rebore clearance

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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cf160
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Joined: Sun Jul 30, 2006 7:24 am
Location: Ottawa, CANADA
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rebore clearance

Post by cf160 »

So Ian, did you recommend .001" per inch of bore? That seems reasonable to me, but I have seized three Model 31 pistons after I fit to the machine shop recommended clearance, so I will always leave a little extra (like .002" - .003" more)
than the recommended machine shop clearance)
I only get piston slap after a seizure.
I am only an Amature mechanic.
But the ring gap is clearly stated in the tech document from Heppolite that I quoted.
Perhaps I rode my model 31 too hard and too long to be considered "standard" usage , (I think it had run over six hours when it seized the first time. The next time it seized after ten minutes.
cf160
David J.Potter
iandusud
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Joined: Fri May 21, 2010 5:05 am
Location: FRANCE

rebore clearance

Post by iandusud »

Hi Dave, your ring gap figures are correct. I didn't recommend 1 thou per inch of bore, although there are some who would, and with careful running in one might get away with it, depending on the bike and the make of piston. What was the recommended clearance you used on your Model 31 and who recommended it? The last rebore I did on one of my bikes was on a Velocette Venom (which have a reputation for running hot). I gave it 4.5 thou clearance (86mm bore = 3.38") giving a ratio of 1.33 thou / inch of bore. I didn't run it in particularly gently, running at 60mph once the engine up to temperature for the first 500 miles. After that riding like a hooligan! BTW I live in the south of France where the ambiant temperatures are high. I recently helped a friend with some work on his Vincent Comet and used the same formula for his rebore. He's just completed 500 miles with no problem. Again riding quite hard. In both cases a Hepolite piston was used. I can't speak for other makes of piston, and I know that some can be problematic. It is preferable to keep piston to bore clearance to a minimum for a couple of reasons. The obvious one is that it last longer before needing another rebore. The second is in order to get a good sealing of the piston rings. A piston with too much clearance rocks in the bore as it changes direction. This is not condusive to keeping the faces of the rings square to the bore, particularly the oil control ring. Just my thoughts on the subject.

Ian
paulvk
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rebore clearance

Post by paulvk »

Hi, the bike is a 600 twin with + 40 hepolite pistons paul
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cf160
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Joined: Sun Jul 30, 2006 7:24 am
Location: Ottawa, CANADA
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rebore clearance

Post by cf160 »

Hello Ian:
From your experience , 1.3 thou per inch seems reasonable and perhaps a little improved oiling on the part of my twin crank will make it work . I blew
out the oil passages which seemed almost blocked
completely. I cannot locate a spare .040" piston , so I will use the smeared one with knurled skirts.
Dave
PS: The Machine shop prepped the barrels for
me and sold me the pistons with rings . They were a London firm , so I thought they knew best and the bike ran well to Liverpool and then
in Canada , but seized again when the clearance was only around 4 thou. So, I decided to allow a little more the next time.
David J.Potter
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