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...
There is sufficient material in the M20 barrel after the standard liner has been removed to do this without any problems..Ian 
email (option): ian@wright52.plus.com
Ian. Does this them mean you could forget the liner all together and fit an even bigger piston? 800cc perhaps
Or is the casting not good enough. Ron
email (option): ronpier@talk21.com
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)...
If you had an unlinered later barrel you could bore to about 95-96mm...Beyond that it would get uncomfortably thin at the narrowest part of the casting...
A 95mm bore with an M21 stroke would make 793cc..a 96mm bore 810cc so it would be possible...
Finding a suitable piston is always the challenge..
There are some big bore motorcycle pistons that might do the job. Car pistons are usually too heavy, have larger gudgeon pins and multi cylinder engines tend to go from 4 to 6 cylinders (or 6 to 8)with smaller bores for a given capacity before hitting those piston sizes...Ian
email (option): ian@wright52.plus.com
800cc.A new challenge Ian! Ron 
email (option): ronpier@talk21.com
Ian, am trying to do this with another sidevalve bike, a Cushman, that is very underpowered, and the barrel is cast in with the engine cases, so had one bored out to my piston size, about 6mm bigger then standard,(66,7 to 73mm) and a very thin spot appeared to one side of the barrel, and was easily pierced through with a screwdriver!
So, it seems the original bore was not in the centre of the casting.
Have you done any measuring of the wall thickness?? we tried afterwards with an electronic device, but with the fins sticking out this does not work, am looking to make some kind of mechanical compass guage, with a dial attached to it, but not sure how to proceed......
I did later ground out a piece of the cylinder wall, to check thicknesses, and found it to be possible, to use the required bore, will take picture of that later.
Another question is, how far out/off can you go, compared with the the crank centre when at TDC. ?
Cheers,
Lex
email (option): welbike@welbiXX.net
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)...
If you had an unlinered later barrel you could bore to about 95-96mm...Beyond that it would get uncomfortably thin at the narrowest part of the casting...
A 95mm bore with an M21 stroke would make 793cc..a 96mm bore 810cc so it would be possible...
Finding a suitable piston is always the challenge..
There are some big bore motorcycle pistons that might do the job. Car pistons are usually too heavy, have larger gudgeon pins and multi cylinder engines tend to go from 4 to 6 cylinders (or 6 to 8)with smaller bores for a given capacity before hitting those piston sizes...Ian
No the material was exactly the same between the linered barels and the unlinered barels.
The difference was in the casting techniques which went forward in leaps & bounds during the war.
The old system required both the cylinder and the linner to bored to size.
And both the cylinder and the linner to be cast using coal and iron, both of which were in short supply.
By using different sand mixes in different places they found they could control the direction of grain growth and the size of the grains.
Thus you can get very fine grains in the bore where they are needed and still keep the mould molten enough for the fins to fill completely.
It is called "chills & shunts".
It was the same with running shafts directly on alloy.
It could not be made reliable till foundries found ways to get better control of grains size shape & orientation.
email (option): bsansw1@tpg.com.au
It will John..but the VW piston is very heavy and like most car pistons I don't really think they are suitable, or at least ideal...
Extensive rebalancing work is required to the flywheels to achieve the correct balance factor with such heavy pistons..
I ran a Triumph car piston in one of my engines years ago but didn't really like it...
A modern forged motorcycle piton at 90mm diameter comes in at virtually the same weight as the original cast M21 piston and has a superior specification, both in the alloy used and the design of the ring set...Ian
email (option): ian@wright52.plus.com
('No the material was exactly the same between the linered barels and the unlinered barels.
The difference was in the casting techniques which went forward in leaps & bounds during the war.')
Whether the difference was technique,material specification or both, in practice the result is a difference between the castings...
Boring out linered barrels frequently reveals faults such as voids and porosity which preclude their use without liners...
I have read the castings had different material specifications..That may or may not be correct...Technical information relating specifically to BSA casting practice at the time would provide a definitive answer but I don't have it...Ian
email (option): ian@wright52.plus.com
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...
There is sufficient material in the M20 barrel after the standard liner has been removed to do this without any problems..Ian
Hi Ian!
I just bought me a M20 barrel with liner today.
May I ask what liner you used for the 90mm piston.
Best regards, Anders Lundqvist, Sweden.
email (option): anders.lundkvist@swipnet,se
Hi Anders...I bought one from 'Westwood Cylinder Liners' in the UK..
I'll post the liner number here tomorrow..The information's in the workshop..Ian
email (option): ian@wright52.plus.com
The Westwood part number for the liner I used is 'WCL 28D FL'...This has 'top hat' design like the original M20 liner...
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...Ian
email (option): ian@wright52.plus.com