Are GPM pistons as bad as their reputation?

Information relating to the Matchless G12 or AJS Model 31 650cc twin
g5wqian
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Re: Are GPM pistons as bad as their reputation?

Post by g5wqian »

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 .
JEAN-NOEL
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Re: Are GPM pistons as bad as their reputation?

Post by JEAN-NOEL »

I agree with Ian. For my G12 CSR with +0.40 GPM pistons, Derrick, from T & L recommend + 0,005 to 0,006.
Andy51
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Re: Are GPM pistons as bad as their reputation?

Post by Andy51 »

Do the manufacturers not specify bore clearance? JP provided this data (I seem to recall a figure of 4 thou) in a leaflet in the box. Andy
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sjr
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Re: Are GPM pistons as bad as their reputation?

Post by sjr »

Thanks for the careful replies, especially Ian from Wiltshire.
I respect Derrick's view, I've used him before on other bikes with good results, so I'll try 0.005" to 0.006" clearance.

I have tried to measure the clearance of the pistons that came out, I think its less than 0.006".
I can get a 0.006" feeler between skirt and bore, but I have to push. I think my technique is wrong - should the feeler slide smoothly?

I've ordered a pair of GPMs from Club Spares tonight, I'll have the bores finished to suit and we'll see.

By the way Ian we may be local. I am near Chippenham.

Thanks
Steve
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sjr
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Re: Are GPM pistons as bad as their reputation?

Post by sjr »

Hurley's in Bath have had a go at measuring them more accurately, the clearance of the new pistons in the bores at the tightest point across the skirt is 0.005" so I have asked them to hone out carefully to 0.006". Hopefully that will be enough to prevent any more seizing, though I'm a bit worried that its only a marginal change so may only have a marginal effect.

I fancy that a higher capacity oil pump might help too. Steve Surbey has some on order, but with no clear delivery expectation yet. I could be an eager buyer for anyone who has a spare set available.

Hopefully,
Steve
g5wqian
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Re: Are GPM pistons as bad as their reputation?

Post by g5wqian »

hi Steve ;
im near to swindon .

i think its fairly well sussed out that the GPM pistons need larger running clearance than original pistons so the rule of thumb with the GPM types [for all makes of british bikes] is .005" to .007" in order to aleviate chances of seizure .

i have seen discussions relating to tighter clearances but they are bordering on the point of danger and its best to stay slack than not i think .

all you do when adding the running clearance is to know what the standard bore size is , then add the running clearance to this and hone it to that dimension .

if you have a rebore then obviously it will be the rebore size plus running clearance .

if std bore is 2.835" then you will be going to hone it out to 2.841" finished size when including a running clearance of .006"

if its a +.040" rebored then it will be a 2.875" bore size honed to 2.881" to include the .006" running clearance .

you can of course go in tighter increments if you wish and have "half a thou " [5 tenths] in it ie .0055" if you want to set it slightly tighter than 6 thou .

machine shops would use a bore gauge to measure the running clearance , and what you do is set the "zero" of the bore gauge at the bore size using a "master", a set of "slips" that are made up to the bore size , or perhaps set against an internal micrometer .

then when you insert the bore gauge into the bore , it will register what the bore is in relation to the pre set dimension , and you can go up and down in the bore and around it measuring the differences from the pre set dimension and view that figure on the bore gauge dial .

the bore gauge will measure in tenths of a thou and so is quite a sensitive measuring tool .

usually the dial will be able to swing both ways around the centre measurement , so can tell you if the bore is over or under wherever you measure it .

if not using a bore gauge then can use an internal mic to measure the bore dimension .

all i did when honing my bores was to have the bore gauge set to my specified bore size , ie 2.875" and then hone the bore out in stages , noting the bore gauge readings each time , until i had the required figure i was looking for , but each time i would also measure the bore using my internal mic .

so far apart from the usual oil weeps , my engine runs fine and have done over 500miles on it , and in fact yesterday i did a 120 mile round trip to near to bristol and back which also used the motorway at 55mph and it ran fine there and back .

if you need any machining work done on the engine then SOUTH CERNEY ENGINEERING have all kinds of eqpt , they can rebore from 2" upwards and crank grinding etc .
they can also do cylinder sleeving but i dont know if they do many motorbikes although they did do my mates 505 cu inch chrysler v8 engine block some years ago .

i have had cranks reground there and blocks bored , for the last 35 years and i guess we are lucky they still exist .
they do work on a lot of vintage and classic car engines , there is a place next door to them that restores old cars for an american business man .

they are on your side of town , ie you come down to them via malmesbury and minety off the M4 chippenham junction 17 .

my father was a proper old school toolmaker and we have all kinds of measuring gear here that he had over his 65 year career , plus some machines , but unfortunately he passed away last august and so even though i can use much of it i am not superb at machining anything like he was .

hope you get the engine sorted and get some use out of it .

my bike is what you call a rolling restoration .

cheers
ian
JEAN-NOEL
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Re: Are GPM pistons as bad as their reputation?

Post by JEAN-NOEL »

Hello Ian,

you wrote :

if std bore is 2.835" then you will be going to hone it out to 2.841" finished size when including a running clearance of .006"

I think you are wrong adding 6 thou to the std bore to have the suitable clearance : for à 650 std bore, according the workshop manual is 2.836" - 2.835". Piston skirt diameter (Top) is 2.8287" - 2.8295". To set .006 running clearance ]you have to check the actual piston diameter, just above (3/8° - 10 mm) BOTTOM skirt, 90° from gudgeon pin (or the max diameter of the skirt).

But technical data are amazing :

2.835"-2.8295" = .00055"
2.836"-2.8287" = .00073"

ie only 0.5 to 0.7 thou ! Far from 5 to 6 thou...

Cheers
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Expat
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Re: Are GPM pistons as bad as their reputation?

Post by Expat »

Hello Jean-Noel,

I've read with interest the posts in this thread as I like a bit of technical stuff but have to disagree with your running clearance figures in your final post.

If you don't mind me saying so, take another look at the maths. Subtracting the figures does indeed give a clearance of between 0.0055 inch and 0.0073 inch, NOT the tenths of a thousandth figures you suggest. :)

Regards,

Steve
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These are my principles, if you don’t like them, I have others. (Groucho Marx)
JEAN-NOEL
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Re: Are GPM pistons as bad as their reputation?

Post by JEAN-NOEL »

Hello Steve,
yes, you are right.
So .006" seems to be a good value.
Regards.
J.N.
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