piston/bore clearance

[Chris Berry]

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

[clive]

So what piston are you using and did the engineers have it when they bored the new liner? Strange it siezed when you were coasting rather than under power going up the hill. Sure it's not the mag if you have one fitted? How will you confirm that it was an overheating seizure by lifting the head? You will have to remove the barrel to spot any telltale marks on the piston. What ring gap did you use?

[Chris Berry]

So what piston are you using and did the engineers have it when they bored the new liner? Strange it siezed when you were coasting rather than under power going up the hill. Sure it's not the mag if you have one fitted? How will you confirm that it was an overheating seizure by lifting the head? You will have to remove the barrel to spot any telltale marks on the piston. What ring gap did you use?

Thanks Clive,
The piston was supplied by AMOC as a standard piston. I supplied the engineers with the piston when I had the work done. The magneto was rebuilt around 1800 miles ago. The piston came with the rings already fitted and I didn't check the gap. I did think that maybe the HT lead could be a problem but it started first kick when I tried after a few minutes. When the bike stopped the back wheel did not lock up but I applied the clutch straight away. It did sound like something had let go internally but all seems good (it may have been the clutch slipping) accounting for the lack of lock up

[Chris Berry]

So what piston are you using and did the engineers have it when they bored the new liner? Strange it siezed when you were coasting rather than under power going up the hill. Sure it's not the mag if you have one fitted? How will you confirm that it was an overheating seizure by lifting the head? You will have to remove the barrel to spot any telltale marks on the piston. What ring gap did you use?

Clive,
Speaking of the magneto my manuals don't specify the actual gap setting rather refer to the special tool. In line with this I have taken a punt and set the gap to 0.015". This being a common setting for most engines I have come across. Is this the correct gap setting?

[Brad]

My G80 was rebored to plus 20 recently with a JP piston supplied. The bore to piston clearance was given as .004 and I was told that the rings were already gapped. I have done about 800 miles now with no sign of any tightening up. I should say that much of this mileage has been done on Romney Marsh which as you might know is very flat. I have reached the point where I have recently opened up to 60mph for a couple of short bursts before backing off to 45/50 mph with no sign of distress.
I know this does not solve your problem Chris but maybe it is useful for guidance/comparison.

[Chris Berry]

My G80 was rebored to plus 20 recently with a JP piston supplied. The bore to piston clearance was given as .004 and I was told that the rings were already gapped. I have done about 800 miles now with no sign of any tightening up. I should say that much of this mileage has been done on Romney Marsh which as you might know is very flat. I have reached the point where I have recently opened up to 60mph for a couple of short bursts before backing off to 45/50 mph with no sign of distress.
I know this does not solve your problem Chris but maybe it is useful for guidance/comparison.

Thanks Brad this helps me a lot to the point that maybe I should look elsewhere for the problem

[oldandsmelly]

Assuming the number of zero's is right, 5 thou clearance should be about spot on. I couldn't find any data for the AMC heavyweight engines, but for the big Norton singles like the 16H, the recommended piston skirt clearance is 5 thou.

How sure are you that the engine pinched up? Was there any warning sounds like a dry squeak? It seems unlikely to me that the engine would seize when coasting downhill and I would be tempted to ride it gently around some more and to check obvious things like oil circulation and unusual noises. These are low stressed engines with lashings of oil splashing about under normal conditions so shouldn't seize unless something is very wrong or you try to ride them like a seventies two stroke.

[SPRIDDLER]

I have heard of a couple of rebored engines seizing when coasting downhill and I recall reading an engineering article which suggested that with the throttle open the piston crown and inlet valve benefit from the incoming 'cool' fuel/air mix. For this reason inlet valves are usually of different material from the hotter-running zorst valves. When coasting with the throttle shut there is no cool incoming fuel/air mix and a heavily finned finned cylinder (although of greater mass than the piston) will cool more quickly than the piston crown. Make of that what you will.

WRT to clearance. With an unknown 350 or 500 piston I've several times adopted a rough and ready rule of thumb of 1.5 thou clearance per inch of bore diameter (for an air cooled cylinder) which has been fine, so your 5 thou looks about right.

When you inspect the piston the ring gap should be checked.

[1608]

Too advanced or running too weak may have aided a new re-boreed engine to run hot. Check the timing and the plugs. A bit on the rich side wouldn't hurt for a while.

[leswaller]

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.