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#384602 - 07/15/11 7:27 pm 650SS pistons. What is available?  
Joined: Jun 2009
Posts: 292
CAB Offline
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CAB  Offline
BritBike Forum member

Joined: Jun 2009
Posts: 292
UK
Hello
I'm posting for a friend with a 650SS (I have a 1961 Matchless G12 CSR and a 1966 MK11 Spitfire).
He is having his 650SS rebuilt for the third time. The engine with new JP pistons seized after about 70 or 80 miles. He is not sure why this happened but it could have been down to the bad workmanship of the restorer not the pistons....
The engine was stripped after the rebuild by a different restorer to cure an oil leak and he found a scored bore on one side with a perfect piston and a perfect bore on the other side with a scored piston.... It looks as if it seized whilst being run-in by the first "restorer" and was hastily bodged back together before my friend had it back.
It was rebuilt with a second set of JP pistons by a different engine builder and to be on the safe side my friend specified the maximum clearance recommended by the Norton Owners Club technical advisor for JP Pistons. Now it has piston slap.... The second engine builder wasn't happy with the condition of the conrod little ends that had been re-sized by the first engine builder to suit the larger JP gudgeon pins.
My friend now wants the second engine builder to rebuild it with new rods and different pistons.
What type of pistons do you recommend? Is there any alternative to JP pistons? The only other type of piston apart from JP that we can find, are listed as supplied by a company called PHC. Any ideas who this supplier is? Could it be Peter Hepworth?
Any advice would be gratefully received.
Thanks
CAB

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#384603 - 07/15/11 7:36 pm Re: 650SS pistons. What is available? [Re: CAB]  
Joined: Oct 2007
Posts: 722
norton bob Offline
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norton bob  Offline
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Joined: Oct 2007
Posts: 722
bromley uk
I rebuilt my 99 with hep 650 pistons with 4.5 thou clearance as reccommended by Norton factory service engineer, I also have piston slap when cold.Its not going to sieze though.

#384974 - 07/18/11 2:43 am Re: 650SS pistons. What is available? [Re: CAB]  
Joined: Sep 2008
Posts: 193
Tobin Peever Offline
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Tobin Peever  Offline
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Joined: Sep 2008
Posts: 193
Pullman, Washington, USA
Sounds like you need to find someone who knows something about Norton engines!! That is total bullshit to have to rebuild a motor three times! Get someone who knows what they are doing and can do it right the first time. I used the Italian GPM pistons from Norvil in my '61 Dommie restoration and they have been fine. You need to make sure the piston clearance and ring gap is correct for whatever piston you decide to run. As Norton Bob says, better to run a bit wide than too tight. I think my machinist used a 0.040 clearance for the GPM pistons and I set the ring gaps in the upper part of the working range.

edit 7/18/11 - beware of brain fart above - that would be 0.0040 clearance!!

Last edited by Tobin Peever; 07/18/11 1:47 pm.

47 Norton ES2
61 Norton Dominator 88
67 Triumph TR6C
68 Moto Guzzi V700
74 Norton Commando
08 Moto Guzzi 1200 Sport
#384985 - 07/18/11 9:21 am Re: 650SS pistons. What is available? [Re: CAB]  
Joined: Apr 2006
Posts: 1,396
johnm Offline
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johnm  Offline
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Joined: Apr 2006
Posts: 1,396
New Zealand
I have also used GPM pistons with good results in my 500. They have been in my race bike for way too long and are about to be retired.

You should be able to get them from the normal British suppliers. Personally I prefer Mike Hemmings and he will be able to advise on clearance, running in etc.

Whenever I see piston seisure I always check igniton timing on BOTH cylinders.

John

#385000 - 07/18/11 11:24 am Re: 650SS pistons. What is available? [Re: johnm]  
Joined: Oct 2007
Posts: 722
norton bob Offline
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norton bob  Offline
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Joined: Oct 2007
Posts: 722
bromley uk
I have had an 88 sieze at 90 mph and seen a few motors with sieze marks and they have all been on the drive side,i always put it down to less oil getting to the farthest piston from the pump but its also possible that the sludge trap plays a part in this.


Moderated by  Dave Comeau 


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