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JP pistons. Any one had problems with seizing?
#313381 05/14/10 9:20 pm
Joined: Jun 2009
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CAB Offline OP
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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

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Re: JP pistons. Any one had problems with seizing?
CAB #313387 05/14/10 9:47 pm
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Yes seen one seize in 50 yds due to missed machining, don't go near them, heavy paperweights is their only use.

Re: JP pistons. Any one had problems with seizing?
kommando #313457 05/15/10 11:17 am
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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; 05/15/10 11:18 am.

73 850 Commando
75 Ducati 750gt
78 Laverda 1200
97 Ducati 900 SS
61 BSA A10
62 Triumph T110
Re: JP pistons. Any one had problems with seizing?
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Thanks gents.
Not the replies that I had hoped for but what I expected.
What are the options with regards to other types of pistons?
CAB

Re: JP pistons. Any one had problems with seizing?
CAB #313622 05/16/10 12:18 pm
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Other than the commonly available Asian made pistons, your choices are:

NOS Hepolites, if you can find them. Hepolite went out of business a few years ago.

Jim Schmidt carries some trick pistons, but you have to use his connecting rods too.

You can get Venolia pistons, they don't stock them, but they can make them up.

-Eric

Re: JP pistons. Any one had problems with seizing?
ewgoforth #313623 05/16/10 1:03 pm
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i had good luck with CycleCRaft pistons girded with genuine Hepolite rings


1972 Triumph T120
1968 BSA A65
1968 MGB Roadster
1979 Chevrolet Camaro Berlinetta
1969 Honda Mini Trail
1939 farmall f30 tractor
2004 Honda Shadow Aero
1972 BSA Thunderbolt
1975 yamaha xs650b
1972 Norton commando
2 olive drab WWII military bicycle replicas
Re: JP pistons. Any one had problems with seizing?
jim schmidt #313946 05/18/10 9:18 am
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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?
CAB #314014 05/18/10 4:57 pm
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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 1300ยบ 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. mad

You might as well start making your own pistons...humm I might give it a try... :bigt



dynodave
BSA 3 1961-1963
Ducati 3 1992-2002
Norton many 1951-1975
87 Serv-Equip 100HP MC brake dynamometer,
Re: JP pistons. Any one had problems with seizing?
Dave Comeau #314047 05/18/10 7:46 pm
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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.


Ron L
1973 MkV 750 Roadster
1974 850 Interstate
1975 850 Cafe Racer
1968 750 Fastback
1967 P11
1969 Ranger 750
Re: JP pistons. Any one had problems with seizing?
Ron Leisner #314072 05/18/10 10:25 pm
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OOps
JP is definitly not JE

I'm deleting my earlier post.

Re: JP pistons. Any one had problems with seizing?
jim schmidt #314083 05/18/10 11:43 pm
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Yes JP pistons are definitly Australian.

http://www.jp.com.au/JPPistons.html

They are cast. On the positive side they make a very wide range.

On the negative side they are not that great. As kommando says they can be made to work by someone with engineering facilites but you shouldnt have to do that.

A friend with lots of experience descibes them as a last resort for street use. Not for any high performance work - and they do need to be checked.

For a Norton 650 SS you probably have better options. Try Mike Hemmings in England.

Shame because the Ozzie stuff is usually very good.

John



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