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Discussion Starter · #1 ·
So based on an old post by Rick, I'm trying to estimate my pushrod length to order some. I will get a checker, but if I can make an educated guess I'll order some up so as not to delay my build. The key piece of information I did not know was that LS cams provides addition lift by going to a smaller base circle, thus requiring a longer pushrod. However, it is said the LS7 lifters are .050 taller than stock....

So, the highest lobe lift of the L76 cam is .287 (E, AFM lobe). The cam I am looking at has a .356" lift on the exhaust lobe. So that means my base circle will be .356-.287 smaller, or .069". I would then add this to the stock pushrod length of 7.400:
7.400+.069 = 7.469.
However I need to subtract off the additional height of the LS7 lifter:
7.469 - .050 = 7.419
I suppose this is why I see many people ordering 7.425" pushrods, and that is what I should get. I am not touching the valves or heads, and assuming the L92/LS3 head gasket I'm getting with the kit is the same as on the car....
Any errors with this that anyone can see?
 

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i run 7.4's and am switching to 7.425 for quieter (hopefully) valvetrain
 

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Tin Indian
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I am quite confused with your calculations. You are referring to lobe lift, and that has nothing to do with the base circle of the cam. The base circle is simply the portion of the cam that has no lobe on it. Small base circle cams were developed for large stroke crankshafts as they have less of a chance of interference with a rod. The only thing that will effect pushrods length is going to be your new lifter height. Correct pushrods length is CRITICAL as side loading of the valve and severe spring deflection will occur with the wrong length and cause premature wear and breakage. Also, noisy valve train has nothing to do with pushrods. There is only one correct length for a given combination, changing that to an incorrect length will start to damage parts very quickly. Just my .02 from years of building drag racing engines....
 

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Discussion Starter · #4 ·
Apparently the LS motors need to go to a smaller base circle to increase the lift of the cam.... Thus the general need for longer pushrods.

See post #4:

http://www.g8board.com/forums/showthread.php?t=24246&highlight=base+circle

As far as the noise - the LS motors valvetrain is non-adjustable, so if you use pushrods that are a bit too short, like the stock ones might very well be, you will probably not have enough preload on your lifter which I'm assuming could cause the noise. I am coming from a traditional Gen 1/2 SBC background so this is all new to me....
 

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Guessing on pushrod length is not really a good idea IMO. Best to just get a length checking tool, get your measurement with the cam at its base circle and the tool adjusted to zero lash. Then just add the recommended amount of preload to the overall length of the measurement. If your using LS7 lifters, i believe the recommended lifter preload is .030" If you do this, you will also need checking springs to install in place of the valve springs to avoid collapsing the lifter when taking measurements.
 

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Everything depends on what you are running. I ended up having to go with 7.100 because of my shaft rockers and lifters.
 

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Contact your cam manufacturer and find out EXACTLY what is the base circle of the cam you intend to buy. GM doesn't even use the same base circle on all LS cams. Your calculations do not include the amount of lifter preload or the change in base circle.
 

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Contact your cam manufacturer and find out EXACTLY what is the base circle of the cam you intend to buy. GM doesn't even use the same base circle on all LS cams. Your calculations do not include the amount of lifter preload or the change in base circle.
That's exactly it. I never compared the 2 cams but i would bet a paycheck that is why i had to go with a 7.100 rod. I had to shim it it get it to that length because if I did not,my length would be right smack in the middle of 2 different rod lengths. This was a problem with my shaft rockers because these rockers have oil passages for the bearings, and you don't want to overextend the adjusters or you will block the passages. So I set my adjusters out 0.050. With no shims 6.950 would extend them too far and 7.100 would leave no room for 0 lash. With the shims, I am about an 8th turn past 0.050 then add 0.010 preload for the lifters.Did my wipe check and all looks good.
My lesson learned here(which i already know but with the advancement of technology i did not think i needed to) is do not trust what the vendors/package sellers tell you because 1. I was told i would not need to clearance my heads for interference with the rockers and 2. I was told that my base circle would be smaller than stock(most aftermarkets are) and 3. 7.400 rods will be perfect for my setup.

I'm just wondering now how many of those so called packages are running very close to coil bind.
 
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