Momentary seizure

It could be just the skirt of the piston that nipped slightly, but the concern would be that aluminium has been wiped over the rings.

One way to check would be to do a compression test and if you get two different readings for each cylinder then a top end strip may be prudent.

Hi George,

I had a similar issue; albeit with a different bike.  I bought a '68 Bonneville in a dilapidated state (a habit of mine) when I was 17 and about to join the RAF.  The bike had a ratty, semi Slippery Sam appearance, and wire-locked drain plugs and fittings suggested it had been used for racing.  The rrearsets, clip-ons, racing tank and seat came off and standard parts fitted.  I got the bike MOTed, but as soon as started riding it, it was clear all was not well - even my mum (!) asked "is it supposed to smoke that much?".  So with my limited experience, I took the head and cylinders off to find worn-out 11:1 pistons and, far worse, an exhaust valve guide broken in half and the valve operating directly in the head - the combustion chamber now formed part of a very large valve seat!!  The cylinders were rebored for new standard 9:1 pistons and I got a replacement second-hand head.  With those parts fitted, the smoke was gone, the bike ran well and it subsequently became my sole transport for getting to and from my RAF training camp.

 I travelled at weekends from home in NE London, around the North Circular and up the A41, about a 1.5 hour trip, generally at 60-70 mph max as I ran the bike in and got used to it, and it performed faultlessly.  One summer Friday evening, I left camp to head back home, got onto a short section of Tring by-pass dual carriageway and decided to open the bike up a bit more, so gently took it up to 80 mph.  Everything seemed fine until suddenly it felt like I had put the brakes on hard.  I pulled the clutch lever in and coasted to a halt in a panic.  I shut the engine down and let it cool for a few minutes, checked oil level, then gingerly turned the engine over and then restarted it.  All seemed fine - no noises or smoke - so I carried on home without further problems.  I had no time to look deeper into what had happened, so ran the bike for another few weeks and everything was still okay and I almost forgot about it until once again I tried a bit more speed and exactly the same thing happened.  This time, I took the cylinders off and found plenty of oil in all the right places, but the piston skirts were quite heavily scored from the two partial seizures.  I stripped the engine and when I removed crankshaft sludge trap plug (no easy task in itself, requiring a large easyout), expecting to then remove the sludge trap tube, I was instead faced with a flat, hard surface; so hard, it resisted all initial attempts to shift whatever it was. I finally resorted to a threaded bradawl to screw into the surface, which was like hardwood, and slowly was able to gouge out years of compacted, hardened sludge that filled the entire sludge trap void.  It took quite a while to be able to extract the tube.  Quite how any oil had got through that seemingly impenetrable mass to let the engine run at all, never mind cruise at 60-70 mph, amazed me.  Suffice to say that after a crank grind, cylinder hone and another set of new pistons, the fault never reoccurred.

So that was my experience - worst case, perhaps you may be experiencing something similar?

Regards, Andy 

Hi George, As said if the compressions read the same I would continue to use the bike gently for a while and check carefully the timing and mixture . Usually the  pistons rattle and you have to start again. Do you know what make pistons and the clearance used?. A little 2 stroke in the fuel may help.I use 41/2 thou on the skirts as a minimum . In theory less should be possible .but it gets chancy. Did you powdercoat the barrels?

Don't do what I did many years ago when my Dommie nipped up on one side (one of the twin carb needles was in the wrong groove). I went for what I am now sure was a completely unnecessary rebore. Alloy was smeared on one bore. I'm sure caustic soda would have shifted it. I wasted a lot of money and time.

I was surprised to read that Dave Degens used 8 thou clearance on his High comp Tritons !, Thats what I would use on a Rudge Ulster  that is well known for its ability to seize . I am in contact with a highly skilled and vastly experienced engineer who is building a Rudge  and insists his engine will be fine on half the clearance. I think he is making a big mistake ,I hope I'm wrong, I'm still willing to learn, not sure he is.