Lightweight engine re-assembly problem

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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Phamiltoneddy
Posts: 22
Joined: Thu Aug 28, 2008 11:18 am
Location: GLOS UK

Lightweight engine re-assembly problem

Post by Phamiltoneddy »

I've been overhauling the 1965 AJS Model 14CSR engine fitted to my recently acquired 1966 Matchless G2CSR to investigate the presence of oil in the combustion chamber and high oil consumption. After running the bike for 800 miles or so,I stripped the engine completely except that I did not separate the crankcases as the bottom end was not giving cause for concern. The piston had seized at some time in the past and I assumed this to be the cause of the problem. I bought a + 20 thou GPM piston from JSL and gave both barrel and piston to a specialist re-bore company to carry out the rebore.
I am now re-assembling the engine. Before fitting the barrel, I checked that the crankshaft could be turned freely by twisting the driveside mainshaft for a dozen or more rotations. I then fitted the piston and barrel and repeated the check. The piston stuck at the bottom of the stroke with a slight “clonk”and firm hand pressure on the mainshaft failed to carry the piston beyond bottom dead center although reversing direction brings it back to TDC.
I suspected that there might be a problem with the rebore so I removed the barrel and piston, fitted the piston to a makeshift con rod and, with the barrel secured on the workbench, inserted the piston in the barrel and moved the piston up and down over its full stroke. The piston showed no inclination to stick.
Next I fitted the barrel to the crankcase without the piston fitted to the conrod. The crank turned freely so the problem seems not to lie with the barrel
Finally, I wondered whether the skirt of the new GPM piston might be fouling the flywheels. I compared the new piston with the old and concluded that the GPM piston was perhaps 10 thou longer in skirt length. So I removed about that amount of metal from the skirt and re-fitted barrel and piston. Alas, the piston still sticks at the bottom of the stroke. I've now run out of ideas. Has anyone out there any suggestions?
itma
Posts: 7721
Joined: Tue Nov 22, 2005 6:01 pm
Location: UK

Lightweight engine re-assembly problem

Post by itma »

yes, you are exactly on the right track, these bloody pistons are too long, I had to remove near a 1/4 inch to remedy the same problem; also The one I had, had also the valve recesses reversed, as if it was a mirror image of the old one.
check it and remove metal as necessary.

the thread about it is here

http://www.ajs-matchless.com/topic.asp? ... ral+Topics

the photobucket pics have been deleted thoughEdited by - itma on 01 Jul 2010 6:07:39 PM
Phamiltoneddy
Posts: 22
Joined: Thu Aug 28, 2008 11:18 am
Location: GLOS UK

Lightweight engine re-assembly problem

Post by Phamiltoneddy »

Thanks for your comments, itma. Much appreciated. The valve recesses on my piston look OK so I'm hoping all will be well once I've had the piston skirt shortened by the appropriate amount. This will also have the beneficial effect of lightening the piston. The GPM piston is significantly heavier than the one it is replacing.
itma
Posts: 7721
Joined: Tue Nov 22, 2005 6:01 pm
Location: UK

Lightweight engine re-assembly problem

Post by itma »

valve recesses are skewed off centre so make sure yours are on the correct side for the valves.
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