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poor valve clearance

Submitted by eddie_simpson on Friday, 23 January, 2026 - 20:11
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Heavy Twins

i am after a 2mm thick head gasket for my dommi 88. anybody making them to order?

Submitted by stan_thomas on Saturday, 24 January, 2026 - 01:28

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A compression/base plate under the barrel would be better, then you can get the same effect with a normal head gasket. Available from the usual suppliers.

Submitted by eddie_simpson on Saturday, 24 January, 2026 - 01:52

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hi Stan,
i ve not seen anything on Andover Nortons website. so who do you suggest.
My engine is a little bit mix and match from different sources and the compression would be too high as it is.
Also a 1mm thick copper head gasket seems a little weak to me. i have read of using 2 head gaskets on a commando engine with similar clearance problems. Also, I would feel happier with 2mm thickness.

https://andover-norton.co.uk/en/shop-drawing/891/dommi-engine-500-600cc
See p/n 06.7171

https://www.rgmnorton.co.uk/buy/barrel-base-compression-plates-dominator-030-75mm_148.htm
out of stock but useful comment on fitting

I think you're asking for a blown head gasket if you go very thick or double up.
A 0.75mm compression plate + extra base gasket + standard head gasket will give you the equivalent of the 2mm head gasket you were after.

Submitted by robert_tuck on Saturday, 24 January, 2026 - 07:46

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It should be possible to find someone with scanning and lazer cutting facilities to make cylinder base compression plates . Many years ago a private individual did just that to provide us with a 5mm alloy spacer for a complex primary cover for a Kawasaki race bike ,allowing the use of a wider clutch.  A common modification at that time. Pistons and valve lifters travel  will need checking .

Submitted by eddie_simpson on Saturday, 24 January, 2026 - 21:00

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Thank you for replies. i didnt see the A N compression plate Stan , so thanks for the heads up. RGM seems to be having problems at the moment . i ll try clay with the base compression plate in place  next and go from there.

Submitted by david_cooper on Sunday, 25 January, 2026 - 07:57

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Have you measured compression ratio?

(I did it on another engine using Evaporust fluid which is a bit more viscous than water but won't corrode anything)

It's easy in theory but in practice it has to be done very accurately because the unswepped volume is so small.  And if the block has been over bored, the ratio goes up a bit.

 



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