Shim or turn down cylinders.

[stevepugh]

Have a set of pistons and cylinders and the piston sticks out the top. So it has negative deck. (This still confused me. I'd say positive. )
Put a .090 base spacer and now have measured a deck of .023

So to get a near zero deck I will need a base spacer of .070.
You can't buy this size.

I have the option of some long cylinders. But they are around 6mm longer than the std length ones.

So.. Do I get the long cylinders and have them machined down to give me a zero deck. Or buy 2 base shims to give me .070?
(A .040 and a .030 for example. I don't like the idea of a .060 and a .010 as the .010 is thin and more likely to distort...)

I've had mixed thoughts about machining cylinders and getting them a the same.

Cheers.

[Marc]

Piston deck height is by definition the measurement of the distance between the end of the cylinder and the top of the piston, which is always assumed to be below it. In the rare cases of engines which rely upon the head gasket alone to prevent the piston from smacking the head (some diesels, for example) the term used is "piston protrusion" which is another way of saying negative deck height.

Why do you want zero deck? Unless you're going to run head gaskets, that means the pistons will be smacking the heads at anything over 2 RPM.
If you are going to use head gaskets, they have the same effect on the unswept volume as the same amount of additional deck height, so nothing's gained.

Minimum safe piston-to-head clearance is affected by the piston rock (so both bore and piston-to-cylinder clearance matter), crankshaft/case flex, bearing clearances, even connecting rod elasticity. What's "safe" at 5000 RPM could have disastrous results at 6500 - can you be sure that you'll never, ever, miss a shift?

The tightest piston-to-head clearance I've ever used was .040" on an offroad 1600 - head CCs were regulated so it was necessary to run as little deck as possible to keep the C.R. up, and .040" was the minimum allowed. Stock 85.5mm bore with .003" piston-to-cylinder clearance & stock rods. You wouldn't think the RPM could reach extremes breathing through a 30PICT-1, but every time I tore that engine down for maintenance (sometimes with as little as 100 miles on it) there was NO carbon buildup on the heads or pistons in the flat "quench" areas - they were that close to making contact. With a larger bore, or for longer service intervals, it'd be folly IMO to run much less than .050".


<edit> I must've misread something the first time. If you have .023" deck with .090" shims, .120" would be needed for .053" ...If you want to push your luck and run .043", you'd need .110" in shims. Doesn't make any difference to the combustion chamber shape whether it's all under the cylinder or split between there and a head gasket - do you have a good reason to run head gaskets? IMO, you're better off without them for a N.A. engine, unless you need to be able to change displacement and/or C.R. easily (as in when racing at tracks at greatly different elevations). For a turbo or nitrous application the gasket can be your "fuse" - hopefully when one blows out/sucks in you'll be able to shut down before doing any more damage.
Custom-length cylinders seem more elegant than juggling shims/spacers/gaskets, but they can be a real P.I.T.A. if you need a replacement in a hurry.

[shepherd]

you can have your .090 shims ground down to whatever you want, far cheaper than cutting new long cylinders down. I've done it, with a magnetic bed on a surface grinder, to 'fix' some scat shims that weren't consistent in thickness. Or, you can run a .060 bottom spacer, and .050 top copper shim. This would shim your cylinder the .070 and give you a .040 clearance at the top. Or you could have a .145 CB performance shim ground to .011 and achieve the same thing with one part, not two. Or deck the case, and use different gaskets.. many ways to skin this cat

[Piledriver]

You should always at least carefully check the case deck first...

I ran >200K miles on a T4 with .028" deck, while running it to 6200 RPM redline at every opportunity.
(admittedly, stiffer crank etc) It ran the previous 150K miles with .038" deck...

True-- no carbon in the squish areas when taken apart, but no evidence of contact either.

On a non-super high RPM motor IMHO the only benefit a loose deck allows for is wiping a rod bearing and preventing contact.

If the engine is a high RPM-only drag motor squish doesn't really help much.
Otherwise, I go as tight as I can within the range of sanity, even at the frayed edges...
For street engines, it helps. For racing engines, I can see the .050" safety net.

Having said that, I'm at .055 on the current setup due to step cut heads I had handy, seems to run pretty stout for a bus motor, and it seems ~det proof.

I'll argue a 94mm piston is not a large piston.

If you are running 4" bore with silicon free Wisecos I can see needing more deck to allow for cold rock, but not much.