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[m1ks]

OK,
So in my previous post I said B****rd the bullet!
Sooo, it may be more a case of dozy b****rd the owner, remember I said the oil level was low?, now I assumed this was as a result of the aforementioned loss of compression and stuttering, and the odd little 'hmm, that doesn't sound right' noise prior to stopping should have rung some old dusty bells from my 2 stroke days!
Stripped the top end after trying everything I could whilst in place to see if the prob was there, all OK, head gasket, OK, bit of a hot spot exhaust side but no blow past it seems.
(It all came apart a lot easier this time BTW, lack of super sticky black ATV silicone would do that)
Barrel off and this was what I saw.
Pic 1 piston rear:-

Pic 2 piston front:-

Pic 3 barrel rear:-

Pic 4 barrel front:-

So, as you can see I had a partial seizure, the top two rings, (which weren't aligned, been easier if they had been), have welded themselves into the rear of the piston groove, the oil control ring is still free in it's groove.
The rear of the barrel is badly scored and the front has a couple of new score marks clearly where it had started heating up, all this most likely due to oil starvation.
While it was pumping through quite vigourously when kicked over on the kickstarter into the sump and onwards from the sump to the head the fact that I was only reading just above the L mark on the dipstick indicates the small amount in there simply couldn't keep up with oiling the bore.
I've been mentally kicking myself up the arse as I am solely to blame for not checking the oil frequently, especially when I know it was just over 1/2 last time I checked prior to going for MOT, being used to more modern machines for a while that use little if any oil and have a sight glass hasn't helped me remember either.
Some pics of the sump area and (hopefully), all seems OK here though circled in one pic is, what appears to be a piece of ally debris that needs investigation, I didn't notice anything while having a good look with the inspection torch so it may be one of those weird photographic things.
Pic 5 Sump rear of conrod:-

Pic 6 Sump front of conrod:-

Last pic Sump:-

[m1ks]

Next question to those in the know.
I'm looking I imagine at either a rebore to the next size, +10 and the appropriately sized piston and ring assembly, or a replacement barrel and piston/ring assembly.
I'd like to retain the iron barrel for the authenticity and am assuming, (though i've never had this done), that a rebore would be cheaper than a replacement barrel?
Do i need to provide the new piston to the machine shop to enable them to measure and bore accurately or can this be done without the new piston?
If anyone has any suggestions as to places that can do this i'd appreciate it.

[Bullet Whisperer]

That looks exactly the same as what happened to a 1939 AJS 250 I have almost as a resident here. The owner decided to dyno it after some tuning work was done and they seized it when it nipped up due to getting too hot, but can't have throttled off as soon as it began to get tight. There were great reams of alloy from the piston welded to the face of the bore and the piston rings had to be broken out. Pre war AJS pistons are not as readily available as Enfield Bullet ones, so it was cleaned up [looked just like yours], new rings fitted and the bore cleaned up with coarse emery. It ran fine again afterwards and has done hundreds of miles since. The piston won't seize in the same way again, because what got hot and stuck to the bore is no longer there to do it again!
Must add the owner is fully aware of how it was brought back to life!
Paul.

[m1ks]

Wow, good, ol british engineering eh, don't build em like they used to.
I could take another look at the barrel on the offchance it'd clean and I'll bear that in mind as a last resort for mine,

Hopefully I can get it rebored for not too much and one oversize piston (and some oil this time), and I should be all set to listen to the music through my new stainless short silencer.

[Les H]

It won't have been the low oil level, unless the oil tank was nearly empty, exactly the same amount of oil would have been pumping round as if the level was at the full mark, The oil might have been running slightly hotter but not significantly so....no need to blame yourself for the low oil level. I would check that the oil pump and spindle are all ok...make sure the pump faces are not scored and leaking pressure and you are getting the full quota pumped around......It's hard to see but the top piston photo seems to be showing the carbon has been blitzed off the top of the crown, so it could indicate that the ignition was too far advanced and overheated the piston by pinking, so I would double check the timing after the rebuild....actually it is easier before you put the head back on... the piston carbon also indicates that it was not overheating caused by a weak mixture. I reckon the bore will clean up with some honing and as Paul suggests, maybe the piston will do also. I would deffinitely put some new rings on it though...after cleaning the ring lands up a bit.....You have to remember the rings do the sealing and the piston condition won't matter too much apart from having a little more slap...but it could have been set up too tightly in the first place and the clearance after all the cleaning up might not be that far in excess to what it should be. Good news is that if you do use the original barrel and piston after cleaning them up, you won't need to run the engine in very long...just need to bed in the new rings...it's not likely to seize again.

[m1ks]

OK, that gives me something to look at, really don't know i'd be happy re-using the piston but I guess a clean to see can't hurt.
The carbon on the piston crown is a good coating, no blitzing off and certainly no audible evidence of pinking at any point when I've been riding it.
I'll have to see if i can beg or borrow a hone locally and see if I can revive the barrel.
Will the timing cover need to be removed to check any part of the oil pump or just the front and rear covers?

[m1ks]

Don't know if any relevance here but at the time it occurred, it was a cool evening and it was about 1/2 throttle registering a wavering 55-60 on the speedo, no idea how accurate it is but I didn't feel that I was stressing the engine at all and previous runs have been at about 50-55.

[Norm]

As Les says, lack of oil wasn't the problem, even at the bottom of the dipstick you still have 70% of oil left in the tank.Is the underside of the piston crown burnt brown.Lack of oil pumping around could be the problem but this usually causes big end failure and you haven't mentioned anything about this. Check in the rockers for any sign of whitemetal, this is important before you go any further

[m1ks]

Underside of piston is a bit on the dark side yes, more black than burnt brown but the rear thats had the most surface contact is brown.
Pics below.




The rockers appeared fine on removal, no signs of any metallic particles in the top end or sump when I drained it. No excessive sloppines or strange noises from the crank end, all seems quite firm.
Will double check the rockers and such tomorrow though.
Can provide further pics of anything that may be of use to you?

[Norm]

Underside of piston indicates it has been hot, most probably caused by running lean at higher revs