Vengeance

What is the average CC of the top ring land clearance? If this guy's stats are correct his is around 4.5 CC. From what I can find elsewhere the average is around 1-1.5 CC. That's a far cry from 4.5 CC.

Vengeance

I finally found one that calculates the top ring land into the compression. It only changes the compression by 0.1 at max, not .411 so that guy's math is off.

joe_padavano

I was going to say, just do the math. That volume is half the piston-to-wall clearance times the distance to the top ring times the circumference of the piston. I actually get less than your number.
On a stock bore 455, using round numbers, that's about 0.0015 (half the clearance) x 0.1 (distance from piston crown to ring) x 4.125 x pi = 0.0019 cu in, which converts to 0.03 cc

BillK

Joe,
You have to remember that most pistons are a lot smaller at the top.

I just measured a completely stock GM 425 piston. The diameter at the top is 4.100 The top ring is .250 down from the top.

Doing the math I come up with right about .671cc There is a slight chamfer on the top of the piston too so that will add a little bit. Either way its less than 1cc.

joe_padavano

Bill,

Thanks for that, I did overlook that point. The top ring is a quarter inch down? I wouldn't have guessed that and I didn't have time to go dig out a piston. Ignoring the chamfer, I get 0.66 cc, so close enough to your number.

BillK

Joe,
I just happened to have a 425 that I am working on otherwise I would not have had a clue This engine was supposedly "completely rebuilt" last year ?? I have my doubts if even .040 over is going to get the bores straight and round. Might have to go 60. The car itself is beautiful and if you looked at the engine externally you would think it was new but that was about as far as it went

Vengeance

I meant 0.1 compression points, not cc. It's so small it's probably not worth calculating.
That's only while it's cold. At operating temp the side clearance should be pretty even top to bottom, since the top gets hotter.