Piston seizure on big bore engines


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I wonder if I could benefit from any practical experience members might have on this subject.

The only intimation of trouble I had of seizure was when coming back from a rally in Germany some time ago. The Comet started “pinking” on the M62 out of Hull & couldn’t maintain 60 mph against a headwind. I rolled off the throttle & took it easy the remaining 40 miles back home to Wakefield on the M62. I thought maybe the ignition timing was out for some reason but then again it hadn’t ever pinked before.

For various reasons I haven’t use the bike since then but I got it out recently to prepare for the Italian VOC rally next month/September & removed the head to try to solve an oil leak which had gradually become serious. I suspected the front push rod tube seals but couldn’t be certain as oil covered nearly everything. I thought I had better clean the cylinder also as it was covered in oil too. It was only when I removed it that I saw the piston seized on the inlet face. Damage to the piston was severe on the inlet side skirt including deep gouges forward of the gudgeon pin holes. The rings were slightly trapped, otherwise no damage here. The exhaust side skirt was fine. The cylinder was scored. The combustion chamber was pristine with no sign of whitish grey high temperature combustion on the piston top or in the head. According to my reading of piston failure analysis guides on the internet, this damage typically indicates a failure of lubrication on the piston caused by temperatures so high the oil film would instantly have evaporated off it.

The engine had seized at 7300 miles after a complete, professional rebuild in 1996. The piston was a JE M89 in a Bob Dunn cylinder liner with standard size stud holes, etc. It ran for almost all of that mileage with a single plug, a “Hamilton” cam, a BTH electronic magneto set to 34 BTDC & a 34 mm Mikuni carb which was jetted on a rolling road by Paul Klat (Ducati racing bike tuning & servicing). The rear sprocket is 46T.

Obviously, when repairing the engine, I would like to ensure as far a possible that seizure will not recur which means being sure why the seizure took place. Also big bores were in their infancy when this engine was rebuilt in 1996 and I would appreciate the benefit of the experience others have gained since then.

I’m advised that ignition setting of 34 BTDC is away too advanced for a big bore & almost certainly the explanation for failure in this case. I have heard different suggestions for ignition timing on big bore engines. Recently Dick Craven of Cravens Motorcycle Museum near York told me that the big bore Vincent twin & single he campaigned in sprinting had been timed at 18 BTDC at times!

The practical problem about radically late ignition on a purely road going bike is getting the engine started. You need an ignition device which will provide a spark powerful enough at kick over not to be quenched in a high compression big bore combustion chamber and which retards starting ignition to no more than on a standard bike. I have struggled with this. I have used the original magneto (starting impossible although device completely rebuilt by the best), manual magneto (starting possible sometimes with much effort & severely damaged knee), the Grosset distributer (excellent but where are you if the battery/generator fails on an old bike and they do) to the BTH magneto (excellent - at 34BTDC anyway - but I don’t know yet if it too shares the original magneto’s problems of coping with really radically late ignition timing). A distributer with its auto advance mechanism locked & single point ignition timing has been suggested. However, I’m afraid there’s not a lot left of my knee joint for experiments of this sort &, anyway, hens teeth are common place compared with single distributors nowadays.

I am not into racing or blowing anyone into the weeds. I just want a Comet that starts easily, can hold its own in modern traffic & cruise & pass lorries on the autobahn at 65 to 75 MPH & stay in the middle lane if that’s necessary. That’s my only reason for going big bore but if big bore reliability is impossible then I may have to consider returning to standard as reliability comes before all else.

Does anyone know :-

  1. a source for a standard JE M89 piston?
Or have opinions/experience on (for the purposes of a touring, road going bike):-

  1. the right ignition advance for a big bore?

  2. whether twin plugging is really necessary on them?

  3. whether a cylinder muff, milled or cast, with smaller stud hole is really necessary with a big bore conversion?

  4. best method of fitting the liner in the cylinder muff?
  5. is reliability possible with big bore conversions?


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We raced big bore till we saw more points with 500cc no problems 3 seasons lots of hot paddock waits 32deg. Btfc max Suggest a barrel packer and try a HD or chain saw decomprossor for starting dual plugs waste of time


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You can buy JE pistons direct from JE if you have some type of trade name or through someone that deals with them. If they don't have the M89 in stock they will make two for $500 with rings. I believe they will do different oversizes for both if necessary.

I don't think the size of the bore affects your timing. Generally, it would be the size and shape of the combustion chamber. The more efficient the combustion the more retarded the timing can be. If you have modified your engine the stock settings are only good guesses. I would find the correct timing on a dyno. 18 degrees sounds like a twin plug ignition with a highly ported head.

Twin plugs are not necessary. It is still a 500 cc head and although twin plugs will allow you to retard the ignition because they introduce some combustion efficiencies, you can only guess at the number unless you set it on a dyno.

I believe that the smaller diameter crank case studs and smaller muff holes are only necessary if you go larger than 600 cc. I have a 600 with a stock cylinder muff.

Someone else will have to give you the fit of the muff on the iron liner. I use aluminum liners and Nicasil for their superior wear with no air filters and higher heat transfer.

The big bore kits should be just as reliable as the stock top ends.

It sounds to me like you have a carb problem. I also think that you may be expecting a little to much from the big bore kit and the Comet. The Comet is an 80 mph machine at best. If you are riding at 75 mph, you are at 7/8 throttle. This is quite extreme, much more so than racing. If you are road racing, you are running up and down constantly and drawing oil into the top end with the vacuum of every throttle closing. What you are doing is more like running at Bonneville for extended periods and that will cause seizures. If you were on a twin there would be no problem. But, a Comet on even the slightest grade and/or a headwind will start to over heat very quickly even if it does not seize. This is exacerbated by the masking of the front cylinder by the front wheel as speed increases. The rear cylinder stays relatively cool, but the front cylinder is in a dead zone at speed. This causes the cooling to be very spotty on the Comet and I believe that it needs much more clearance if you are going to run at full throttle on the highway.


Bill Thomas

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I was going to ask Vibrac what clearance he ran, I was thinking maybe he could clean up the piston and hone out the bore a few thou, If it's not too bad. I bet the clearance was not a lot as a road bike. As Vibrac says, Fit a packer to the barrel to de tune it a bit. Maybe it was a hot day + plus some bad petrol ?, Always use the best petrol, He does not have a lot of time, Good Luck, Bill.

greg brillus

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There is probably no one answer to your problem, on account of the many changes you have made.......However.......Are you running the original oil feed through the liner to the piston skirt. This would definitely help, as there are many who advise it is unnecessary......If this is the case, why did the factory bother. I would think as David has suggested that the main culprit is in the carburetion, as the results given on a dyno will NOT give the required results needed on the road, it is only a benchmark "Test Piece" if you like. I know racers who set up their carbs on the dyno, only to have suffered holed pistons down the main straight of a large race track. Holding an engine whilst loaded at half to full throttle for long periods of time is not what anyone does on a dyno. So I would say your mid to 3/4 throttle range is too lean. Do you run an air filter....? As again this would help. I am not convinced that 34 deg advance is TOO much, but retarding it back to 30 or 32 may be worth trying. Sometimes when an engine is putting out a continued higher power output than at a lower output, the difference between it running fine, verses detonating and overheating can be a very fine line. Yes having a big bore kit is going to cause some form of weakness in the cylinder, how can it not, the distance from the piston to the hold down studs is reduced, so the chance of the piston "picking up" on all four corners will be higher than standard, due to there being less heat absorbing material. But these engines are prone to picking up in this area anyway. So rebuild your top end, new piston, give it the right amount of skirt clearance, I would make sure the cylinder oil feed is working and aligned carefully. Reset your ignition with slightly less advance, and look carefully at trying to richen up the carb, say lift the needle two notches, perhaps you need to go smaller in the slide cutaway.....If it is say a number 4 now, go back to a 3 1/2 or whatever. You will find the changes need only be minor to make your bike reliable again. Do NOT do any radical changes, but again, I feel the carburetion needs looking at. Finally, When torqueing down the head Nut/studs, do NOT over torque them, as this will distort the liner and cause high spots at the area's where the hold down studs pass through, thus a possible cause for another seizure. Cheers for now.........Greg.

Black Flash

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On my comet I am using a 600 cc big bore Terry Prince toped kit from his first series ( I have been told that he changed the design of the combustion chamber in his later kits). I am using twin sparks on a Boyer Bransden micro digital system and have the engine timed at 28°and the engine runs great.
I started with 30°and a far too rich carb, but when tweaking /weakening the carb the engine finally started pinking under hard acceleration and so the timing was retarded to 28°and feels happy now.
I have no idea about the advance range of this unit, but the bike starts first time(usually) and ticks over like a grandfather clock on TPV mark IV race cam. I am convinced that the rocksteady idle and running is mainly due to the good spark at low idle speed and the new Mikuni carb. I find the parts for the carbs (VM series) extremely cheap and very well made. on my bike changing the needle position by one groove makes a noticable difference, so the carb reacts far better to tuning than I was used to from old and worn Amal carbs.
I am using a 36 mm carb but if you want I can search for my settings and let you know.

Black Flash

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Oops sorry one thing I forgot.
When talking to Bob Dunn about his bored and stroked engines he told me that the bigger bore on a standard head forms a small squish band and the pistons have to be checked and machined to suit the required clearance. He also said that in his believe the squish band had a very positive effect on the swirl of the mixture and the burning of it. He suggested on these engines to retard to less than my 28° !

cheers Bernd