AJS & Matchless Owners Club

I lifted the barrels of my recently acquired 1960 Model 31DL, not expecting any problems but just to check that all is in order.

Nasty surprise, the nearside piston is showing partial seizure marks all the way down the back of the skirt, and a bit on the front too. The pistons are from GPM, and I see from an internet search that several people have complained about them going out of round when hot. I'd love to hear from anybody who has any experience with using GPMs on twins, especially if you have had seizure problems.

I see that Club Spares has pistons in stock for the 1960 engine. Do we know who manufactures them?

Thanks for your help,

Steve

GPM - some people have had bad experience due to rebore clearance being too tight.
I've run them in two bikes over the years and with gentle run in they're fine.
Lots of car/bike racers swear by them.

It's a love/hate relationship. I ran them is an outfit which naturally has to work harder than a normal twin without any trouble whatsoever. Covered several thousand miles. They do require a little more clearance than AE or Hepolite pistons but I'd use them again. Others loathe them and wouldn't ever fit them and swear by JP pistons. Others however swear at JP pistons as they find they're too heavy.

Stuoyb wrote:GPM - some people have had bad experience due to rebore clearance being too tight.
I've run them in two bikes over the years and with gentle run in they're fine.
Lots of car/bike racers swear by them.

Has the rebore clearance ever been advised by GPM?

G15 Roy wrote:

Stuoyb wrote:GPM - some people have had bad experience due to rebore clearance being too tight.
I've run them in two bikes over the years and with gentle run in they're fine.
Lots of car/bike racers swear by them.

Has the rebore clearance ever been advised by GPM?

Nope and despite several attempts by the local contact in Italy I doubt it never will be.

Thanks for the quick responses!

I've cleaned things up a bit, the pistons are GPM 7223 (cast in letters under the crowns) with stamping 73 5 (on the crowns).
Tried to measure the bores, I think they are +0.040" or +0.060"
Does anyone know what the 73 5 stamping means? Is it 73.5mm? (That would indicate +0.060")

Presumably if I could get Hepolite that would be better than either GPM or JP?

What does the spares scheme have?

Looking closely at the two pistons, the LHS (seized) one has obviously been running much hotter.
Oil has carbonised into a brown glaze under the crown and also under the oil ring, whilst the RHS piston stayed completely clean in these areas.
I suppose that could be a poor camring on the (K2F) mag, or maybe a manifold leak on one side.

Steve

I wouldn't use them unless there was no other option.
They seized up in my CSR after 1500 miles of careful running in, first time I opened it up to over 60mph it seized, had the barrels honed to increase the clearence a little and then suffered horendous piston slap.
Was fortunate to find a set of Hepolites to replace them with and have had no further trouble.
Avoid if you can.

Laurie

Wassell own the trade name for new hepolite pistons , actually made by JCC and great quality
Unfortunately they don't do Matchless or AJS pistons
I use them in my Norton 750 and much better than JP or GPM
About time the club changed to the new hepolites and commissioned a batch to be manufactured
just need Russells , the club , AMC classic to stump up for a batch of a 1000 pistons. Haha not much chance of that

Like Robs says like marmite you either love GPM / JP or hate then

personally I would not use a JP piston even as a door stop , and I don't like marmite either

g80csp11 wrote:Wassell own the trade name for new hepolite pistons , actually made by JCC and great quality
Unfortunately they don't do Matchless or AJS pistons
I use them in my Norton 750 and much better than JP or GPM
About time the club changed to the new hepolites and commissioned a batch to be manufactured
just need Russells , the club , AMC classic to stump up for a batch of a 1000 pistons. Haha not much chance of that

Like Robs says like marmite you either love GPM / JP or hate then

personally I would not use a JP piston even as a door stop , and I don't like marmite either

You're correct there's not much chance of ordering 1000 pistons and I'm afraid that even GPM have dropped some of the AMC pistons in their range and have no plans to produce anymore!

I have a +40 thou JP 8.5 to 1 piston in my '54 18S with HS cams and it has served me well with no problems, with many miles at 60+ mph. I have not weighed it, but vibration is no more than my standard G80 with original wire wound piston. Cheers, Andy