Piston deck height discussion.

David G

Well-known Member
I am checking how much gap is desirable between the top of the piston and the bottom of the head. I am measuring the piston deck about .045 above the height of the block. The gasket ring has compressed to .050 and there is about .005 cylinder liner protrusion, so that should give me about .010 clearance. That seems a little snug to me.

These are custom pistons, so questioning the tolerances.
 
I prefer .030/.032 th.in. Brave people might try low .020s. I don't think .010 will work.
My #s are for my Chevy. I don't know what your Massey needs . I want my squish /quench tight as possible with out piston hitting the head.
 
That is _rather_ tight! Typical recommendations are for around .045 for typical hot rod type motors, although it has been a while since I've built anything. On a stock small block you would likely find about .060 (production tolerances and all). I don't know how tight the modern engines are built.

Now, all that said, sitting in my driveway is a 350 sb that I built that is running .028 piston to head clearance. It has about 30K miles or so on it. I did not build it as a hot rod motor but was experimenting with fuels vs. compression ratio. It has true 9.5:1 with cast iron heads. I would not want to turn it any tighter than 4500.

In an ideal world your clearance would approach zero while the engine is running but of course the question is at what rpm. What are you working on? .010 static seems too tight for anything I can think of.
 
That confirms my calculations, I had the machine shop that ordered the pistons install them. I am checking everything now. I believe that .030-.040 is a tight but safe number for good squench.
 
The engine has had sort of a faint knock in the top end since it was built, which I thought was due to the high compression. I have the engine out to fix some leak issues, so checking everything now. It did not look right to me to see the piston deck above the block deck, so I took some measurements. These pistons were built by Arias to clone the M&W, I am wondering if they got the piston deck height off. I have a set of stock flat top pistons that I can compare to, once these pistons are out. I have pictures of each cylinder, will post them.
 
If there is no evidence of them hitting, and you run at governed RPM, shouldn't hurt for them to be close.

Not sure what will happen when carbon builds, will it accumulate or get blown away by the squench?
 
(quoted from post at 09:51:32 09/28/15) You may want to grind those little grooves in the head or pistons. Seems to help performance I heard.

I second that motion. They will help with detonation issues, and help propagate the burn. They will not however build horsepower themselves. They will allow you however to do other things to the engine that will. i.e. tighter squish, higher compression, increased timing etc. etc. while running a lower octane fuel.

Singh Grooves
 
(quoted from post at 21:34:48 09/27/15) I am checking how much gap is desirable between the top of the piston and the bottom of the head. I am measuring the piston deck about .045 above the height of the block. The gasket ring has compressed to .050 and there is about .005 cylinder liner protrusion, so that should give me about .010 clearance. That seems a little snug to me.

These are custom pistons, so questioning the tolerances.

Crank flex, rod expansion, growth from warming and crank/rod clearances considered. That is tight for a small motorcycle engine let alone a tractor engine.
30 thou cold is probably the close limit. Any more than 40 thou and the squish/quench that reduces detonation is lost.
 
Also depends on what rods are being run as for stretch factor.
We used to do SBC with slightly worked (debur and weight match) factory steel rods to .030 P2H clearence. These would be run to 7500 without issue.
 
(quoted from post at 18:34:48 09/27/15) I am checking how much gap is desirable between the top of the piston and the bottom of the head. I am measuring the piston deck about .045 above the height of the block. The gasket ring has compressed to .050 and there is about .005 cylinder liner protrusion, so that should give me about .010 clearance. That seems a little snug to me.

These are custom pistons, so questioning the tolerances.


I'd cut about .030" off the top of the piston. On a performance engine for street, and some racing use (turned up to 8,000 rpms) I set the head/piston clearance at .035" with no sign of the piston hitting the head. A stock V8 in a car the top of the piston is usually around .025" to .040" below the top of the block plus the thickness of the compressed head gasket.
 
(quoted from post at 07:51:32 09/28/15) You may want to grind those little grooves in the head or pistons. Seems to help performance I heard.



That doesn't do anything to help performance, you can't even see a difference in power with a engine on a dyno.
 
Following up, the owner of the machine shop that ordered and installed the pistons is coming over next Monday night, real good guy.
 

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