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
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!!
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.