Does anyone know what the Cylinder Pressure is on a Mk I or Mk II lemans fitted with a Big Bore 950 cc Cylinders & Pistons. A standard 850 lemans would be between 170 PSI and 180 PSI
The big bore should increase compression ratio by 11% (increase in swept volume + no increase in unswept) so i would be looking for a best of 180 to 200 psi. But I would be happy with 170 as the increase in pressure will cause increase in ring leakage at the speed you are turning it over.
Thank you for the information , very interesting. I am viewing a bike fitted with a 950 big bore kit , however the pressure is 140 PSI both cylinders, which I believe to be slightly low for a lemans. I appreciate you confirming my suspicions, at best leaking valves, worn rings or bores. This will be reflected in the purchase price, should I decided to go ahead.
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With that PSI and it starts sounds ok.
Most important that it is similar on both sides.
Problems that need sorting NOW like burnt valves show on one side more than the other.
Have fun with it!
Have you tried squirting a bit of oil down the plug holes?
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As suggested, a bit of oil down the bores and see if the pressure comes up. I would almost certainly be looking at harden seats, replace the guides and exhaust valves.
The big bore kit will probably have the lower compression pistons fitted as used in the T3’s G5, SP1000 etc. I’m not aware that a high compression 950 round barrel kit has ever been made.
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Thanks Don Spada useful.
That explains why they came out so quickly in the Le Mans life and why they had more grunt but less ‘pep’ than the 850
There was a 950cc Le Mans in the States, the CX100. BUT this was basically a G5/SP engine with down rated carbs and not the 850 Le Mans large valve heads. I believe MG had a wobble about the emissions of the 850 Le Mans
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A high compression 950 kit was available, I have one fitted to my Le Mans engined Cali. I can’t remember who made it, I’ll have a trawl through my records.
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All I can find is a sales receipt for the conversion kit specifying 10-1 pistons. I believe some VW airhead pistons will fit and Crosthwaite & Gardiner sell an 88mm 10-1 piston which looks very similar to mine, might be worth a word.
Well I’ve finally got round to doing a compression test on my high compression 950 pistons. Thanks to you lot I now have a winter project. Both were only just over 100 psi. I have other bikes and my T3 has fallen to the back of the queue over the years. I’ve had suspicions about its lack of performance and the fumes at tickover but it was always fun to ride so I put up with it. I have three piston and barrel kits of various compressions and over the past 30+ years, depending on my mood, I’ve swapped them around. They are all on the original rings but I haven’t worked out the mileages and also, true to the original spec Le Mans, I run bell mouths without air filters. So, time to investigate.
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Did you do the test with the carbs fully open? It can make quite a difference.
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………no, and I’m in a pub with friends who just suggested the same thing. Brain fade is becoming a real issue……
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Test redone, cold engine with throttle full open, 150 and 155 psi. Phew!! I’ll do another tomorrow with hot engine.
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Ok, Le Mans 950, 88mm, 10-1 pistons, unknown mileage, hot engine, throttle open - left 175 psi, right 186 psi. I’m happy with that, any performance issues I’ve got may be carb related after all.
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