would need would be 7.410", but since they only come in increments of .050", a 7.400" pushrod will work fine.

If you have milled heads, stock thickness gaskets, and a smaller base circle, then you need to subract the difference between your mill and your smaller base circle. Lets say you have a cam with a .030" smaller base circle, and your heads are milled .030". Those two would offset one another, this requiring 7.380" pushrods. But once again, there is no such thing as an aftermarket 7.380" pushrod, so you'd want to stick with 7.400"s.

Lets say you just put milled heads on your car, and keep the stock cam, or you have a cam with a stock LS1 base circle, and you have stock thickness gaskets. Then in that case, you would be closer to 7.350" pushrods.

As you can see, a lot of this is guesswork. The only way to know for sure what your base circle is is to measure it yourself. You need to take into consideration the compressed thickness of the head gaskets your using, and you need to make sure your head deck height. If this sounds mind boggling, I'll throw one more twist into the mix. Hydraulic lifters, like the ones used in an LS1 can compensate for slight variations in valvetrain geometry, so that is why its not super critical to be EXACTLY correct on pushrod length. You could be off as much as +/- 0.025" and the lifters will still be able to handle the difference.

Of course, an experienced engine builder will take all these things into consideration and come up with an exact geometry and use solid lifters with lash caps to get the perfect setup. However, most people who will read this are not pro engine builders and using the guessing method we all use is good enough for a street engine.

In conclusion, if you are on one extreme of milling heads, or having a really small base circle, you'll need to change your length, but the majority of us are safe to use stock length 7.400" pushrods.

[Darkhalf]

:beer: Good work Mike like the explanation.

[jrp]

damn mike, just dropping some game huh? i forgot about the gasket thickness. would comp R lifters be any different, i know they have a different pre-load then stock lifters.

and i can already tell this will be going in the archieves once we establish them :)