Different base circles for intake and exhaust? Reground cam base circle tolerance?

[av8rbray]

I had a camshaft reground for a Gen1 AMC V8 and am measuring it with simple calipers and am finding it has different base circles for the intake and exhaust lobes, Is that normal for a split pattern cam? If not then it has quite a variance in base circle across the all the lobes of .031" :

The intake has a 1.377 average base circle (smallest measures 1.374, largest 1.380 - .006 difference)

The exhaust has a 1.354 base circle (smallest 1.349, largest 1.361 - .012 difference)

Between all of them there is a .031 difference from smallest to largest (1.349-1.380) with a 1.367 average base circle.

Not really a difference I was expecting because the AMC 327's have no adjustment in the rockers.

My original cam base circle was 1.453. So essentially about .045 of material was removed off the base circle: (1.453-1.367)/2. Which would mean I would have to use a .045 longer pushrod to maintain the original pushrod geometry and lifter preload. However with the variances of the base circle, keeping it all exact and the same will be impossible. Should I use different length pushrods on intake and exhaust and accept the .010 differences in lifter preload? Or is .031 variance nothing to worry about with a hydraulic lifter and go ahead and use all the same length pushrod? Or is this way out of spec and see about a different grinder?

[Tom68]

They have to grind the base circle to get the lift, lobes can't be higher than the bearing journal diameter, high lift cams have small base circles, they've left as much material as they can to give you the dual pattern you desire.

That's how I think it works ??

[rebelrouser]

A lot of the cheaper like RV cams are reduce base circle camshafts, and I have had to use longer pushrods many times. For just a mild build I have never used two different lengths, most hydraulic lifters have a pretty big tolerance for preload depth.

[av8rbray]

That is all correct. I suppose my main question is when comparing all the lobes, the diameter of the smallest base circle to the largest base circle has a difference of 0.031”. Is that normal to see such a large difference?

And again the engine does not have adjustable rockers. And no after market options exist. They are shaft mount that bolt to the head and whatever preload you get is what you get. You cannot change it. If the lowest heel of the cam and the highest heel of the cam is on one side of the engine, and I get pushrod length to give me zero lash on the “thinnest” lobe, then I will have a push rod pushing .031 difference into the pushrod cup on the “thickest” lobe. Wouldn’t that have the potential to hold a valve open when the lifters are fully pumped up?

Or I’m thinking about this wrong? Perhaps the heels of each lobe would be at the same “level” relative to the bearing journals? Is that what base circle runout is?

[Walter R. Malik]

I had a camshaft reground for a Gen1 AMC V8 and am measuring it with simple calipers and am finding it has different base circles for the intake and exhaust lobes, Is that normal for a split pattern cam? If not then it has quite a variance in base circle across the all the lobes of .031" :

Almost any camshaft with different lobe lifts will have different diameter base circles.
Especially when regrinding one ... the lobe being reground is usually finished when "it cleans-up all the way around" which usually will have differing base circles. Minimum metal removal is usually the target.

[CamKing]

Almost any camshaft with different lobe lifts will have different diameter base circles.
Especially when regrinding one ... the lobe being reground is usually finished when "it cleans-up all the way around" which usually will have differing base circles. Minimum metal removal is usually the target.

Exactly.