Tin Indian

Seems like some folks have a little problem understanding pushrod length requirements.

The aftermarket cams being installed today will usually have a smaller base circle than the stock LS1 cam they are replacing.

Base Circle is actually the distance accross the cam's lobe. This is 90 degrees from the top of the cam's lobe at the point of highest lift.

You can easily measure this distance with a micrometer. Measure across the cam's lobe (the lobe's smallest distance). You will need to cut this distance in half in order to get the cams "centerline to lobe heal" distance. This centerline to lobe heal distance is what is important to you in determining lifter preload.

The stock LS1 cam has a 19.7mm (.776") centerline to lobe heal distance.

By the way, someone posted a while back that the TR-224 actually was measured to have a .030" smaller centerline to lobe heal distance than the stock cam.

If your aftermarket cam has lets say a .746" centerline to lobe heal distance, that would mean that it is .030" (30 thousands inch) smaller than stock. Remember, the stock cam was .776"

To be very accurate, you would then need a pushrod that is .030" longer than stock to achieve the same spec lifter preload as you had with your stock cam.

These aftermarket pushrods come in .050" length increments, so in this case, a longer 7.45" rod would get you closer to your factory spec lifter preload than using the stock 7.40" pushrod.

Remember, hydraulic lifters, due to there design have a + - preload variance that the lifter can operate within. It don't need to be exact. Just get it as close as possible with the various pushrods that are available (in .050" increments)

Be advised, milling the heads will require shorter pushrods equal to how much is milled off the heads.

So, to re-iterate all this:

1. If your cam has a .030" smaller centerline to lobe heal distance than stock - You would need a .030" [longer] rod.

2. If you just milled the heads .030", you would need a .030" [shorter] rod.

3. Combine the two, and your stock 7.40" pushrod would be perfect.
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Lets say you can't measure you new cam's base circle; What to do?

Turning the engine, bring the exhaust pushrod on any cylinder up until it is at peak lift (valve fully open). Now, install a pushrod on the intake of that cyl. (that valve will be fully closed) and turn the rocker arm bolt down until the lifter is at "zero lash" (pushrod will start getting harder to turn when spun with your fingers). Mark the bolt with a marker. Count how many turns it takes to completely tighten the bolt to 22 FT/LB. From what I have read, the bolt should have turned between 1 to 2 turns for the acceptable GM lifter preload.

Hope this helps,
Ron
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<small>[ October 28, 2002, 06:18 PM: Message edited by: Kimchee and Rice ]</small>

DravenMyers

Good info, Ron! Thanks! <img border="0" alt="[cheers]" title="" src="graemlins/gr_cheers.gif" />

ibjamin

iTrader: (6)

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">To be very accurate, you would then need a pushrod that is .030" longer than stock to achieve the same spec lifter preload as you had with your stock cam.

These aftermarket pushrods come in .050" length increments, so in this case, a 7.35" rod would get you closer to your factory spec lifter preload than using the stock 7.40" pushrod.
</font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">I think you mean the opposite here don't you? If the base circle is smaller and you need a longer pushrod then you would need a 7.45" length to get closer to the factory preload. You could stay with the 7.4" but you certainly wouldn't want to go shorter. Right?

George E.

Tin Indian

George...

Your right. I changed it

Tin Indian

ttt

Reckless

iTrader: (9)

Leave it up to Ron to complicate things <img border="0" title="" alt="[Big Grin]" src="gr_grin.gif" />

AFTICA

iTrader: (8)

Good info. There has been alot of confusion over this. Just one question; Why are the aftermarket cam manufacturers making cams for our cars with smaller base circles? Do they NEED to do this to achieve the lobe designs used these days?

myss2638

iTrader: (10)

Great info.

Has anyone actually ran the longer pushrods?

I have stock heads with the tr-224 cam. The valvetrain is noisey and it is starting to bug me a bit. I am happy with the power the car makes but headwork is not in my current budget. I would love to cut the valvetrain noise down.

LunaticTA

</font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by myss2638:
<strong>Great info.

Has anyone actually ran the longer pushrods?

I have stock heads with the tr-224 cam. The valvetrain is noisey and it is starting to bug me a bit. I am happy with the power the car makes but headwork is not in my current budget. I would love to cut the valvetrain noise down.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Did you purchase the valvetrain upgrade from TR at the same time you Got the cam?

I have the same cam and the noise is pretty annoying, so I went back through my pics to see what TR sent me for a replacement pushrod and this is what I found.

<img src="http://www.ws6transam.com/images/SLPrSS/TRProd.JPG" alt=" - " />

So if the above is true then TR sent me stock length Push rods instead of longer ones to make up for the difference in base circle. <I wonder what they are thinking>

Is .030 really going to make that much of a difference in valvetrain noise though?

Interesting thread.

Chuck

myss2638

iTrader: (10)

I purchased the cam seperately, right now I am running the cam with rev 1116 dual springs and stock rods.

If I wasn't gettting knock retard I would be able to live with a noisey valvetrain.

I am not sure whether the longer pushrods would lessen the noise hopefully someone can help out here.