AJS & Matchless Owners Club

My 1952 AJS model 18s with a new piston and bore seized. At the time of seizure, the engine had about 150 miles on the new bore. It was lightly loaded doing about 55mph on a level road in cool weather conditions and about 10 miles into the ride. After the seizure it restarted, and I was able to ride it home. The engine has the original carburetor and jetting and was running well at the time.

The piston was purchased from the club. A picture of the box label is attached. It is +1.5mm and the cylinder was bored to 0.0045" clearance. The piston seized at both the front and the rear. Very little damage to the bore. Pictures attached.

Any thoughts on what caused the seizure? I don't want to risk damaging a new piston or my bore by trying again without some reason to believe that it will not happen again.

Thanks

just running a bit tight clearance , you will need a new set of rings and provided they are free in the grooves , clean up the piston where the damage is with wet/dry and make sure you wash it well affterwards

I always fit new rings if this occurs

bore will most likely be ok if you clean out the aluminium with caustic soda ( eats aluminium ) then wash well

modern pistons often expand a bit more than originals , hence clearance wants to be at least 0.015" per inch of bore

not a disaster , it will live again


Make sure the ring gaps are correct when you go to refit , 0.006" - 0.010" from memory

Depending how you get on with your engine borer, when mine had a partial nip, I took it back and he put it back on the honer and took another fraction out of the bore for free as it took about 10 minutes.

As previously said, clean the piston up and it should go again. I was able to reuse my rings as they had no evidence of damage, scoring or marking at all. And double check your ring gaps as said.

Good luck with it.

Thanks for the suggestions.

I have cleaned the bore with muriatic acid and a light hone. Bore looks good to me. I have cleaned up the piston. It has obvious damage but should be O.K. Rings look O.K. and are free and correct in their grooves. I assume the piston is now a looser fit in the bore and perhaps that is what it needs. I will put it back together and run it some more and see what happens.

sounds good , careful running in
I seized a twin once 5 times when on the way to a international Jampot in Sweden , nearly 2000 miles before i got home and that was after 1000 miles running in without issues .
Looks like you have got away with it , good luck

52M18S wrote: ↑Fri Mar 01, 2024 7:05 pm ...... the engine had about 150 miles on the new bore. It was lightly loaded doing about 55mph on a level road in cool weather conditions

Just a point (probably unnecessary as I guess you know) but it's more a question of keeping the engine spinning when under load and not letting it labour, e.g. changing down earlier than usual on hills. With the engine spinning faster in a lower gear the oil pump is working faster and moving more oil, but that may just be my misguided logic as all the other bits are also moving faster.

52M18S wrote: ↑Sun Mar 03, 2024 6:32 pm Thanks for the suggestions.

I have cleaned the bore with muriatic acid and a light hone. Bore looks good to me. I have cleaned up the piston. It has obvious damage but should be O.K. Rings look O.K. and are free and correct in their grooves. I assume the piston is now a looser fit in the bore and perhaps that is what it needs. I will put it back together and run it some more and see what happens.

I dont think that because there was a seizure and you cleaned up the damage on the piston that the piston is now a looser fit in the bore . I think to be sure you need to hone the barrel to give a bit more clearance snd run it in gradually. These pistons need additional cclearance on the bore.