Valvetrain questions!

[Turbo6inKY]

Ok, searched a bunch, didn't find much with as much specificity as I'd hoped.

I'm doing a hydraulic roller cam on my engine, I've mocked up the engine with some 8723PT1's I had laying around. This block and heads have been decked a couple of times, so the measurements obviously aren't stock anymore.

My pushrod measurement is 7.980" with the 8723PT1 head gasket, which I've measured at .050" thickness.

Now, there is no 7.980" pushrod I can find anywhere. I can find 7.975, 7.990, and 8.000. I checked Summit and Smith Brothers.

Here's the wrinkle, I'll be putting Cometic MLS gaskets on this thing when it goes together for real, the .027" thick ones. They're just not here yet, and I was bored so I mocked it up and measured. Subtracting the .023" difference between the 8723PT1 and the Cometic from the measured length gets me to a 7.957" rod.

How much longer than the measured rod can I go before the lifter starts pushing the valve open? The lifters themselves have 0.120" total travel in them. I measured with the lifter fully extended, no depression. Obviously I don't want to go anywhere near that, but can I get away with a 7.975" length rod?

Or do I need to just bolt it together with the right head gaskets, measure again, and stop trying to math it out?

 

[earlbrown]

The lifter won't hold the valve open unless you bottom it out (or it pumps up at high RPM due to bad springs and a few other things)

If you don't trust the math you did, just get a set of feeler gauges and put a stack adding up to .027" on all four corners then measure for real. Just in case you can't get exact, Heavy Duty Reynolds Wrap is exactly .001" That's what I do to level large diesel heads on the surfacing machine when I want to get it dead nuts (along with subscription cards out of catalogs)


To answer your original question, pushrod length doesn't need to be dead nuts down to 1/10 of a thou. That's just not possible. ESP considering you're going to have 12 different answers do to varying valve tip heights.

 

[Turbo6inKY]

The lifter won't hold the valve open unless you bottom it out (or it pumps up at high RPM due to bad springs and a few other things)

If you don't trust the math you did, just get a set of feeler gauges and put a stack adding up to .027" on all four corners then measure for real. Just in case you can't get exact, Heavy Duty Reynolds Wrap is exactly .001" That's what I do to level large diesel heads on the surfacing machine when I want to get it dead nuts (along with subscription cards out of catalogs)


To answer your original question, pushrod length doesn't need to be dead nuts down to 1/10 of a thou. That's just not possible. ESP considering you're going to have 12 different answers do to varying valve tip heights.

Thanks Earl! I didn't even think of just shimming the head off the block at the proper height. I'll do that and re-measure in the morning.

 

[earlbrown]

And if you have test springs, you can go ahead and clay the pistons too.

 

[dank GN]

What kinda rockers are you using? Stock ?

 

[Turbo6inKY]

What kinda rockers are you using? Stock ?

The TA Heavy-duty stamped ones on the TA heavy duty shaft.

 

[Turbo6inKY]

Renamed the thread to answer another question, but then answered it myself. Thanks for looking!

 

[haywire4130]

You'll find the tolerance on some pushrods are pretty large when you buy them, I bought a 16 pack amd they varied almost .010 between the longest and shortest, so i just used the closest ones. Measure them and don't assume a 7.980 is exactly that.