JP pistons in 88

Hi Paul.

About 9 years ago I got a pair of JP pistons for my 650SS following relining back to standard (supplied by the engineering company who did the work) and they were way heavier than the old +60 pistons by about 70g each. Vibration was too bad so I took them back to the engineering company and they attempted to remove weight from them (about 30g from each) and whilst things improved it was still too bad to live with so I bought a new set of lighter pistons from RGM (Gandini) which were much closer to the originals in weight and actually a bit lighter so they are what are still in the bike now. I would say you will need to get the crank rebalanced if you use the JP pistons as they are, though since in my case some 30g was able to be removed if this could be done with yours you should be close enough to the original weight to be OK. Either way some more work involved! I can't now remember just where the weight was taken from but I seem to think the heads were pretty thick so it could well have been from the underside. Or perhaps you could get a closer matching set (I recall RGM quoted the weight of their pistons so you can check before you buy) and send the JPs back or sell them, which would cut out the extra work though obviously increase costs, at least temporarily.

The weight of a conrod is 340g, of which about 120g is effectively at the little end with the piston.

I have a 318g Heppolite and a 328g JP.   So mine are closer than yours.  With conrod that adds up to about 450g total.  If you have a 30g difference, that's about 8% change in balance factor. But people argue about the best balance factor anyway.

Have you had a rebore? The bigger piston will be heavier anyway.

I have just had my 650 rebored for a pair of new +40" Gandini Pistons which were miles heavier than the original +20" Hepolites (Heplexes). 

While the engine was apart I took out the crankshaft and had it rebalanced to match the new pistons. Checkout the photos to see what I consider some very shoddy engineering.  When I complained I was informed that this often happened with Norton flywheels. 

Paul,

I put a JP Piston in my ES2.

It was 70 odd grams, maybe more, heavier than  the previous piston. BSA I think. It did vibrate quite a bit more with the JP Piston.

But the reason why I changed to a Gandini was the rings would not bed in. The smoke screen when I started up was embarrassing!

Don Anson

Melbourne 

Don's comment about rings bedding in reminded me that I also had that problem with my JP pistons and forgot to mention it in my earlier post. Even after the best part of 1,000 miles I still had plenty of smoke and the silencers had filled up with oil so they smoked like mad if I was stuck in traffic as the silencers heated up more! Had to swill them out with petrol when I realised what had happened. When I switched to the Gandini pistons they were absolutely fine from day one.

That was my first thought.  So I asked about the balance factor used and was quoted 68%. I was expecting the answer to be 72%.

It also struck me that rather than drilling holes everywhere the engineer could have just ground the centre rib off similar to a 750 crankshaft.

Anyway......the motor is all back together and definitely runs a lot smoother. Now it is just a question of a putting up with a 1000 miles of running-in.