commando 850 pistons and rings

I don't know if the JPM pistons you mention are the same as the JP pistons I bought 5 years ago for my 650SS but if so beware their weight. The JP pistons were really heavy, even after some machining to lighten them, and still much heavier than the Gandini pistons I eventually bought from RGM, which were much closer to the original weight. Unless you want to go to the extreme of getting your crank rebalanced to suit! I'd be tempted to ask Norvil what weights the two makes are and compare to your existing ones, then go for the closest match assuming you don't mind the price difference if the JPM turn out to be closest. Even at that price they're a fair bit cheaper than the JP ones I bought all that time ago!

I bought and fitted +20 Hepolite pistons to my 850 with no issues.

Have to admit I did not check the weights.

I think JPM is really GPM who are based in Italy, the faux Hepolite marketed by Wassell's are made by JCC Pistons of Taiwan and are a very good copy of the original Hepolites made at Bradford. They do not come with JCC rings, worth asking whose rings they are supplied with, Hastings were the initial supply but then it changed. 

I was told by a vg Italian engine builder last year  that the modern Hepolite rings are not good, and he won’t use them. 

I rebored some 850 barrels about 5 years ago, and the engine shop man told me they were Volkswagen Beatle pistons Norton fitted. There are many sizes of pistons and rings for the Beatle in the Hepolite catalogue, have a look.

JP pistons have a major flaw, well a few flaws really.

1. Their rings are far too stiff and take too long to bed in running the risk of the bores glazing.

2. They make a lot of different pistons from each core castings, so most of them are much heavier than the original piston.

3. They have a habit of missing complete machining operations. One I saw on a Matchless the ring land relief had not been machined and was the same diameter as the rest of the piston, Seized engine 20 ft off the driveway. 

Engine was dismantled, fingers cut removing rings, relief area machined, proper rings refitted and the bike ran well if a bit more vibratory.

They have a use of being the last resort due to their low batch size economics, but it really needs to be the last resort.

I buy rings on andover last year and works very good

it is this tipe

tomas

Thanks, those are the ones I have been looking for, that, I am told , are a better bet with modern Hepolite pistons.