Piston up grade

Information relating to the Matchless G5 or AJS Model 8 350cc Lightweight
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Re: Piston up grade

Postby Martin.S » Wed Feb 13, 2019 5:35 pm

Rob, if I remember correctly you said that only the 7.5:1 pistons fit as the 8.5:1's dome was too big. Can you remember where it fouled or just needed the valve pockets enlarging?

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Re: Piston up grade

Postby ajscomboman » Wed Feb 13, 2019 6:47 pm

Martin.S wrote:Rob, if I remember correctly you said that only the 7.5:1 pistons fit as the 8.5:1's dome was too big. Can you remember where it fouled or just needed the valve pockets enlarging?


Blimey now you're asking a question. It was 20 yrs ago, from memory it fouls on the spark plug the valve clearance isn't an issue. Also the 59 HC are not easy to come by.

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Re: Piston up grade

Postby Martin.S » Fri Apr 19, 2019 4:28 pm

I have now machined the pockets out of a post 61 G12 csr piston I bought from Rob (thanks) after having the barrel bored to suit.
I also re profiled the crown to suit the combustion chamber

Piston Valve Cut outs.jpg


Crown re profile.jpg


I now have a compression ratio of 10.67:1 - oops!
I could skim the piston crown and or use a copper base gasket to lower the compression but by how much?

The later G2csr's went to 9.5:1 but the G12 speed kit used 10.5:1(?)- what do we think I can get away with?
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Re: Piston up grade

Postby ajscomboman » Fri Apr 19, 2019 7:10 pm

Martin.S wrote:I have now machined the pockets out of a post 61 G12 csr piston I bought from Rob (thanks) after having the barrel bored to suit.
I also re profiled the crown to suit the combustion chamber

Piston Valve Cut outs.jpg


Crown re profile.jpg


I now have a compression ratio of 10.67:1 - oops!
I could skim the piston crown and or use a copper base gasket to lower the compression but by how much?

The later G2csr's went to 9.5:1 but the G12 speed kit used 10.5:1(?)- what do we think I can get away with?



It comes down to ride ability in the end as opposed to compression. The twins went to 10.25, which I'm running with twin carbs but the bike only likes to run at too high a speed for pleasant club runs. Low down at slow speed it's lumpy and not nice, it doesn't like to bumble along. Whilst the H/W singles went up to 12 on the comps it came at a price. Realistically you'd be better off trying to get it to the mid 9's. Alternatively try it and see, it won't take long to swap out a piston of lower comp.

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Re: Piston up grade

Postby Martin.S » Fri Apr 19, 2019 9:40 pm

If my calculation are correct, using some gasket paper I have that's a touch over 0.5mm thick as a base gasket I should get 10:1 or as you say something a little thicker and aim for perhaps 9.5:1.
Cheers.

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Re: Piston up grade

Postby Martin.S » Sat Apr 20, 2019 7:02 am

A 1,2mm base gasket would give me a ratio of 9.48:1 - with such a thick gasket would it be better to use copper instead of a fibre material?

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Re: Piston up grade

Postby ajscomboman » Sat Apr 20, 2019 7:45 am

Yep, copper or ali would be better at that thickness.

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Re: Piston up grade

Postby Martin.S » Sat Apr 20, 2019 8:12 am

thanks - 1.2mm plate ordered

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