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Old 09-18-2004, 07:34 PM
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cam base circle vs. lobe lift question

Hi All,

My son and I are building a 383" Chevy with 6.0 "H" beam rods (cap screws) and 1.125" compression height. I've posted the question about rod / lobe contact here before and the concensus opnion was that you needed 0.050" minimum clearance.

Today, we attempted to verify the clearance on an old worn out cam and we could NOT figure out a way to do this! We tried putting a pair of rod / piston assemblies in and there is no way to get any kind of feeler gauge in there.

BUT! In the process, we noticed that on this old (factory) cam which measured 0.405" lift on the good lobes, the lobes are almost "even" with the bearing journals. This is making me think that a "high lift" cam MUST be ground with a smaller base circle. Otherwise the cam would not go into the block.

That being the case, would we be OK with the cam (currently in his driver) we'll be using? It's a Comp XM-276-12 with 0.503" / 0.510" lift. Obviously, we can't check with this cam (but will for sure) until it's swap time, but we'd sure like some assurance it will be a go.

How about some peeps with high lift cams measuring "base circle" vs. the "over the nose" dimension?

Thanks!

Russ

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Old 09-18-2004, 08:46 PM
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i have the same compression height and rod length as you do in my 383. my rods are I beam though. the cap screws would come very close to the cam lobes in about 3 or 4 cylinders. way too tight for the .050" clearance.

I'm not sure if the lift you gave for your comp cam is at the valve or what but my lift at the cam is .3454 int and .3574 exh. I was actually able to get a small feeler gauge in between the cam lobes and the rod during my mach up. in order to do it I had to leave the opposite piston/rod assembly out from the one I was testing so I could stick my hand down that cylinder. for example: if I was checking the clearance on the #1 rod, then take out the #2 rod and piston and reach down the #2 cylinder with your gauge. it's a tight fit, but possible. Maybe someone has a better method of double checking.(maybe the modeling clay method) I had to take material off the rods in order to abide by the tolerences. I had to do it to all the rods to make sure I didn't mess up my balance job.

oh and by the way SCAT said that my rods were "clearanced" so they wouldn't hit the cam. well they didn't hit but they had about .015" clearance. if you can avoid all of this with a smaller base circle then I would do so to save you the hassle. I wish I did.

dave
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Old 09-18-2004, 09:52 PM
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Plumbers putty on the rod works great for checking clearances, dissect with an exacto blade and measure the thickness.
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Old 09-18-2004, 09:57 PM
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Quote:
Originally posted by 4 Jaw Chuck
Plumbers putty on the rod works great for checking clearances, dissect with an exacto blade and measure the thickness.
BINGO! Here we were trying to figure out how to stack some kind of crap on the cam lobes and you hit on the perfect solution! We'll try that tomorrow.

Later,

Russ

Oh, any ideas on the base circle stuff?
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Old 09-19-2004, 11:31 AM
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I was also interested in a small base cam. Do they have any neg
compared to a normal base one?
seems like if you can avoid all the clearence hassle why not do it
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