I see some people on here running 7.35" and 7.45" instead of 7.40". Why?

 

Differences in rocker boss height relative to the top of the lifter. This can be the result of milling heads, thinner gasket, different length lifter roller to pushrod cup (Morel for instance) and cam base circle. It is also a method to modify lifter preload with the stock rocker arms.

 

That about covers it. The biggest reason is probably milled heads, followed by a different base circle of the cam.

 

In non adjustable rocker system such as stock LS systems, the preload on the lifters is determined by the pushrod length.
So when the base circle of the cam is diminished, thinner gasket used or heads are milled, it is necessary to modify the length of the pushrod to achieve as close as posible to stock preload.
In general:
cams with smaller base circles will require longer p-rods
Milling heads (up to a certain value) will require shorter p-rods. (keep in mind aftermarket cam + milling can offset eachother and stock length could still apply)

In short, if you didn't modify you p-rod length you could face geometry problems
such as:
Too long will result in lifter collapse on high rpm pump up (can result in cam damage as well)
Too short could cause the valvetrain to be noisy and cause unwanted harmonics and a fluttering valvetrain at high rpms.

 

Thanks for the info! I'm gonna be running stock lifters (new), patriot heads which were barely milled like .003-5" for cleanup, Stock Gaskets (from SDPC) and a XER Comp Cam. Should I go ahead and use the stock length 7.40" pushrods or get the 7.35"?

 

with XER lobes i would use 7.425, or atleast stock.

 

Yeap, XE-R, 7.425's would make things a little quieter.

 

running 7.40s with my Tourquer 2 X-er lobed cam

 

Think of it as the distance from the rocker to the lifter when the lifter is on the base circle of the cam minus the amount of preload you want to run.

 

How do I determine the amount of preload? Thanks!

 

ttt...

 

0 lash + .080 preload