Pistons to sleeve or not to sleeve

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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Pistons to sleeve or not to sleeve

Postby ebolastu » Fri Jan 26, 2018 9:06 am

Hi I have a m20. 57

I have a very good set of original wire wound pistons, but the cylinder is really quite worn.

I either have to have a rebore and new pistons or resleeve and use the originals.

1. What is the standard bore for the wire wounds and what size would the normal pistons be?

2. Can you get original rings to replace those on my original piston?

3. When I asked about replacement pistons I was told that the modern ones are mostly rubbish.

Now i’m Not sure what to do?

Any advice appreciated.

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Re: Pistons to sleeve or not to sleeve

Postby clanger9 » Fri Jan 26, 2018 9:40 am

If it's any help, T&L Engineering did a resleeve for my Lightweight; it seems to have been straightforward enough.
It was a bit more expensive than a rebore, but meant I was able to re-use my original, lighter piston rather than a heavier replacement one.

If you are stuck for rings, try Cox & Turner. They seem to have everything...
1966 AJS Model 14 250 CSR "cafe racer"
1993 Ducati 750SS

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Re: Pistons to sleeve or not to sleeve

Postby Groily » Fri Jan 26, 2018 9:42 am

This - and much else on the site and in the archives referred - will help you: http://archives.jampot.dk/technical/Gen ... chdata.pdf

Not sure about finding rings but there are various specialists who are referred to in various threads on here that could help. Thorntons is one, there be others.

Not all modern pistons are 'rubbish'. But they do need to be a correct fit so you're right to be pernickety, which is where a decent engineering firm is needed if reboring/resleeving.
I haven't run wire-wounds in 30+ years but have had no trouble with AE / Hepolites in my M20, nor in fact with (often-maligned) JPs in my M31.

I think it comes down to getting the job done right by people with accurate measuring sticks and experience of the particular parts. Wire-wounds run closer tolerances than other pistons and hence make for a very quiet engine, so maybe worth resleeving if necessary to benefit from a good pair. But if that's not an option, I wouldn't worry too much about using other parts as they can be perfectly OK judging from my own experiences.

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Re: Pistons to sleeve or not to sleeve

Postby 56G80S » Fri Jan 26, 2018 2:02 pm

I've only recently "finished" running in a re-sleeve and new standard JP piston nicely done by T&L. All's good.

But if I could've found the standard piston I might've used. I could only find the +20 and +40. Groily has it right in my book.

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Re: Pistons to sleeve or not to sleeve

Postby g5wqian » Wed Jan 31, 2018 6:53 am

the GPM pistons from the club are fine as long as you make sure you have allowed .005" running clearance when having a rebore .
my twin is using .006" and it is loose , no seizures , it can be a little noisey when cold but once all warmed up its quiet .

you will have later barrels for the larger bore crankcase i think .

if you sleeve it youll be machining out more than any rebore your likely to have , id leave a re sleeve until youve exhausted all reboring options .

when you say barrels are worn, how much worn ? , in the state they are could they be honed out another .004" so you could fit STD sized GPM pistons from the club and still have the std bore size .

my twin is +040" rebored and fine , but i also have a few sets of barrels for it for spares .

the original wirewound pistons had a very tight running clearance of .001" , so if you are still on that you might be able to hone the bores to suit the modern piston clearance requirement and at same time save the wirewound pistons for future if you find a good set of std barrels .

there seem to be a fair few 500cc barrels about at times and not expensive , just make sure you get the later ones and not the earlier type with the different stud pattern size .

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