What's the score with pistons?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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What's the score with pistons?

Postby Halfwhelk » Thu Jul 04, 2019 8:23 pm

1955 AJS 18CS longstroke
After a long drawn out process (which I wont bore {sorry} you with) I am about to to get my ally barrel fitted with a standard liner and ready to accept a new STD piston.
I'm told by a few people whose judgement has been reliable in the past, that currently available new pistons are not much good. They are too heavy and need to be overbored to +.006" otherwise they will seize.
The recommendation is to get a new-old piston. Theres a wire wound split piston on ebay at the moment, that looks to have been well used but is not, as far as I can see, scored or scuffed. I'm waiting for information on the part number.

What do you think? Avoid new pistons? Is there a secret stash of Hepolites somewhere?

I thought this restoration was going to be quite good when I started, but now, 6 months, 300 hours and a few thousand pounds in, I think I'm going to be lucky if it ever runs again.

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Re: What's the score with pistons?

Postby Pharisee » Fri Jul 05, 2019 7:43 am

Whilst I can't comment about new pistons in an 18CS, I did have my '53 G3 barrel resleeved and fitted a new STD piston (supplied by T & L Engineering. Sorry, don't know what make it is.) What I can say is that the rebuilt bike has covered around 1000 miles since the rebuild with no problems what-so-ever. It's quite(ish) and smooth, it doesn't smoke and pulls well. All in all, I'm happy with it.

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Re: What's the score with pistons?

Postby Dave T » Fri Jul 05, 2019 10:21 am

Look on ebay.
https://www.ebay.com/itm/Nos-original-H ... 1%26asc%3D

https://www.ebay.com/itm/Nos-original-H ... 1%26asc%3D

https://www.ebay.com/itm/1947-1955-Matc ... 1271957013

I know the +20s use up a rebore but how many miles are you going to do on the bike? I bought a wirewound one from the seller and installed it in my CS with a one thou bore clearance and it ran really quietly. The Hepolites I think need five thou.
From the same U.S. seller I have bought a NOS Robbins piston which I have installed in my current G80 and it runs much better than the previous modern thing as it weighs 80gms less! Fitted with five thou clearance.

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Re: What's the score with pistons?

Postby 56G80S » Fri Jul 05, 2019 10:41 am

My experience matches Pharisee's but with a G80S. There's some tuning to do still but currently performance improving as each 1,000 miles rolls by.

On the mileage question, the Matchless was at +20 when I bought it. After several years but less than 10, rebored to +40. Wrecked bore and piston in Germany one way to the last Polish IJR due to gudgeon pin circlip letting go but was heading for a further rebore(!) before long anyway.

If you don't have a stable of machines any more (most I had at a time was 3) the one can get quite a lot of use. If you are fortunate to have several machines, why worry about mileage, they'll see you in your box?

Johnny B

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Re: What's the score with pistons?

Postby Andy51 » Fri Jul 05, 2019 9:13 pm

I used a + 40 JE piston on my '54 18S, iron barrel but scrambles cams and 8.5 to 1 piston, regularly used at 70mph. Barrel is bored according to instructions that came with the piston. I did not weigh the piston, but vibration no different to my '54 G80S with original wire round piston. Never nipped up, no discernible piston slap. In summary, no problems and works well. Cheers, Andy

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