Enfield re-bore
 

I'm on a 1968 Enfield Bullet (350cc), although my questions are probably fairly generic. So far I have come from India to Turkey, but the bike seems to have had enough now! On arrival here, it became apparent that it required a re-bore, but luckily I was carrying an over-sized piston and rings as spares.

The re-bore has now been done, but it still seems rather tight. To begin with, the piston was seizing every 5km (when the engine gets hot). After repeatedly waiting for it to cool down, and then continuing again, I have over the course of a few hours brought this distance up to about 20km. Since my experience of this is non-existent, and my mechanical knowledge barely any larger, my first question is: should I expect seizing initially after a re-bore? If so, how rapidly should the new piston wear in?

If, as I suspect, the cylinder needs to be bored out a bit more, how tight should the piston rings be when everything is cold? At the moment, the ends of the rings close tightly, presumably reflecting the fact that the piston only just fits inside the cylinder. Should there in fact be a small gap between the ends of the piston rings, to allow room for expansion of the piston? Actually, I've just found some specifications in the manual, where it says that the gap when new should be 0.015-0.020 inches, so perhaps I've just answered my own question. There's not much room for error, though, since the maximum permissible gap is 0.030 inches!

Any advice much appreciated, since I'll have to consider abandoning the bike if I can't fix it!

James

Sorry to hear you have problem.
Seems like your mecanic have done a bad job.
You could try to re bore it, but i have never heard of people open up the bore more. But it may work

I think your best bet is by going for an new oversized piston and have the job done by an better mecanic. And be carfull when you drive it in again.

Two years ago there was an enfield mecanic in Athen. Some overlanders have ordered parts from him. I dont know more of him.

Good luck! Frode

James, the specs on the piston ring end gap sound correct - just file them down until you have the correct gap. Be careful - the rings can break very easily - they are extremely brittle.

I put a file in a vise sticking out to the side, clamped so that it's vertical, hold the ring ends in your hand and file back and forth carefully and slowly, keeping the ring ends very square.

Take your end gap measurements at several places down the cylinder, using the piston to push the ring down and keep the ring square in the bore.

First of course you should check the piston clearance in the bore, but if there is zero ring gap that may be the whole problem.

There should also be a spec for the piston clearance, and where to check it - if not specified, check it on the front to back "thrust" sides of the piston at the bottom of the piston skirt, against various locations in the bore - (should be the same everywhere, if not the bore isn't round or it's tapered and you're in trouble.) Ideally you would use a wire gauge, but a narrow blade gauge will do, just add .0015" or so guesstimate to the measurement.

Somewhere between .002" and .005" clearance will be tolerable for that engine if you have no specs.

Any seize marks on the piston can be FILED off with a very fine file. They will probably be on the "corners" of the piston. Just file enough to remove the worst of the marks and get the surface flat and smooth again. DO NOT use sandpaper - it will leave grit in the piston material. You CAN sand the bore in desperate situations no worries.

WASH thoroughly all parts in gas and then warm soapy water to ensure it's all very clean. Oil the cylinder immediately after washing.

Finally, don't forget to check ignition timing, valve clearance, carb jets etc to ensure there is no excessive advance or lean condition causing overheating and seizing.

Good luck!


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Grant Johnson

Seek, and ye shall find.
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One world, Two wheels.
www.HorizonsUnlimited.com

Many thanks Frode, and especially Grant for your detailed advice. It's interesting that you say that bad timing can cause overheating - I think that's what caused the problem in the first place. Unknown to me, a tappet guide had broken. I guess this happened in Iran when I suddenly had to make a major adjustment to the timing - I thought it was just because the bolts holding down the rotating backplate of the points had come loose, and it had adjusted itself, but now I know that it's because a tappet was wobbling about.

Anyway, I haven't managed to sort out my problems. It was clear when the barrel come off again that the re-bore had been no good. It was done on a lathe, which I now know is a recipe for disaster - shame the mechanic had it done that way, particularly since he knew that there was a re-bore machine in the next town. Had it properly ground out there, and filed the piston as recommended, so it was looking rosy.

Back on the bike, it turned out that this wasn't the only problem. The last piston seizure had indeed felt particularly severe, and it seems that something in the crank assembly broke. I've been carefully considering all the options (including a kind offer to help in Istanbul, if I got the bike there, by an English guy contacted by the Istanbul HU community).

However, weighing up the costs and time implications, I have reached the difficult decision that it is better for me to abandon the bike. It's heart-wrenching, but I have to be back in England before too long, and even if I spent the several hundred dollars required to get the parts here, I'm still not convinced the bike could be fixed, and I would run into severe time troubles, having already spent over three weeks trying to sort it out.

I'm currently trying to persuade Turkish customs at the Iranian border to take my bike and relieve me of the need to leave the country with it (it's in my passport), which of course is easier said than done!

A sad end to what has been a great trip, but I'm sure there will be others.

James