Running pistons above the block

Road-race engines and ancillaries - general discussion
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HFStuart
Posts: 24
Joined: Sun Feb 11, 2007 10:05 pm
Location: Suffolk
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Running pistons above the block

Post by HFStuart » Sun Nov 27, 2011 10:36 pm

I'm building a 'fast road' engine for my 2.0 Lancia Beta Spider using the internals of the one a rebuilt a few years ago (that car was scrapped)

The intended spec is:
150-160 bhp, 7k RPM max. Very tractable.
Ported Head with the larger inlets
9.5-9.8:1 compression
2x45 DCOE
Std cams or std exhaust with a slightly more aggressive inlet
Mapped ignition.

The engine has flat top cast pistons with 84.8 bore. Guy will be getting various bits to prep for me but I've run into a small problem while stripping the block this weekend. I've measured the compression chamber volumes (I can post the figures if anyone wants to check them) and as it stands it would be 8.7:1 compression ratio, too low I suspect, especially if I want to go for a hotter inlet cam. To get it to 9.5 I need to deck the block 1.1mm. This will mean the piston crown is 0.35mm above the block at TDC. Piston to head clearance will be OK at 0.9mm but I'm concerned about the head gasket. I've two old ones and the fire rings measure 84.75 - 84.85 diameter on both. That's too tight to the pistons which are 84.76mm.

I don't want to skim the head as it's already very close to the edge of the inlets (any thoughts on this?).
It seems I can either look for head gaskets to suit an oversize bore or get a small chamfer machined on the outer edge of the piston crown - say 1.3mm deep at 30deg.

Any ideas on which way forward is best? Custom dome pistons would solve the issue but I do have some budget constraints here!

Stuart
GC 158

Urbancamo
Posts: 318
Joined: Mon Aug 08, 2011 2:04 pm
Location: Finland
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Re: Running pistons above the block

Post by Urbancamo » Mon Nov 28, 2011 1:31 am

Greetings Stuart.

I would do chamfers for pistons. Very easy and effective. Can be easily made in lathe.
I've run pistons over deck by 0.4 mm but with original bore. I know TC engines wich use 85.5 mm pistons with STD gaskets with no problem. Some chamfering to pistons is of course needed...

Notice that there is several thicknesses for headgaskets. I've seen 1,4 and 1,6 mm uncompressed ones.

0.9 mm quench sounds like a working engine!
GC_25

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