Hello I'm posting on behalf of a friend with a newly rebuilt 650SS. The bike has been languishing in his shed for 20 years so he finally bit the bullet and had a local expert restore it. Cosmetically it looks great but doesn't charge and is now running on one cylinder.... Anyway, he doesn't have the time to fix the bike himself and he visited a professional restorer to suss out his workshop and get an idea of how much it is going to cost to sort out. The first restorer's engine builder said that the engine was very tight and that he should add a capful of 2 stroke oil to each gallon of fuel during the running in period. The professional restorer was surprised to hear this and asked what type of pistons had been used in the rebuild. My friend said JP pistons. The professional restorer said that he had bad experiences with these pistons and wouldn't use them unless he had no other choice. A bit more research found several other people with poor opinions with these pistons (heavy and that they expand more than the original type hence the seizing). Has anyone else had problems with these pistons and is it necessary to increase the clearances from those recommended by the manufacturer? Thanks CAB
Yes the JPs should not be the first choice - a (name not mentioned here but quite trustworthy) big UK shop has mentioned 2 me that even the quality asian pistons are of better consistency/machining than the JPs - shame as they do offer a nice variety of piston/CR for old brit bikes
Last edited by Mike667-neighbor of the beast; 05/15/1011:18 am.
The JP piston I saw siezing was incorrectly machined, the relief land for the rings was not machined, plus the rings were so stiff they drew blood when spread over the piston to fit them. Once the land was machined and the rings replaced with correct rings the bike ran fine. But why have to buy a piston just to really only use it as a core and have to rebuild it. Way heavier than the orginal too.
Re: JP pistons. Any one had problems with seizing?
#314014 05/18/104:57 pm05/18/104:57 pm
Maybe I am naive about some things, but IMO the piston manufacturer should know something about his own product and how it is intended to be used..... If a forged piston expands more than a cast one, then it should be manufactured sufficiently under sized to fit the bore it is intended for. If the engine is a 3.000" bore then the piston should be made to work in a 3.000" hole. If the machinist can bore the hole correct then why can't the piston manufacturer make the piston to the correct size to run in a 3.000" bore, and run to 1300º on the dyno without seizing. Why should we play by their half a$$ engineering and "tweek" the bore to accomodate them? Don't they know what the bore size is they are designing to and the expansion cooefficient of their piston alloy? Why should we have to reengineer their work (or lack of...) OTOH To give them some slack... they don't have control of the air/oil cooling which does affect the actual temperature of the piston. Only the engine builder does and maybe few people actually are qualified to do engine developement work. In the mean time you seize motors.
You might as well start making your own pistons...humm I might give it a try...
dynodave BSA 3 1961-1963 Ducati 3 1992-2002 Norton many 1951-1975 87 Serv-Equip 100HP MC brake dynamometer,
Fellas we might have a misunderstanding going here. CAB is asking about JP pistons and Jim Schmidt responded with JE pistons. The JP pistons I understand are made in Australia for many classic motorcycles, but the JE Pistons are made in California. I don't have any experience with either in Nortons.