piston ring gap and bore size

Information relating to the Matchless G12 or AJS Model 31 650cc twin
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John Carney
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piston ring gap and bore size

Post by John Carney »

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
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1608
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Re: piston ring gap and bore size

Post by 1608 »

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.
John Carney
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Re: piston ring gap and bore size

Post by John Carney »

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.
g5wqian
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Re: piston ring gap and bore size

Post by g5wqian »

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
John Carney
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Re: piston ring gap and bore size

Post by John Carney »

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
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dave16mct
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Re: piston ring gap and bore size

Post by dave16mct »

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.
John Carney
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Re: piston ring gap and bore size

Post by John Carney »

thanks Dave
g5wqian
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Re: piston ring gap and bore size

Post by g5wqian »

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
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