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Discussion Starter #1
Something very weird just happened. I have just rebuilt a 929 engine rebored to 954 with JE pistons. Initial running in period was uneventful, everything seemed OK, and several short trips were made just before and just after new year. It seemed to be running perfectly.

Then I went to start it a couple of days ago when the temperature was around 5degrees C, and of course the Li-ion battery I have fitted was not too happy. It started to fire, and was about to clear its throat when suddenly there was a massive CLONK from the engine and it stopped dead. The starter motor couldn’t turn it, and when I put a 14mm spanner on the right hand end of the crankshaft I couldn’t get it past TDC on no. 1 cylinder, actually about 20degrees before. It would turn freely in the other direction, again until about 20 degrees BTDC, and it was a solid resistance, like there was a bolt or something between the piston and the cylinder head.

Took engine out, removed cylinder head – nothing. Just the valves blinking in the winter sunshine, perfectly intact. No debris, no sign of any damage whatsoever.

Turned engine over – still the mystery blockage. Stethoscope told me the clonk was coming from No 1 cylinder. No marks on cylinder bore, either above or below the piston.

Inverted the engine, removed the lower crankcase half. So now the crankshaft is running free of the primary gears. Still blocked, but when I applied a bit more force, the crankshaft lifted a little out of its bearings, the no 1 piston started moving and the crankshaft settled back down. From that moment on the blockage disappeared.

Removed the piston on No 1 cylinder, not a mark on it, and no marks on the cylinder. Nor on no 4 cylinder, or any of the cylinders for that matter. No debris to be found anywhere. Checked to see if anything on the starter ring was fouling on the crankcase- nothing.

It is without doubt the weirdest thing I have ever come across. If I was a bit more New Age I might suspect that the engine knew something I didn’t and made itself jam to avoid some terrible accident.

I have no doubt that there is a perfectly ordinary mechanical explanation, but I can’t begin to imagine what it might be. I’ll put the piston back in and re-fit the crankcase lower half and try to reproduce the fault. Unless I find it I dare not use the engine again.

So – any ideas anyone? Anyone had anything remotely like this? All ideas welcome, no matter how crazy.
 

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2,712 Posts
Strange one that @oldbastard , Hydrolocking on one cylinder (any fuel in the oil)?, maybe leave the Spark Plug out on No1 cylinder when you turn it over next time, nonsense but I'm clutching at straws.:unsure:
 

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Discussion Starter #3
Yes I thought that at first but it continued to do it with the head off:unsure:. I'm sure I'm going to kick myself when I eventually discover the cause but at the moment it's driving me NUTS.
 

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Discussion Starter #5
well I found the cause of the problem. It wasn't that one piston didn't want to go up, it was that another didn't want to go down. This cylinder was the one where I had a valve drop in because the collets weren't 100% in/place. Took a few hundred kilometers before it dropped, but the effect was devastating. I thought I could get away with putting a new piston into that cylinder after I had buffed out the dings in the cylinder wall, but now I discover that is not a workable solution. Alusil is delicate stuff, and there is a definite limit to the amount of damage it can tolerate. But above and beyond all this, my wife and I are tonight toasting my immense good fortune that this did not happen while I was out exploring the remote countryside of, say, Luxemburg.
So the block is scrap, I don't fancy shelling out for another rebore on 2 cylinders and 2 oversize pistons, Instead I'll recycle all the useable bits in an old block I have in the garage loft. Summer project. Meanwhile I have another old faithful 929 that STILL burns no oil and has good compression. That will go in shortly to keep me mobile.
"Always look on the bright side of life...🎵🎶"
 

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Discussion Starter #6
well I found the cause of the problem. It wasn't that one piston didn't want to go up, it was that another didn't want to go down. This cylinder was the one where I had a valve drop in because the collets weren't 100% in/place. Took a few hundred kilometers before it dropped, but the effect was devastating. I thought I could get away with putting a new piston into that cylinder after I had buffed out the dings in the cylinder wall, but now I discover that is not a workable solution. Alusil is delicate stuff, and there is a definite limit to the amount of damage it can tolerate. But above and beyond all this, my wife and I are tonight toasting my immense good fortune that this did not happen while I was out exploring the remote countryside of, say, Luxemburg.
So the block is scrap, I don't fancy shelling out for another rebore on 2 cylinders and 2 oversize pistons, Instead I'll recycle all the useable bits in an old block I have in the garage loft. Summer project. Meanwhile I have another old faithful 929 that STILL burns no oil and has good compression. That will go in shortly to keep me mobile.
"Always look on the bright side of life...🎵🎶"
well I found the cause of the problem. It wasn't that one piston didn't want to go up, it was that another didn't want to go down. This cylinder was the one where I had a valve drop in because the collets weren't 100% in/place. Took a few hundred kilometers before it dropped, but the effect was devastating. I thought I could get away with putting a new piston into that cylinder after I had buffed out the dings in the cylinder wall, but now I discover that is not a workable solution. Alusil is delicate stuff, and there is a definite limit to the amount of damage it can tolerate. But above and beyond all this, my wife and I are tonight toasting my immense good fortune that this did not happen while I was out exploring the remote countryside of, say, Luxemburg.
So the block is scrap, I don't fancy shelling out for another rebore on 2 cylinders and 2 oversize pistons, Instead I'll recycle all the useable bits in an old block I have in the garage loft. Summer project. Meanwhile I have another old faithful 929 that STILL burns no oil and has good compression. That will go in shortly to keep me mobile.
"Always look on the bright side of life...🎵🎶"
 

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Discussion Starter #7 (Edited)
Sorry about the repetition. I wanted to post these pics but got into a rage with my mobile. What you see at the bottom of the nearer cylinder is a ledge where the bottom of the piston has been digging away at the cylinder wall. While the engine was running there was enough force to get past the ledge, but at cranking speed and a sluggish battery, it wasn't having it and just dug in. Probably for the best it happened this way, the damage would have been terminal eventually, probably as I was out using the bike in anger.
IMG_20210126_192203.jpg
20210126_192420 (1).jpg
 

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as you say the bore is quite damaged, better to look for a second hand cylinder block!! I personally would not have put it back together with that level of marking, but everyone is entitled to their own opinion, glad you found the issue whilst trying to get it started, rather than whilst riding it!! Will make a great summer project..
 

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Discussion Starter #9
as you say the bore is quite damaged, better to look for a second hand cylinder block!! I personally would not have put it back together with that level of marking, but everyone is entitled to their own opinion, glad you found the issue whilst trying to get it started, rather than whilst riding it!! Will make a great summer project..
Well every cloud has a silver lining...the event described above forced me to look again at the pile of parts I have accumulated in my garage loft, and not only was I able to find pistons that bring the cylinder-piston clearances back into a standard range (1/4 of the way from standard to service limit) for a block which I had stopped using because it was a bit piston-slappy, but I also found some unused main bearing shells that are the right grade to complete the refresh. I only wish I could do the same for my knees.....
 
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