I'm currently in the process of have a 383 built for me and the guy and I got on the discussion of small base circles cams to use for clearance. And he said that Comp has some good selection of small base flat tappet cams. I told him that I didn't wanna use a small base cam, and the H beams and the pistons were too much money for an ungrade for me. I told him that I didn't wanna use a small base cam because I already had a cam picked out. Then he asked which one? And I told him that it was a GM Performance roller cam with .558 on the exhaust side. So then he put me on hold for a minute and came back and told me that a roller cam is already ground on a small base. Then I just said "ok" and left it at that cause I didn't know for sure if he was right or not, so I couldn't tell him that he was wrong. Can anyone that knows what they're talking about shed some light on the base circles of off the shelf roller cams?

Thanks,
Tom

 

Where did all the pros go?

 

I don't know about any "pros", but.... I didn't feel like trying to touch that one with a 10-foot pole.

Every off-the-shelf roller I've ever worked with was on the same base circle as any other cam. That is, at peak lobe lift, the tip of the lobe was about the same distance from the cam centerline as any other cam. Of course, as you increase the lift, the lobe doesn't get taller; it can't get any taller than the diameter of the cam journal, otherwise it wouldn't be possible to install it. Instead, as the cam gets "bigger", the lobe gets smaller on the side opposite the tip, since lift is the difference betweeen the 2 heights.

So, one could say, that the "bigger" a cam is, any cam, the smaller the base circle actually is. The limit to how "big" you can grind a cam blank is how small the opposite side of the lobe is. So to make a "big" cam that has overall smaller lobes, in order to provide a little more clearance to rods, the whole cam has to be made smaller; which can either be done by using a smaller blank, or grinding it down more which is not very economical.

I think your guy is a little bit misinformed, or doesn't understand cams very well.

 

There are standard base circle roller/and flat tappet cams.
There are "small base circle" cams too.
There are different small base circle cams. .800" .875" .900"
are common.

Even a small base circle cam may not clear your rods.

If yu are building a 383 or 406 with a 3.75 stroke crank and useing stock 5.7" GM rods ( or simular) you will have to check and verify cam/ rod clearance yourself and clearance grind the rods/ rod bolts.
The best thing to do is borrow a cam the same/bigger or simular in lift/duration and lobe separation and base circle dia and trial fit it to your motor.
Depending on how the rest of your motor is machined, you will have to check and verify proper push rod length with a non standard base circle cam.

When we did my 406 we clearanced the rods for a 265deg .650" lift roller and then checked it again with a big solid cam.
Check it advanced and retarded too as this affects cam/ rod clearance as well.

(the cams I'll be using will never be this "big")

Just part of the joys.