Original hepolite piston/bore clearance

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
moley
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Original hepolite piston/bore clearance

Post by moley »

Hi all


I am trying to find out what clearance should be between piston and barrell
Using an original hepolite piston , the piston is NOT wire wound.
For a 55 g80s . I know replacement gpm's piston are 5/6 thou but I would have thought Ann
Original piston would be less.


Cheers

Andy
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Rob Harknett
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Re: Original hepolite piston/bore clearance

Post by Rob Harknett »

You should find your bikes instruction book page 6 states, clearance .001" front to back ( tolerance .0005" )
moley
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Re: Original hepolite piston/bore clearance

Post by moley »

Wasnt tht one thou clearance for the wire wound pistons only .?
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ajscomboman
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Re: Original hepolite piston/bore clearance

Post by ajscomboman »

moley wrote:Wasnt tht one thou clearance for the wire wound pistons only .?
Most definitely, try running a non wire wound piston with that clearance and I'll offer to buy the bike at scrap value. Quoting what should be in there according to the original manual is a total waste of time as that would have been for a wire wound and you quite clearly stated that your replacement was a Hepolite un-wound. Rob, you really must read the original post before replying as someone without more experienced knowledge could have taken your word as gospel and trashed a good motor.
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Rob Harknett
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Re: Original hepolite piston/bore clearance

Post by Rob Harknett »

ajscomboman wrote:
moley wrote:Wasnt tht one thou clearance for the wire wound pistons only .?
Most definitely, try running a non wire wound piston with that clearance and I'll offer to buy the bike at scrap value. Quoting what should be in there according to the original manual is a total waste of time as that would have been for a wire wound and you quite clearly stated that your replacement was a Hepolite un-wound. Rob, you really must read the original post before replying as someone without more experienced knowledge could have taken your word as gospel and trashed a good motor.
It was stated the piston was original Hepolite, not wire wound. Rob. So it is a bit confusing when referring to clearance and tolerance. Hepolite pistons are machined to give the correct clearance, when bore to the correct size on the piston box label. Assuming our member was not going to rebore the barrel, who ever did that job would have bored it to the correct size. So should really not need to worry about clearance. However if he was going to actually fit the piston himself, he should be aware he would need to get the ring gap correct. ( have had to grind enough of them to get ring gap correct. ) Ken de Groom put me wise to that, not having tools in my youth I must have been lucky in my youth with just putting rings on with a gap of whatever. Hepolite make no reference to piston clearance except the piston are machined to give correct clearance for the bore. They do make reference to piston ring gaps. Assuming a bore or re bore has been done correctly, and the piston machined correctly, we should be OK in just checking ring gaps. However I do appreciate what you say Rob, about piston clearance. Its good advice to check what ever piston you may have at hand these days, when an exception to the old rule is not followed.
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ajscomboman
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Re: Original hepolite piston/bore clearance

Post by ajscomboman »

Not quite sure what you're saying Rob.

If you're agreeing that the original post stated it was not wire wound why then did you quote the clearance for the 1955 wire wound spec original piston?
The poster merely wanted to know the running clearance of piston to barrel using a non wire wound Hepolite piston.
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Rob Harknett
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Re: Original hepolite piston/bore clearance

Post by Rob Harknett »

I did miss understand Rob, The post said original piston then, not wire wound, that threw me into thinking the poster had an original piston for the bike. So the hand book info should be correct. I was not aware 55 G80 had wire wound piston. I have only ever had a 350 from 1955. I do not recall my 55 350 having a wire wound piston back when it was almost new. But that was very long ago. I was only 17 when I first had it apart when the bike was a year old. I changed the piston to hi comp. Since then I have always trusted the people to rebore correct and supply the correct piston to suit. Apart from just replacing rings and used pistons. I never had mic's then to measure piston and bore. Only feeler gauges. I think I use to use .004" for 350. Having never had a bore seize, perhaps I have been lucky. Only been unlucky when some one else fitted a piston or rings for me. Piston fitted for me in a 350 broke in many bits after just a few miles. Twin nipped up on test when a person fitted new rings, so he changed them back to those I had fitted before I took the bike back.
Bottom line is, if there is a miss understanding, it is good when some one like yourself spots something they feel is wrong and makes a comment about it. Nothing wrong with that, and no one should feel offended about it.
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Rob Harknett
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Re: Original hepolite piston/bore clearance

Post by Rob Harknett »

I believe it is also important the clearance measurement position is in the correct place. Front to back?? As I believe pistons are oval, and expand front to back making them round when warmed up. Plus rings gapped correct. The person who fitted my piston that blew up was of good reputation. But he used a blow torch on the piston to fit the pin. He also ground the rings a lot to gap them. This was also a Hepolite not wire wound piston. At that time I heard from two other people that had the same thing happen with the piston breaking in half. The pistons broke on the oil ring groove, They were lucky, they ended up with the bottom half still intact on the con rod, top half stuck up in the bore. Where as mine was revving a bit and did a lot more damage. Bits everywhere in the engine, carb and exhaust. I am trying to recall something about these Hepolite pistons that broke. I am sure they had a diamond or the word diamond cast inside the piston. So take care if you find one like that. Mine was a 60 + piston. I replaced my barrel with 20+ barrel I had spare and S/H auto jumble piston, which is still Ok after some 20 years.
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ajscomboman
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Re: Original hepolite piston/bore clearance

Post by ajscomboman »

I'm currently running Hepolites at 3-4 thou without any issues mind you that's on a twin, whether a large single would be different I can't say but I'd recommend a call to Ken De Groome who advertises in the back of the Jampot.
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Rob Harknett
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Re: Original hepolite piston/bore clearance

Post by Rob Harknett »

That does sound about right to me Rob. I think there is however some mis understandings about " clearance " e.g the book states .001" This does in fact agree ( almost ) with Hepolite recommendations. Perhaps the miss understanding is, this figure is for per inch of the diameter of the piston in question. Again where you measure must be correct, even more so on a worn bore/piston land in the barrel. My old bikes running on well worn pistons, perhaps .015 clearance are very happy bikes. You open the throttle without any fears for many years. They may smoke a bit. Have a rebore with new piston. You could be nursing them for a year or more running them in, and get problems. Something I have never had with my well worn engines for the last 30 odd years. The next owner may worry and rebore them with new pistons. Even though the old bikes will last many more years being used only on club runs etc. if left alone. My little 36 AJS 250 is still as found, smokes like a good un. I have only done about 12, 000 miles on it in 30 years and still never so much as had the head off.
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