Original hepolite piston/bore clearance

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
g5wqian
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Joined: Wed Nov 18, 2015 7:43 am
Location: wiltshire
Location: near swindon wilts

Re: Original hepolite piston/bore clearance

Post by g5wqian »

i would give it .005" minimum .

i would treat hepolites the same as GPM if i were you .

i have a twin [56 model 30] and when i bought it , it had been rebored +.040" but had a little knock when i ran it and rode it , i took the barrels off and found the nearside piston had seized on its rear of the bore , ie where it gets the least cooling .

i measured the bores with my bore gauge and internal mics and they were only .001" clearance .

i bought new set of pistons and rings and i honed the bores myself to +.006" clearance over the std bore size+040" , ie std bore was 2.835" , plus .040" = 2.875" and then honed for +.006" running clearance = 2.881"

i ran it for one year and then stripped it again , no problems with seizure and i had clean bores, but a fair bit of oily carbon build up on the piston crowns , so i deglazed the bores and also used some oversize rings filed down to give it a tighter ring gap than before , ie now its .012" ring gap on the compression rings but was previously .025" due to the addition of the increased running clearance honed into the bores ..

what i did in my case was to use +060" rings and file them down to get a tighter ring gap , because the +.040" rings showed a .025" gap without any filing [ although .025" ring gap is still in spec as the manual says range is .006" to .030" ].

if you have rings that already show a fair gap when installed straight out of the packet , when you use increased piston running clearance you will find the ring gap will englarge by 3.14 times the amount of bore diameter added , ie the bore circumference will be increased by 3.14 times what you honed the bore over , so if you put and extra .004" on youll get an extra .012" or so on the ring gap .
so long as you have rings that dont go above the specified tolerances then it will be ok but if you end up with ring gaps over .030" then you may need to think about a way round it which in my case was to use +060" rings and file them down .

with a 60 plus year old engine you do whatever you can do with whatever you can obtain .

i have been running my engine for the second year now with no problems , and i am not frightened of giving it revs and also riding on 100 mile trips .

see attached pdf i copied and youll see recommended clearance ranges between .004" to .006" for hepolite pistons , or even .009" for racing .

hope this helps you decide .
all the best
ian
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