850 le mans, 950 conversion

Hi all, i’m considering doing a 950 conversion on my le mans 2, partly because you cant seem to get new pistons or cylinders any more in 83mm (850cc), I was wandering if you have to widen the aperture in the crank case to allow the cylinder to pass through it, Gutsibits cant be sure and Moto-mecca seems to think the 950 cylinders will fit straight in without any modification, but I would prefer to hear from someone that has done this conversion already.
many thanks Chris

Think you will need to get the crankcase skimmed if you are starting with an 850 to make the bigger barrels fit.

i was told the modern plated 940 barrels fit straight in the same hole as the 850 cast ones will try and measure some for you as i have some at work i think
and you can get the 940 ones with high compression pistons

and when you do it i will be interested in the worn out 850 parts

The cylinders in my le mans are cast and i’m going to get them re-measured on Friday as they had new liners fitted 20 miles ago and I think they may have been over bored as I suspect piston slap, The left cylinder has scores caused from the gap in each piston ring, however I am very suspicious that the piston rings maybe the cause of the piston slap problem due to lack of support to the piston, as the rings I used were from Gutsibits and were meant for cast and nickasil cylinders.
when I rebuilt my other lemans at the same time with the same company re-sleeving the cylinders, I fitted piston rings from motomecca which were for cast barrels only! and they were a lot stiffer and a pig to fit as apposed to the ones from Gutsibits that seem to have no spring to them and fitted in with my fingers with ease. The le mans with the cast only rings fitted runs perfectly and sounds sweet as a nut!.
As with the 940 cylinders you mentioned, I thought you could only get 950 cylinders?.
Cheers Chris.

Wouldn’t have thought rings would be the underlying problem with piston slap ?.

Sounds like too much piston to bore clearance to me ,( if it is slap)? .Â

that’s what I will find out Friday with any luck! :confused:

these are what i was thinking of and they also do a high compresion version

Thanks very much for the info, that’s exactly what I was after, if all fails on Friday these will be just the ticket!

Any updates ?

My neighbour has just had Nigel at NBS do the same job to the Mk111 he has bought off me.
He is planning an article in Gambo around the rebuild.
So far looks good.

That is going to be a nice motorcycle when its finished
Nigel does a lovely job
be nice to see it being used again
will you blub and get all tear stained when you se it being ridden again ??

No.
I have been told I can take it for a run.

i have just aquablasted a complete big bore mk 111 for nigel



Did you notice a broken sump fin on the rear lhs.
If so you will probably be getting the correct unbroken sump to do.

Not sure