Hi,
I'm doing a top end rebuild on an 1961 AJS 650 de Luxe and wondered if anyone would know what a reasonable max tolerance on the piston ring gap
and bore would be or how I could get this info ? The piston is part number 026325. as far as I know the bore is 72mm ( 2.836 ) and the ring gap should be .006. I don't really want to go up a size with a rebore and oversize pistons and rings unless I have to and the bike will have a fairly easy life with me so not trying to get a ton up out of it. Also what do you think would be a reasonable rule of thumb cold engine compression measurement be ? thanks
John
piston ring gap and bore size
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Re: piston ring gap and bore size
Just comp tested my 1960 G12 with fairly recent new rings, so not fully bedded in yet but gave 98 psi cold. Be interested to hear a general consensus on this though
The bike rode well for quite some time on + 18thou ring gap. I know the book says 6th on new bore and rings but I believe that is a bit ambitious and 8 thou would be safer.
The bike rode well for quite some time on + 18thou ring gap. I know the book says 6th on new bore and rings but I believe that is a bit ambitious and 8 thou would be safer.
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Re: piston ring gap and bore size
Hi, thanks, that's very helpful When I measured before taking it apart it read 95psi on one cylinder and 25psi on the other. I think the low one was due to carbon built up on the valves maybe. ( I don't think the engine has run for about 40 years before I bought it ) An engineer measured the bore and it's about 7thou bigger after honing which I did myself with a honing tool in a drill that I borrowed to cut through the glaze on the bore. Now I've found someone locally who can do these things properly I wouldn't try this again ! The ring gap is about 49thou ! so I'm going to try with new rings and see what they measure at.
Just for interest, the piston 026325 is the 8.5 to 1 for the 31CSR and the parts list I've just received from BMS ltd says the de luxe model normally has 026324 7.5 to 1 so they must be interchangeable which should give me a better chance to find oversized pistons and rings if I have to.
Just for interest, the piston 026325 is the 8.5 to 1 for the 31CSR and the parts list I've just received from BMS ltd says the de luxe model normally has 026324 7.5 to 1 so they must be interchangeable which should give me a better chance to find oversized pistons and rings if I have to.
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Re: piston ring gap and bore size
hi john ;
on my 56 model 30 ajs i have .010" ring gap when ring is 1" down the bore and the bores are honed for .006" running clearance [gpm pistons swell by .003"].
only thing is that if you hone the clearance like this , then some rings may already have a much wider gap without touching them .
i used +.060" rings and filed them down with a ring filer to give me the reqd gap in the +040 bores .
technical manual says you can have .006" to max .030" ring gap , so the margins are quite wide .
compression is around 125-145psi , youll need to kick engine over about 10 kicks with throttle wide open and watch the compression gauge climb , it should get somewhere near this or more i think in your case [might need a squirt of oil in the bores if they are dry ].
im using GPM pistons in +.040" over .
i can hear some piston slap when its cold but when warmed up its fine .
i can rev it without worrying if it will seize .
ian
on my 56 model 30 ajs i have .010" ring gap when ring is 1" down the bore and the bores are honed for .006" running clearance [gpm pistons swell by .003"].
only thing is that if you hone the clearance like this , then some rings may already have a much wider gap without touching them .
i used +.060" rings and filed them down with a ring filer to give me the reqd gap in the +040 bores .
technical manual says you can have .006" to max .030" ring gap , so the margins are quite wide .
compression is around 125-145psi , youll need to kick engine over about 10 kicks with throttle wide open and watch the compression gauge climb , it should get somewhere near this or more i think in your case [might need a squirt of oil in the bores if they are dry ].
im using GPM pistons in +.040" over .
i can hear some piston slap when its cold but when warmed up its fine .
i can rev it without worrying if it will seize .
ian
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Re: piston ring gap and bore size
Thanks Ian;
That's really helpful. I got some new rings which are about .021 gap so it should be OK. I don't suppose you'd know what the torque setting on the cylinder head bolts should be ? Have you any idea where I could get a technical manual or sheet with these sort of settings in ?
John
That's really helpful. I got some new rings which are about .021 gap so it should be OK. I don't suppose you'd know what the torque setting on the cylinder head bolts should be ? Have you any idea where I could get a technical manual or sheet with these sort of settings in ?
John
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Re: piston ring gap and bore size
Here's one of the best manuals:
http://archives.jampot.dk/book/Workshop ... models.pdf
Torque on cylinder head nuts with 5/16" studs is 22 lbs ft.
Dave.
http://archives.jampot.dk/book/Workshop ... models.pdf
Torque on cylinder head nuts with 5/16" studs is 22 lbs ft.
Dave.
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Re: piston ring gap and bore size
thanks Dave
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Re: piston ring gap and bore size
hi john ;
cylinder head nuts should be 25ft/lbs for 3/8" studs if its a later engine and 22ft/lbs for 5/16" but i do mine at 25ft/lbs , make sure you use a 3/8" torque wrench as i dont find that the 1/2" ones go down low enough nor accurate enough at the low value . .
be carefull if you have acorn nuts as they might bottom out , but normal full nuts which allow threads to pass through will not be a problem .
tighten the head down in 3 stages of tightness , then leave overnight and re torque again the next day .
then when youve given it a run out and about , re torque them again afterwards .
just make sure your head gasket is sufficient thickness for proper sealing and not interference from the barrel spigot depth , i had some solid copper head gaskets a few years ago that were only just over 1mm thick and they didnt seal because the spigot was touching the head before the gasket made contact , hence i had a lot of oiling of the plugs and top of pistons and had to sort it out with new gaskets from the club , the copper ones should have been 1.5mm minimum thickness . .
i use the clubs composite head gaskets which are 1.7mm thick when compressed , they work fine for me .
there are 2 types of head gasket for the 650, early and later , one has bigger spigot hole , i think one is 3" and other is slightly larger , so check which you have .
there are 2 thicknesses of base gaskets , i use the thick ones but you can use either .
club spares are selling solid copper head gaskets , in 1.5mm thick but if you use them just make sure the barrel spigot doesnt bottom out to the head recess before the gasket gets pinched up , if they do you might have to get some of the barrel spigot machined off .
you can get some composite head gaskets from amc classic spares from steve surby which have viton o rings where the oil ways through and go towards stopping the dreaded oil weep there , i dont know if club spares do similar gasket or not .
mind you if you do find you have oil residue appear from the joint between head and barrel its a common thing and not to worry too much , i have a can of brake cleaner handy and give mine the once over every so often if i spot any oil there .
normally i find that at speeds up to 55mph the engine is nice and clean , but if you rev it and go over 60mph for long periods the oil pressure is a fair bit higher then its likely to weep oil .
i have good oil pressure and the higher the revs the higher the pressure , the matchless/ajs twins have very good oil pumps compared to other makes .
my tell tale in the oil tank shows a good return flow of oil but if you look at my ARIEL NH tank it is just spits of it most of time , but the oil pump is rotary on the AJS and a piston pump on the ariel .
hope you get it all done and running nice
ian
cylinder head nuts should be 25ft/lbs for 3/8" studs if its a later engine and 22ft/lbs for 5/16" but i do mine at 25ft/lbs , make sure you use a 3/8" torque wrench as i dont find that the 1/2" ones go down low enough nor accurate enough at the low value . .
be carefull if you have acorn nuts as they might bottom out , but normal full nuts which allow threads to pass through will not be a problem .
tighten the head down in 3 stages of tightness , then leave overnight and re torque again the next day .
then when youve given it a run out and about , re torque them again afterwards .
just make sure your head gasket is sufficient thickness for proper sealing and not interference from the barrel spigot depth , i had some solid copper head gaskets a few years ago that were only just over 1mm thick and they didnt seal because the spigot was touching the head before the gasket made contact , hence i had a lot of oiling of the plugs and top of pistons and had to sort it out with new gaskets from the club , the copper ones should have been 1.5mm minimum thickness . .
i use the clubs composite head gaskets which are 1.7mm thick when compressed , they work fine for me .
there are 2 types of head gasket for the 650, early and later , one has bigger spigot hole , i think one is 3" and other is slightly larger , so check which you have .
there are 2 thicknesses of base gaskets , i use the thick ones but you can use either .
club spares are selling solid copper head gaskets , in 1.5mm thick but if you use them just make sure the barrel spigot doesnt bottom out to the head recess before the gasket gets pinched up , if they do you might have to get some of the barrel spigot machined off .
you can get some composite head gaskets from amc classic spares from steve surby which have viton o rings where the oil ways through and go towards stopping the dreaded oil weep there , i dont know if club spares do similar gasket or not .
mind you if you do find you have oil residue appear from the joint between head and barrel its a common thing and not to worry too much , i have a can of brake cleaner handy and give mine the once over every so often if i spot any oil there .
normally i find that at speeds up to 55mph the engine is nice and clean , but if you rev it and go over 60mph for long periods the oil pressure is a fair bit higher then its likely to weep oil .
i have good oil pressure and the higher the revs the higher the pressure , the matchless/ajs twins have very good oil pumps compared to other makes .
my tell tale in the oil tank shows a good return flow of oil but if you look at my ARIEL NH tank it is just spits of it most of time , but the oil pump is rotary on the AJS and a piston pump on the ariel .
hope you get it all done and running nice
ian