piston/bore clearance

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Pharisee
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Re: piston/bore clearance

Post by Pharisee »

I've just had my 350cc G3 barrel resleeved by T & L Engineering. They supplied the STD JP piston and that specified 0.004" clearance. The rings weren't fitted so I checked the gaps before fitting (0.008" - 0.011"). The two compression rings were perfect, the one piece oil scraper was just a little tight so that was 'adjusted' with a fine Swiss file.
I'm from the Fens.... Gimme six.
Chris Berry
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Re: piston/bore clearance

Post by Chris Berry »

leswaller wrote:
Chris Berry wrote:I have just had my G80 res;eeved and taken back to standard. The engine rebuilders advised me that they had bored the sleeve to allow 0.005" piston clearance. On my first run after around 6 miles i came over a reasonable hill in third gear (so as not to labour the engine) at around 20 mph and the was coasting down the other side in top gear at 30 mph when the engine stopped. It appears the engine temporarily ceased (to be confirmed when I lift head). After a minute or two I was able to restart the engine and return home with no apparent consequences. I m now thinking that 0.005" clearance may not be sufficient or I should avoid hills like the plague (unfortunately my area has lots of hills) until the engine loosens up some more. Can anyone advise please what is the recommended piston clearance?

regards Chris Berry
I had my engine rebored and fitted with new pistons 2 years ago. It was done by a club member who does this kind of thing for a living. Before I got the engine back he advised me that "due the fact that the pistons supplied by the club were designed for a water cooled engine" the engine was likely to try and nip up in the first 1000 miles, after which time it would be fine. This has proved to be the case, it did keep trying to nip up, however some 3000 miles later it goes very well with no rattles and no oil burning.
Thanks Les,
what did you do to prevent it nipping up?

Chris
leswaller
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Re: piston/bore clearance

Post by leswaller »

I rode with my hand covering the clutch lever and listening to the engine. As soon as I heard or felt it tightening up I pulled the clutch and killed the engine. After about 5 mins I restarted and it was then usually ok for the rest of the ride/ day.
Les
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Chris Berry
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Re: piston/bore clearance

Post by Chris Berry »

Thanks for all your help. As it now turns out it was not the piston seizing rather the inlet valve. Having just gone for a short run covered around 6 miles and the bike stopped this time I was in a safer environment which enabled me to get around the bike and diagnose the fault. The inlet valve is seized tight. I haven't yet removed the head but sounds like a lube issue. I know I am getting a flow fro. The front of the pump after that who knows
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clive
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Re: piston/bore clearance

Post by clive »

Check the valve seat has the hole in it for the oil supply aligned correctly and is not full of crud. check the oil supply needle adjusting screw is set 1/6 of a turn back from fully closed. Is it possible a previous owner closed it up because a worn valve guide was already allowing too much oil into the head and since a new guide has been fitted? Has the guide dropped so that the valve is binding on the belled out part of the valve stem ( the earlier guides have no circlip and if one is fitted the valve spring platform needs a slight rebate to accommodate it)? If all is good try opening the needle a small amount more until you have worn it in for a good few miles especially in what I imagine may be summer heat over there. .
clive
if it ain't broke don't fix
Chris Berry
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Re: piston/bore clearance

Post by Chris Berry »

clive wrote:Check the valve seat has the hole in it for the oil supply aligned correctly and is not full of crud. check the oil supply needle adjusting screw is set 1/6 of a turn back from fully closed. Is it possible a previous owner closed it up because a worn valve guide was already allowing too much oil into the head and since a new guide has been fitted? Has the guide dropped so that the valve is binding on the belled out part of the valve stem ( the earlier guides have no circlip and if one is fitted the valve spring platform needs a slight rebate to accommodate it)? If all is good try opening the needle a small amount more until you have worn it in for a good few miles especially in what I imagine may be summer heat over there. .
Thanks Clive,
All good info. I haven't lifted the head yet will be doing that tomorrow. I had new valves and guides put into the head and pointed out the oil gallery to the engineer when I took the head in. He assured me that he lined it all up when he assembled it (as I picked it up assembled). I didn't check the adjustment so I don't know if they screwed the needle right in on assembly but will be checkng these things tomorrow when I lift the head. I was concerned with the oil pump as I am only getting an ooze out of the front but the return flow to the oil tank is fine. From memory an ooze is all I ever got out of the front of the oil pump (the oil pump is new also)
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clive
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Re: piston/bore clearance

Post by clive »

yes supply to the top end is slight
clive
if it ain't broke don't fix
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