MikeGyver

I'm almost done building my turbo'd 383 which will be running 10-15psi and hopefully making about 700rwhp. I bought a short block from a member of this forum and the machine shop paperwork says the block was decked to 8.990".

The stroke is 3.75", 6" rods, and piston compression height is 1.125" giving a 9" max piston height. This should put the piston .010" out of the hole, but I've run a straight edge across and its not. The piston is about .010" IN the hole.

I've tried tapping on the bottom of the piston while watching a dial indicator centered on the piston's top, and it only raises it about 2 thou, as would be expected. So whats the deal? is 9" a hot measument after the piston grows? Or is my deck height not actually 8.990"?

The reason I ask is because I need as small a gasket as possible to raise my compression from around 8.6:1 to 9:1. I need to somehow accurately conclude where the piston is so I can set my piston-to-head clearance to about .040" with the head gaskets. Any ideas?


Here's some pics for sheetsengeeggles. I checked valve to piston clearance using clay. I threw on the factory .050" head gasket I pulled off my old motor, and used about half the head bolts and torqued them to maybe 50ftlbs or so. The Dime is .050" thick. The clay squished down to about .075" which is the piston to head clearance, and shows that the piston is in the hole a little bit.

Revolutionary

You really need a bridge with a dial indicator to determine exactly where the piston sits.

MikeGyver

So where it sits is where it sits, reguardless of what the peperwork says? All I need is at least .040" clearance from where the piston sits to the head?

I'm going to set the piston to TDC with a dial indicator, then check how far the piston is in the hole using a straight edge and feeler gauges.

MikeGyver

Ok, I measured the 2 side points above the wrist pin (so the piston doesnt rock, as much anyway) and I got .006" and .012", so if you level the piston and split the difference, it's .009" in the hole.

Is this conclusive enough to say that I can safely run a .040" gasket?

Revolutionary

yep

WS6 TA

Mike, isn't that always the case, it doesn't seem to ever be as easy as it should be?

Have you checked more that one piston? it almost sounds to me like one of your numbers is off or the machine shop screwed up, since you're getting an error in the range of .020" deck height vs assembly height. If one piston is _that far off_ I wouldn't be really surprised if you found that the decks are cut crooked or uneven side to side, I mean that's a big error somewhere. It also bothers me that you're getting that much variation over the wrist pin on one piston, are you saying there is that much slop in it or that the 2 sides are that different? If you're sure about the numbers on the parts than I'd be measuring the thing as many ways as I could come up with looking for machining errors (are the decks straight, are they 90* to each other, are the 90* to he bores...)

Assuming that something else isn't wrong, then you're right, the height you measure is the height you have, don't do any special stuff trying to get it to move, just turn it over, find TDC and measure, the typical >.035 or .040" quench clearance is to account for things trying to stretch while the engine is running, that is why it's not uncommon that when you get below the .030" range you start seeing evidence of the piston touching the cylinder head when you run it.

As long as you don't find other problems and you're finding that all the pistons are at least .006" in the hole, I'd feel safe running a .040" gasket, heck, it's just an engine, right? If I had a reason to I'd consider tempting fate I would even _consider_ running something as think as .028" (.034" quench, that's about as tight as I've ever tried going and didn't have any signs of any problems).