Quick question on pushrod size.
TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
Throwing a few pennies into the pot...
We just did a H/C swap using LS7 lifters. VRX3, Patriot 59cc milled heads, MLS gasket. I somehow screwed up the measurement on the pre-load the first time we did it and used 7.400 pushrods. NOISY! Could not live with the sewing machine ticking. Pulled the covers and re-measured using a slightly different method and found we had 115 mil (0.115 inch) of pre-load. Seemed we should go to a 7.350. Asked questions, got no help on Tech, but a local shop recomended we go down to a 7.300 pushrod. I was skeptical of it, but the difference is incredible. Night and day difference in the noise. We now have 15 mils of pre-load, and the engine is almost as quiet as stock. I am not educated enough in the valve-train dynamics to understand why this would be a problem. The empirical evidence says it works good. Also, those clearances are measured in a garage at 50* F. Once warmed, the Pushrods, Lifters, Valves, everything will want to expand, not contract. I don't see how this could be a problem. Just wanted to throw that out there...
Oh, and I think I screwed up in locating the zero lash the first time.
We just did a H/C swap using LS7 lifters. VRX3, Patriot 59cc milled heads, MLS gasket. I somehow screwed up the measurement on the pre-load the first time we did it and used 7.400 pushrods. NOISY! Could not live with the sewing machine ticking. Pulled the covers and re-measured using a slightly different method and found we had 115 mil (0.115 inch) of pre-load. Seemed we should go to a 7.350. Asked questions, got no help on Tech, but a local shop recomended we go down to a 7.300 pushrod. I was skeptical of it, but the difference is incredible. Night and day difference in the noise. We now have 15 mils of pre-load, and the engine is almost as quiet as stock. I am not educated enough in the valve-train dynamics to understand why this would be a problem. The empirical evidence says it works good. Also, those clearances are measured in a garage at 50* F. Once warmed, the Pushrods, Lifters, Valves, everything will want to expand, not contract. I don't see how this could be a problem. Just wanted to throw that out there...
Oh, and I think I screwed up in locating the zero lash the first time.
64cc-59cc= 5cc
Each 5cc are (.005x 5= .025') so allready there you should have .025' shorter p-rod.
Let us continue: Base circle of cam smaller:
VRX3 is XE-R lobes on intake, XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .025' shorter on p-rod due to mill and .0475' longer due to base circle, so subtract the two and you have .0225 longer than stock pushrods (so 7.4225)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Substract that amount:
7.4225 - .060 = 7.3625 (That alone should have told you your 7.400 are too long)
So (on paper) and this is in no way accurate,
BUT here this is LS7 lifters and not LS1/LS6 and you chose the shorter p-rod but the 7.350 would have worked as well.
Last edited by PREDATOR-Z; Apr 21, 2009 at 11:58 PM. Reason: error in calculation
Are the Caddy lifter the same pushrod cup height as the LS1 or the LS7 lifters?
I gotta say that the most important thing I have learned in all of this is that you either need adjustable rockers, or you REALLY have to measure for pushrods. The engine is very sensitive to the length of the pushrod. Throw a few tolerances about, and even a few slight machining errors, and you can quickly be too long/short. Funny how 50 mils makes such a difference in this engine. Sometimes you do have to miss how simple things used to be with a mechanical or hydraulic flat tappet. Also noted that there is probably a fairly large practical range that will work just fine (seems to be 10-100 mil) but that you will get different noise levels from each. Also have noted in other threads that oil presure, viscosity, and quality seem to really be sensitive issues as well with the stock lifters.
I gotta say that the most important thing I have learned in all of this is that you either need adjustable rockers, or you REALLY have to measure for pushrods. The engine is very sensitive to the length of the pushrod. Throw a few tolerances about, and even a few slight machining errors, and you can quickly be too long/short. Funny how 50 mils makes such a difference in this engine. Sometimes you do have to miss how simple things used to be with a mechanical or hydraulic flat tappet. Also noted that there is probably a fairly large practical range that will work just fine (seems to be 10-100 mil) but that you will get different noise levels from each. Also have noted in other threads that oil presure, viscosity, and quality seem to really be sensitive issues as well with the stock lifters.
Last edited by 67rsss; Apr 21, 2009 at 11:28 AM.
Ok, just a rough calculation: (I'm assuming these are LS6 castings). This is not to prove anything but just to show that it makes sense.
64cc-59cc= 5cc
Each 5cc are (.005x 5= .035') so allready there you should have .035' shorter p-rod.
Let us continue: Base circle of cam smaller:
VRX3 is XE-R lobes on intake, XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .035' shorter on p-rod due to mill and .0475' longer due to base circle, so subtract the two and you have .0125 longer than stock pushrods (so 7.4125)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Substract that amount:
7.4125 - .060 = 7.3525 (That alone should have told you your 7.400 are too long)
So (on paper) and this is in no way accurate,
BUT here this is LS7 lifters and not LS1/LS6 and you chose the shorter p-rod but the 7.350 would have worked as well.
64cc-59cc= 5cc
Each 5cc are (.005x 5= .035') so allready there you should have .035' shorter p-rod.
Let us continue: Base circle of cam smaller:
VRX3 is XE-R lobes on intake, XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .035' shorter on p-rod due to mill and .0475' longer due to base circle, so subtract the two and you have .0125 longer than stock pushrods (so 7.4125)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Substract that amount:
7.4125 - .060 = 7.3525 (That alone should have told you your 7.400 are too long)
So (on paper) and this is in no way accurate,
BUT here this is LS7 lifters and not LS1/LS6 and you chose the shorter p-rod but the 7.350 would have worked as well.
Last I checked it was. 
Using Predators Numbers look how close just will 2cc diff it makes.
My Setup:
64cc-61cc= 3cc
each 1cc are (.005x 3= .015') so already there you should have .015' shorter p-rod.
Let us continue: Base circle of cam smaller:
XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .015' shorter on p-rod due to mill and .0475' longer due to base circle, so Subtract the two (.015 - .0475 = .0325) and you have .0325 longer than stock pushrods (so 7.4325)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Subtract that amount:
7.4325 - .060 = 7.3725
My Setup:
64cc-61cc= 3cc
each 1cc are (.005x 3= .015') so already there you should have .015' shorter p-rod.
Let us continue: Base circle of cam smaller:
XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .015' shorter on p-rod due to mill and .0475' longer due to base circle, so Subtract the two (.015 - .0475 = .0325) and you have .0325 longer than stock pushrods (so 7.4325)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Subtract that amount:
7.4325 - .060 = 7.3725
Using Predators Numbers look how close just will 2cc diff it makes.
My Setup:
64cc-61cc= 3cc
each 1cc are (.005x 3= .015') so already there you should have .015' shorter p-rod.
Let us continue: Base circle of cam smaller:
XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .015' shorter on p-rod due to mill and .0475' longer due to base circle, so Subtract the two (.015 - .0475 = .0325) and you have .0325 longer than stock pushrods (so 7.4325)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Subtract that amount:
7.4325 - .060 = 7.3725
My Setup:
64cc-61cc= 3cc
each 1cc are (.005x 3= .015') so already there you should have .015' shorter p-rod.
Let us continue: Base circle of cam smaller:
XE-R lobes are about 1.455 in base circle, stock is 1.55 so that is 0.095 difference. Take 1/2 that amount as effective base radius (.0475)
Now you are .015' shorter on p-rod due to mill and .0475' longer due to base circle, so Subtract the two (.015 - .0475 = .0325) and you have .0325 longer than stock pushrods (so 7.4325)
Now the LIFTERs: LS7 have the cup .060 taller than LS1//LS6 OEM lifters.
Now you have too long a p-rod.
Subtract that amount:
7.4325 - .060 = 7.3725
Only when MEASURING to zero lash are you doing anything other than guessing.
yes there are many variables, but it gets people an idea of whats going on, and hopefully lets people know that the only way to know what length pushrod to use is to measure their exact setup.
Why? Changing the pushrods is easy, takes us about 15 minutes including torqueing everything back up. I can't imagine it is much harder to do when the engine is in a 4th gen...
Now the real world issue here that NOBODY is talking about with the faux guessing math is how much of that milling was needed to flatten out the cylinder head? They don't just automatically clean up when you take .001" off of them... you would have to physically measure the setup for that. Or how about the depth and diameter of the VJ relative to the valve?
Then again if you have never actually milled heads or machined valves and valve jobs you can't understand the tolerance stacking that is going on here. That's also why the only way you can set the proper spring installed height is measuring EACH spring, not just throwing the same parts at every one.
As for preload... nobody is mentioning power output numbers from any tests they have done here.
I know WHY I recomend the amount of preload I use, because I have data behind that testing those lifters.Bret