[Bell040]

Step #55

The methods I used for checking pushrod length were to find the pushrod length with a rocker torqued down and at zero lash then adding my targeted lifter preload to the length checker and verifying its length by bolting the rocker just down to zero lash and then getting 1.5 turns out of the rocker pedestal bolt to get to 22ft/lbs.

Some things are better explained by watching them so here is a video of the entire process I went through to measure and then verify the pushrod length I was going to use.



So after all my measurements I came up with a 7.325" base pushrod length, added 0.075" preload (GM spec on the stock lifters is 0.060" so I erred slightly on the high side of spec) and came up with a 7.4" pushrod which makes sense as the new cam's base circle was about 0.033" smaller than the stock intake cam lobes and 0.055 smaller than the stock exhaust cam lobes (base circles on the stock cam are different for intake and exhaust yet all the stock pushrods were the same length so the exhaust lifters were carrying about 0.020" more preload than the intake lifters). I picked up some Comp Cams Hi-Tech 5/16" diameter 7.400" length hardened pushrods after having determined the proper length I needed.

I figured I should add some information that I found. The stock intake valves are actually 0.024" longer than the stock exhaust valves to keep the valve train "geometry optimized" because of the difference in the cam base circles. http://www.camarohomepage.com/ls3/page4.htm

So I am pretty sure that the comment of your's that I bolded is not true.

Therefore, to keep the exact same preload as the stock system (whatever it may be... you noted a GM spec of .060") you would need to go from 16 of the 7.375" pushrods (@ stock cam base R=.748" int and .760" exh) to 8 intake pushrods of 7.392", and 8 exhaust pushrods of 7.403". I assumed that the difference in base circle that you measured (of .033" and .055") were diameter measurements, therfore your cam had R=.732" on int and exh.

I used this EQN:

cam base radius + valve deltaL/rocker ratio + pushrod length + C = the LS3 valvetrain constant, where C = other valvetrain stuff that isn't changing.

the stock intake valvetrain formula would be .748" + (.024/1.7)" + 7.375" + C = 8.147" = stock distance that matters
... for the exhaust, the formula would be .760" + (0)" + 7.375" + C= 8.135"

add your cam that has R=.732" into both equations, leaving pushrod length as a variable:

int pushrod = 7.392"
exh pushrod = 7.403"

if I am correct, do you did pretty good ... but maybe unknowingly if you were trying for .075 preload, and didn't account for the longer intake valves.

... I think the valve train can handle an 0.008" change (or 8 mils in engineer speak)


btw, I am printing your DIY to help me through the cam swap in a couple weeks (ordered all the parts, waiting for them to come in). Thank you for doing such a meticulous job on it!