Sqiush clearance is generally 0.9mm

Stick them in the fire Steve when your misses isn't looking. The flame anneals them so that you can use them again. Apparently you can only do it two or three times though. I have come across the two different thicknesses of gasket as well.

You can anneal copper very easily by getting it red hot then quenching it in cold water. Unlike carbon steel where doing this will harden it, it actually softens the copper.

You can also take some of the work-hardening out of stainless steel in the same way.

Originally posted by rattle Thanks to both off you any Idea if the gasket thickness changed over the years

I would also like to know the answer to that, as I have just changed ONE of them !

The rate of cooling doesn't affect the annealing process of copper, but it does flake off the oxide layer that will have formed, giving a cleaner surface.

curiosity, why did yamaha do .5 and 1 mm gaskets, presumably the barrels are the same height and teh base gasket is teh same ?

Don,t ask me I think some ones been giving me wrong info ,I just put a micrometer (thanks steve) on my very old gasket and would you believe it 0.20 just like dale alexander said .Ill be having a word with some one tommorow two sizes my arse.Still It got me to check my squish clearance so far so good.

this is what i know about rd400 heads and gaskets i hope it makes sense
rd 400 c/d use a 1mm gasket and if you look at the head you would swear it has been skimmed as there is no defined squish or recess in top of head
400 e/f use a 0.5mm gasket and have a defined squish area or recess in head
so i would guess the more defined squish area lost a bit of compression hence the thinner gasket
these are only my views on the subject and i could be wrong

Originally posted by old rd user this is what i know about rd400 heads and gaskets i hope it makes sense rd 400 c/d use a 1mm gasket and if you look at the head you would swear it has been skimmed as there is no defined squish or recess in top of head 400 e/f use a 0.5mm gasket and have a defined squish area or recess in head so i would guess the more defined squish area lost a bit of compression hence the thinner gasket these are only my views on the subject and i could be wrong

I never knew that - thanks for some very helpful info.

Originally posted by cargers I am running genuine Yamaha pistons (dykes ring type)in my 350 engine and the top of the pistons sit virtually flush with the top face of the barrels. I have to have base gaskets fitted or else the pistons sit too high up.On my 250 motor i have to remove the base gaskets to get a decent squish clearance Ian.

never thought of removing the base gasket.
i know now.
ive just filed the lip off my 350 barrel this evening and then lapped the head to the barrel. i can now use rd400 gaskets and ive got a 0.5mm gasket to get my squish down to under a mm.
i too have got dykes ringed pistons and mine were almost a mm below the top of the barrel
the acid test is to check with modelling clay and solder.

ian what sort of squish clearance do you aim for?