M20 engine seized when cooling

Information relating to the Matchless G9 or AJS Model 20 500cc twin
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M20 engine seized when cooling

Postby EgLarsen » Fri Mar 31, 2017 3:45 pm

Got my M20 1957 last autumn and had a few rides before Winter. I fitted new gaskets in the top end and tigthened all accesable bolts and nuts.
Last weekend I took her out for this years first ride.
After the ride I had to tigthen a few nuts - and now comes the problem. The kickstarter was very heavy to tread Down. And no start. I left the bike to cool Down, but then it was seized as if the engine had been filled with rubber.
Now the primary chain is off, the top is off and the oil pump is off - but I can only turn the engine using all my weigth.
No strange sounds - just seized.
Does anyone have an idea of what is wrong, or do I have to use the springtime dismantling the whole engine to check crankshaft bearings and so on.

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Re: M20 engine seized when cooling

Postby Mick D » Fri Mar 31, 2017 4:25 pm

Hi

Try removing the barrels to diagnose a potential partially seized piston.

Regards Mick

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Re: M20 engine seized when cooling

Postby 1608 » Fri Mar 31, 2017 4:29 pm

You will need to remove the barrels at least. Having got as far as you have with stripping I can only assume a seized piston. Has the bike been rebuilt recently? I would check the clearances of the pistons, rings and bore. I presume no oil tap fitted and that oil is circulating ie. are the bores dry.
Looks like Mick beat me to it.

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Re: M20 engine seized when cooling

Postby EgLarsen » Fri Mar 31, 2017 8:02 pm

Hello Again
Thanks for the suggestions but I have the barrels off - oily and OK. But the engine still seized and almost impossibly to turn.
Other good advices to get me running again :)

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Re: M20 engine seized when cooling

Postby Rob Harknett » Fri Mar 31, 2017 8:22 pm

Drive shaft bearings seized ??? I assume you have the primary chain off, to prove its engine not gearbox.

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Re: M20 engine seized when cooling

Postby EgLarsen » Fri Mar 31, 2017 8:32 pm

Yes Rob - that was my first attempt to spot where the failure is - its the engine.
I think it is a bearing somewhere that is seized: it is my first try with British bikes - - my daily bike is a Yam SR500... just kick and go (OK sometimes kick, kick, kick....and go)

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Re: M20 engine seized when cooling

Postby Rob Harknett » Fri Mar 31, 2017 8:46 pm

Just noticed you did say you removed the chain. Some one will come along that's had the same trouble and advise. I had something similar happen, after a few weeks winter standing idle in the shed. My engine sounded like it had gravel in it, when turned over. Main bearings had gone rusty. Have you drained the sump or found any odd colour oil? rust, alloy, bronze colours.

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Re: M20 engine seized when cooling

Postby EgLarsen » Fri Mar 31, 2017 8:54 pm

Hello Rob.
No strange sounds and no discoloured oil. But what did you do with the rusty bearings- just let the motor run and afterwards change the oil ? or was it the big overhaul.

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Re: M20 engine seized when cooling

Postby clive » Fri Mar 31, 2017 9:10 pm

Surprised no one has suggested that the Magneto may have seized. Have seen a single where the engine cannot be moved because the mag had seized (due to shellacing?). presumably you have not removed the mag from what you have said. You will need a special puller tool for the mag sprocket but worth trying before you disassemble any more. I guess the same holds for the dynamo but I have never heard of one seizing the engine, they simply break up and sound awful!
clive
if it ain't broke don't fix

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Re: M20 engine seized when cooling

Postby dave16mct » Fri Mar 31, 2017 10:16 pm

That's a good suggestion Clive. Easy to check as well. Just pull the magneto gear off and you should be able to spin it in your fingers. Here's hoping!
Dave.

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