Pistons protruding

[christenlin]

I am putting together an engine for use in a special, it has a 2 bearing 1.5inch Phoenix crank, Phoenix rods and modern pistons from Tony Betts. It also has a high compression head.

A trial assembly, has revealed that my pistons protrude the deck by 8 thou.  I would welcome advice on the best way to deal with this, and what clearance under the head should I really be aiming for, bearing in mind head gasket thickness etc.

Many thanks

[Reckless Rat]

Sounds like you've had the block planed (or someone else has). You will need a spacer shim to go between the block and crankcase or some metal taken off the tops of the pistons. If I remember correctly the pistons should be .020" (?) below the block surface to allow for crank whip at high revs. The spacer shims are/were available at A7c.co.uk.

[Tony Betts]

its more likely you are useing a base gasket that is 17 thou thick, rather than 27 thou thick.

dont waiste your money on spacers. 

fit a thicker gasket, or put two in.

[Ian Williams]

Sorry to disagree with you on this occasion Tony, although what you suggest may solve one problem, personally i believe two paper gaskets (one even!) is a perfect recipe for a leaking block joint.

[Ivor Hawkins]

Perhaps a couple of silicone block gaskets would do the trick?

[Terrytuned]

Silicone gaskets are to soft for this, use a block shim coated on both sides with Three bond or machine pistons. I do a dry assembly to check this before fitting the piston rings. Terry.

[Bob Culver]

If the gaskets do not overlap does a mere .008 matter? Assuming use of a conventional gasket

[andrew34ruby]

If the gaskets do not overlap does a mere .008 matter? Assuming use of a conventional gasket

I think the eight thou might matter if there is a lot of crank whip.

[Charles P]

If the gaskets do not overlap does a mere .008 matter? Assuming use of a conventional gasket

I think the eight thou might matter if there is a lot of crank whip.

It's a 1.5" crank. 
My basic maths tells me that a 1.5" bar is about 1.7 times stiffer than a 1 5/16 so how much more will they really protrude under whipping? This probably isn't the same number as the "20 thou" frequently bandied, probably because Jack French said it in the 50's.
Turning down the piston crowns by a few thou would seem a lot easier than messing about with shims under the block.



c

[Alan]

20 thou might be correct for an original 1 5/16 crank at 6000 revs. I don’t know, not done that for years. The 1 1/2 cranked race engines I’ve built all have the piston crown level with the top of the block. 30+ years since the first one and I’ve not had any problems.