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PistonRings #758209 12/05/18 11:52 am
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photobob Offline OP
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I have a 1963 A65 Star Twin I have the barrels off at the moment to check if they need a rebore. The barrels at present are standard, I have carefully measured the wear in the bore and the maximum wear which is at the top is.003"this is the difference between the bottom unworn part of the bore and the top. The manual states that if wear is over.005" then a rebore is required, as mine are under that it should not need a rebore at present. I have also checked the piston clearances top middle and bottom this also confirms .003" maximum wear. I have a new set of standard piston rings but on trying them in the bore I find that in the unworn part of the bore I have a piston gap .015" and in the worn part of the bore it is .018". The manual states that the piston gap should be.008" to .013" so the new rings are above that recommendation is this normal. I have also measured the cylinder bore in the unworn part and it measures 2.9525" the manual standard bore size states 2.9521"-2.953"
Just to give some background on this bike, the pistons that are in now are the original pistons that it left the factory with in October 1962 it is a 1963 model and was not first registered until June 1963 these pistons have BSA cast into recess where the gudgeon pins go and the number on the otherside is 68-251. The bike has done approx 13,500 miles from new. when I bought it some 6 years ago it had stood dismantled for over 30 years,it only did 4,000 miles in the first 12 years of it's life.


Last edited by photobob; 12/05/18 12:14 pm.
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Re: PistonRings [Re: photobob] #758210 12/05/18 11:54 am
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kommando Online Content
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You can get away with larger than allowed ring gaps, much better than too tight a gap.

Re: PistonRings [Re: photobob] #758212 12/05/18 12:12 pm
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triton thrasher Online Content
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Your ring gap is not too big. Don’t worry about it.

Making the gap in the second ring 30% bigger than the top ring gap improves sealing.


Amateur Loctite enthusiast.
Re: PistonRings [Re: photobob] #758215 12/05/18 1:20 pm
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Tracey Spear Offline
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Is there a risk of metal fatigue in the rings from squeezing and releasing .003 on every stroke?

Re: PistonRings [Re: photobob] #758219 12/05/18 2:11 pm
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No, if they did then I suspect all my engines would need re boring and they are currently fine and most on second set of rings from worn rings not broken rings, the wear limit is 5 thou over original top limit so 3 thou is well within.

Re: PistonRings [Re: kommando] #758236 12/05/18 5:15 pm
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photobob Offline OP
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Thanks for those comments it puts my mind at rest.This all started because I was convinced that I was hearing piston slap but I doubt it now.

Re: PistonRings [Re: photobob] #758240 12/05/18 5:59 pm
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Piston slap?

Racing clearances, mate!


Amateur Loctite enthusiast.
Re: PistonRings [Re: photobob] #758245 12/05/18 6:44 pm
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What make are the new rings you are using? Many of the replacement rings available including Hepolite are actually manufactured in the far east these days, the fit may not be as accurate as the originals.

If your'e seriously concerned about the ring gap It may be worth shopping around and trying a set from another maker.

Don't forget to de-glaze the bore and run the engine hard for the first few miles to get the rings to seat.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Re: PistonRings [Re: gunner] #758247 12/05/18 7:11 pm
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Originally Posted by gunner
What make are the new rings you are using? Many of the replacement rings available including Hepolite are actually manufactured in the far east these days, the fit may not be as accurate as the originals.

If your'e seriously concerned about the ring gap It may be worth shopping around and trying a set from another maker.


There is nothing wrong with his ring gap. New British Hepolite rings in a new bore often had gaps about 18 thou in a bore that size. And he’s got used bores.


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Re: PistonRings [Re: photobob] #758253 12/05/18 8:21 pm
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leon bee Offline
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I suppose one of the scientists could come along and argue, but I agree with all the above. .018 is fine. In fact, ever since reading a John H. post about too close gaps popping the heads off new asian pistons, I do about .018 anyway. I know you have original pistons, but it'll be fine.

Re: PistonRings [Re: photobob] #758262 12/05/18 9:19 pm
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If your hot ring gap attempts to go less than zero, your engine’s top end will meet its doom.

Like wives and sweethearts, ring ends should never meet.


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Re: PistonRings [Re: triton thrasher] #758338 12/06/18 11:47 am
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photobob Offline OP
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They are the new style Hepolite rings have been hearing some adverse comments about these so I think I will buy some from a different maker and compare but most of the main dealers in the UK are supplying these Hepolite rings so are they really that bad.

Re: PistonRings [Re: photobob] #758339 12/06/18 11:50 am
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You can get Hastings rings for the A65 which are excellent quality and also use a three piece oil control ring which helps.


1968 A65 Firebird
1967 B44 Shooting Star
1972 Norton Commando
Re: PistonRings [Re: photobob] #758386 12/06/18 11:01 pm
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I can vouch first-hand that 30 thou ring gaps don’t cause problems on another popular marque of Brit twin.


Amateur Loctite enthusiast.

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