I am doing up a mates 1971 T120 engine, I have found the previous owner had fitted a primary seal on the crank, I understand this should be deleted on 1970 and up? The engine had a top end overhaul, and unfortunately the DS piston picked up and partially seized. Can any one advise if the restriction to crankcase breathing could have attributed to the piston smearing itself on the bore? The only place I could see the engine breathing was through the oil level holes between primary and crankcase, and they are pretty small.
Cheers Richard

With the sump unable to breathe as easily as it should, the oil level would have been higher than normal. Extra oil sloshing on the pistons would NOT be the cause of the damage you describe.

Thanx, now I wonder what eleser could have caused the nip up, aprt form a hot day and a gradual hill climb?

Cheers Richard

Run it in in winter ! Theres a thousand things couldve caused it . Probly lugging it , or not opening both gas taps thrashing it .

It's hard to get a good read on damaged parts, but you need at least .004 and preferably .0045 piston-to-bore clearance or you'll get that partial seizing under heavy load (don't ask how i know this, although the book is crystal clear on the subject)

Unless the bike has recently been re-bored and the wrong clearance has been used, its safe to say that clearance issues arent likely to be the cause of your problems. Much more likely cause is that side piston running much hotter than the other, which may result in pistons getting very hot, especially so if the bike is being ridden hard.

Common reasons for overheating, are ignition timing being out, or weak mixture on that side carb. If the piston has got hot, then the colour of the crown will be distinctly different from the one thats ok, and depending on how bad the seizure actually is, it may be possible to rebuild the motor with a new piston, and simply hone the affected cylinder.

My guess is lean mix would have holed the piston before sticking it...

Well Thanx guys, I am suspicious of the fuel taps and our old friend ethy, the taps were the knock off type with rubber sleeves inside. Fuel flow was ordinary through the taps, ran a drill through and now a solid 1/4" bolt of fuel exits. Engine was a fresh rebore, now she is on size bigger with new pistons. The engine is apart to check the sludge trap and bottom end. Big ends look good, so just new shells there.
A quiet cool break in will follow.
Cheers Richard

Its most likely that the rebore was not done properly..............good idea to check this type of work very carefully before rebuilding.