[robertway]

I am in the process of measuring my pushrod length for the new blower cam I am installing and am needing a little guidance around what "zero lash" actually is. My understanding of zero lash is that it is the point at chich the rocker is tigthened down and the pushrod is lengthened JUST ENOUGH to take the play out of the rocker in that it cannot be lifted even slightly off the tip of the valve. I have my length checker set to the point that there is NO up and down play on the tip of the valve, there is no preload on the valve spring, and there is very slight play laterally on the tip of the rocker where it contacts the valve stem (you can nudge it back and forth a bit but it isn't loose). Is this "zero lash"?

The reason I am asking is because the shop I am dealing with for my parts said they typically come in around 7.450-7.800 (which includes som epreload amount) on their pushrod lengths for their cam I am installing. The problem is I am coming up with 10 turns on the pushrod checker (10x0.050=0.50) which comes out to a zero lash pushrod length of 7.300 (6.800 base checker length + 0.500 from the ten turns) Using what I have found online as well as in the two LS Rebuild How To books I am using to guide me a bit, the standard preload on the stock lifters is 0.60 and the one book even gives the 0.50-0.75 range. Even using the upper part of that preload range it puts me at a 7.375 pushrod length which falls short of the range the shop says they typically measure.

Now I did measure a couple other things and did indeed confirm that the base circle on the new cam is about 0.035 smaller on the intake lobes and 0.055 smaller on the exhaust lobes compared to stock (the stock cam has different base circle sizes for intake and exhaust while the new cam has equal for both). With the smaller base circle of the new cam one would think I would need a longer pushrod than the stock length to make up for the difference lost to the base circle and to maintain the stock preload.

So I am either not setting zero lash correctly or I am using the incorrect lifter preload amount (0.060) to come up with my final pushrod length. Taking my measured pushrod as it is now with my assumed "zero lash", I would need to add 0.150 preload just to get to the lower end of the range the shop stated.

Having said all that, I have meausred the intake and exhaust rods on both cylinders 1 and 2 to take into consideration any difference between int/ext and between two different heads that are "assumed" to be machined to the same exact height from the factory. I have rotated the engine over constantly and always made sure my timing marks were in such a position that the cylinder was in its firing position with both valves closed and lifters on the base circle.

If you have read this far, thanks you and hopefully I haven't confused anyone. Please no replies like "just use what the shop says as it is their cam so they know the pushrod lengths". I would be fine with that if my repeated measurements didn't show me falling short of their range and I tend to get overly anal about doing these types of things as I want to do it once and do it right.

Thanks in advance....