Re: Are GPM pistons as bad as their reputation?
Posted: Fri Mar 10, 2017 6:55 am
i have GPM pistons in my ajs model 30 .
when i first got the bike a while ago it was leaking oil out the base gasket so i took the barrels off etc and then noticed the same as you have with a partial seizure on the rear of the nearside piston .
i bought new pistons and started the process of sorting it out .
the engine had previously been rebored only a short time before and upon measuring the bores i noted it had been bored to the original factory specs , with 0.001" running clearance in the bore , and this is what the problem was .
i then honed the bore out to give it 0.006" running clearance , ie 6 thou over the reqd bore size .
with that done i reassembled the engine and it ran fine for one year and 2000 miles without problem and ridden as and how i like .
i had fitted some solid copper head gaskets from the club spares but they were the wrong thickness , they were supposed to be sold as 1.5mm thick but were in fact only 0.93mm , and this caused an oil weep to come from the oil way between the head and barrel due to the head not sealing fully , so i took it all off again to repair .
this time i fitted new piston rings and used the same pistons as before but have replaced the head gasket with a standard composite type .
because i have enlarged the bore by .006" i found that the ring gap was wide at .028" , so i have used the next size up of compression rings but have kept the oil ring size that corresponds to the specified bore size [ ie mine is .040 over and i used .060 over compression rings and filed the gap to .012" and used the .040" oil ring which wasnt filed and had a .028" gap without touching it ].
engine runs fine and nice and smooth , with little smoke at starting , i have done just over 200 miles at present since assembly and running it in at up to 50mph speed.
the GPM pistons are fine if you make sure you have the reqd running clearance honed in , and it seems that pretty much all the suppliers of GPM pistons for english bikes will say you need .005" to .007" running clearance for these pistons .
i think the same applies to JP pistons as well if using them .
it is probably due to the modern alloy used for pistons not being the same as used previously .
from some other guys observations it seem that these pistons will expand 0.003" when hot , hence the need for at least .005" clearance .
mine are on the looser side of the figure , it all runs fine and not really much noise from them that i can tell , i am happy enough and could rev the engine high when i first rebuilt the top end and i dont see any reason why it wont do the same now with the tighter ring gaps .
unfortunately it seems GPM have made the ring length to suit the bore size and if you open bore out to give it the reqd running clearance to aleviate seizing , then you also end up with a wide ring gap of something like .020" on the compression rings , so if you arent happy running such a wide ring gap you will need to go up a size on the rings and file the gap to suit your needs , i know its controversial to do this but it has worked ok for me and keeps my 60 year old engine running still .
i also have other rings from mitsubishi and bmw which i had bought to try but have not used them as i saw no need to since i have done the top end twice in 2 years with GPM parts and no problems in that area since using the larger running clearance spec .
hope this helps .
when i first got the bike a while ago it was leaking oil out the base gasket so i took the barrels off etc and then noticed the same as you have with a partial seizure on the rear of the nearside piston .
i bought new pistons and started the process of sorting it out .
the engine had previously been rebored only a short time before and upon measuring the bores i noted it had been bored to the original factory specs , with 0.001" running clearance in the bore , and this is what the problem was .
i then honed the bore out to give it 0.006" running clearance , ie 6 thou over the reqd bore size .
with that done i reassembled the engine and it ran fine for one year and 2000 miles without problem and ridden as and how i like .
i had fitted some solid copper head gaskets from the club spares but they were the wrong thickness , they were supposed to be sold as 1.5mm thick but were in fact only 0.93mm , and this caused an oil weep to come from the oil way between the head and barrel due to the head not sealing fully , so i took it all off again to repair .
this time i fitted new piston rings and used the same pistons as before but have replaced the head gasket with a standard composite type .
because i have enlarged the bore by .006" i found that the ring gap was wide at .028" , so i have used the next size up of compression rings but have kept the oil ring size that corresponds to the specified bore size [ ie mine is .040 over and i used .060 over compression rings and filed the gap to .012" and used the .040" oil ring which wasnt filed and had a .028" gap without touching it ].
engine runs fine and nice and smooth , with little smoke at starting , i have done just over 200 miles at present since assembly and running it in at up to 50mph speed.
the GPM pistons are fine if you make sure you have the reqd running clearance honed in , and it seems that pretty much all the suppliers of GPM pistons for english bikes will say you need .005" to .007" running clearance for these pistons .
i think the same applies to JP pistons as well if using them .
it is probably due to the modern alloy used for pistons not being the same as used previously .
from some other guys observations it seem that these pistons will expand 0.003" when hot , hence the need for at least .005" clearance .
mine are on the looser side of the figure , it all runs fine and not really much noise from them that i can tell , i am happy enough and could rev the engine high when i first rebuilt the top end and i dont see any reason why it wont do the same now with the tighter ring gaps .
unfortunately it seems GPM have made the ring length to suit the bore size and if you open bore out to give it the reqd running clearance to aleviate seizing , then you also end up with a wide ring gap of something like .020" on the compression rings , so if you arent happy running such a wide ring gap you will need to go up a size on the rings and file the gap to suit your needs , i know its controversial to do this but it has worked ok for me and keeps my 60 year old engine running still .
i also have other rings from mitsubishi and bmw which i had bought to try but have not used them as i saw no need to since i have done the top end twice in 2 years with GPM parts and no problems in that area since using the larger running clearance spec .
hope this helps .