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[devon john]

went out today bimbling at around 50-55mph then noise coasted to a stop ,,it solid ,,seized !!!!

well i have had this noise problem since rebuild and couldn't find it now its found me!!!

will strip later the week and report back

[Gwilly]

Well i for one am sorry to hear this news John.

Know you put in a lot of work.

Will be interested to read the autopsy report.

[Bet]

Not being a professional, if you have an Electra, I wonder if it is the same problem that I had? (Electra cams)

[devon john]

500 classic with 535 conv, big valves ,amal carb , etc.

can't understand a four stroke with a 1000mls on should seize.

[Davie Hall]

An old work mate had a similar problem with his 350 classic. It decided to seize one day, he dismantled it and found nothing after 2 top end strip downs it was found that when it got hot, as in pulling up a long hill or a long run on a constant throttle the piston had a casting flaw that deformed the piston causing it to sieze. A new piston and a hone of the bore cured it

[simon]

My old 51 350 did the same thing until I split the crank to discover the big end had been changed for a roller but the oil way was still blocked with white metal. astounding thing was that as long as I kept it around 45 to 50 mph the oil vapour was enough to keep it going fine. It would pinch up if I went any quicker.

[Presto]

Just a thought. If you’ve rebuilt with the forged piston and not had the barrel accurately machined to the new piston to give the exact clearance needed you can be sure as night follows day it will seize – usually within the first 500 miles. No fault of the parts but they must have the correct clearance. And then they must be run-in with care for about 800 miles. Another possibility - detonation will give you a 'noise' and after a little time can blast a hole in the piston crown or cause severe over-heating and a seizure. The cause is usually a fuel mixture that's too weak for the higher compression piston.

[PeteF]

The 535 conversion would entail a rebore and piston. You did run it in again?
The big valves and carb would alter the breathing dramatically. Sure you got the mixture right?

[ChrisD]

Devon John. Sorry to hear of it. I had endless problems with fitting several new pistons into an iron barrel and they kept seizing, despite the high cap oil pumps and the bore-piston clearance set as per the specs. BUT the specs for clearance (5.5thou) were too little for the cast iron barrel. So in my 1996 Classic 535 I fitted an alloy barrel and had the machinist set the clearance at 6thou, well honed (so probably nearer 6.5thou) and all is well.
Hope you come right. Cheers, ChrisD

[simon]

That's a massive clearance for a non racing machine. .0035" at the thrust face was what I got told by our hosts for a standard piston and iron bore. Once I had a functioning big end bearing oil feed the bike never looked like seizing again.