Below is part of what I have in my notes on the 720 conversion. I think it is from a post by Ian some time ago. Apologies if I have got that wrong.
cheers
Anyone contemplating an oversize M20 using an M21 crank and 90mm piston has had the problem of finding an unlinered post 55 barrel for the conversion...
That isn't a problem any longer..I have just had an oversized liner fitted to a standard 'linered' M20 barrel which is large enough for use with the large piston... 'Westwood Cylinder Liners' in the UK..
There is sufficient material in the M20 barrel after the standard liner has been removed to do this without any problems..Ian
BSA used a low material spec. casting with a liner initially and then a higher spec. material without a liner later on...
I wanted to fit a 90mm piston to a 'low spec.' barrel after the original liner was removed. The question was whether the barrel had enough material to take the bigger liner required for a 90mm bore..(Thus not having to find a rarer unlinered barrel)...
The Westwood part number for the liner I used is 'WCL 28D FL'...This has 'top hat' design like the original M20 liner... https://www.westwoodcylinderliners.co.uk/products/browse-by-size/
It is slightly over length for the job so there is an option to shorten it either at the bottom, thus retaining the 'top hat'...or at the top, leaving a parallel liner...
I opted for the latter as, once fitted, the liner is trapped between the top face of the crankcase and the underside of the head so it can't go anywhere
Finding a suitable piston is always the challenge..
Semi Finished Bore: 88.10mm Less 1.00 for final boring
Outside Diameter: 93.13mm
Overall Length: 228.60mm
Flange Diameter: 97.03mm
Flange Depth: 3.05mm
With its outside diameter being only 93mm, when bored to suit a 90mm piston you're only going to have about 1.5mm of liner wall left, What I know about liners you could fit on a postage stamp hence why I started this thread but I'd have expected to need more than that, you live and learn.
