Pray

iTrader: (7)

I pulled the heads off my car tonight to prep to sell and noticed a huge gap between my piston and cylinder wall. The car has never made the power I thought it should and sends oil into every thing in and on the engine. I also never had the cranking compression I thought I should have had either. The car still made 497/457rw out of a LS6 383. Well any way, I measured the gap and it is .035. I am sure it is supposed to be between .003 and .004 with Diamond pistons. I guess I should have noticed this before I assembled the engine but I wached my machinest hone the block and trusted his work. He had the pistons measured so he knew how much needed to be removed with the hone.

Here are my questions.

The gap is supposed to be .003-4 correct? Not .035.
How screwed is this engine?
Does it have to come out and the block be replaced and rehoned?

Brett

cncbird

iTrader: (3)

you have to measure the clearance on the piston skirt ,not at the top---they are much smaller around the ring lands

chopper

iTrader: (18)

what He said ^^^^^^^^
the piston mfg's vary a little on exactly where on the skirt to measure but about one inch below the bottom of the oil groove.
Oh btw, nice numbers on the dynow! What heads and cam did You use? Carl.

Pray

iTrader: (7)

Thanks guys. I was about to freak out on my machinist. From what you are saying the crown is .030 smaller on my Diamond pistons? Wouldn't that induce piston rock and put alot of strain on the exposed part of the rings? I have done six heads and cam jobs on stock long blocks for guys and I can usually barely see the top ring at TDC.

I had some Larry Meaux ported AFR 225's with a 239/244 .651/.612 112+2. The cam is still for sale. I think the heads were a little too large and I was shrowding the valves. I am going with some TSP 2.5 LS6 heads and a 244/248 .610/.615 114+4 now. Hopefully I will make around the same power with the smaller runner and larger cam.

KCS

iTrader: (20)

How exactly did you measure the Piston-wall clearance? Did you use a feeler gauge or something?

The crown area of the psiton is smaller because that is where it will absorb the most heat, from combustion. Forged aluminum is dense and expands alot so it needs more room than a cast piston too expand.

Yes, it will cause piston rock, or what is also known as "piston slap" when the engine is started up cold. As the temperature reaches normal operating temps, the clearances are taken up and it is OK to put a little throttle and load on the thing. Nature of beast.

Pray

iTrader: (7)

I first measured with a couple of thing feeler gages to fit around the piston. But the gap was so large that I just used the inside measureing part of my dial calipers to get an exact measurement. I know forged pistons expand more than a cast of hyper piston does but I had no idea it would have that large of a gap. We did measure the pistons one inch down on the skirts when we were honing. That is why I was so confused. I just didn't think the piston crown would be .030" smaller than the skirt. I called Diamond to get an exact answer but they were closed during business hours. I will try again later.