Please forgive my ignorance...

I've been looking for a post-war unlinered cylinder barrel for my 720cc build for quite awhile now without any success, so I've been thinking what would happen if I remove the standard liner then overbore the barrel to take a liner suitable for the 90mm piston I wish to fit ?

Am I better having a thicker cylinder wall and thinner liner wall ?

i.e.
Could I bore the cylinder to 92mm, fit the liner and then bore that out to 90mm leaving only 1mm of liner wall thickness ?

Or is it better to remove more of the cylinder and risk breaking through to the fins so I can have a thicker liner wall ?

Assuming I'm going to break through the cylinder wall to the fins would I be better adding weld to the barrel where it's thinnest before boring for the liner or just weld up the damage afterwards ?

thanks

email (option): bsa@wooky.plus.com

You might find once you press the liner out, the casting may clean at 90mm, you then only have to make a ring to fit the old line flange.

Remember the barrel is held at the bottom, the more you bore it out the better chance you have of loosing your head! ;)

Mark

email (option): pes.sales@btconnect.com

I thought the barrels were fit with a liner because the casting was of inferior quality and only post-war castings were better and were used without a liner ?

I'm happy to be corrected though.

email (option): bsa@wooky.plus.com

What I have been told:

Some cylinders with the liner removed can be bored out to 90mm and work fine. Some can not due to the poor quality of the cylinder casting. As there is no shortage of the early liner version of the cylinder, I would cross my fingers and bore it. If it does not work, find another cylinder and try again.

On my 720cc bike:

After a long time of looking for the late cylinder with no liner, I had to buy a complete 1953 M20 motor to get the cylinder. I never could find just the cylinder. Had I not found the 1953 motor, I was gonna bore out the one on my bike.

Thanks for the advice, if I've not found a unlinered barrel by the time the bike is ready for it's engine I'll bore out a linered barrel and keep my fingers crossed.

email (option): bsa@wooky.plus.com

The problem you might encounter is rapid cylinder wear, as well as porosity...If you do get rapid wear the material that is worn off has to go somewhere and there has to be a risk of damage to the thrust surfaces of the (expensive) piston...I would carefully check the diameters of the standard barrel and see if you can get a liner in....Don't forget the barrel is supported by the substantial tappet chest as well as being held down at five points.(and theres a head steady)..I'm about to go away for a couple of weeks so won't have time to look at a barrel myself...It's something I've frequently thought to look at though...:face_with_rolling_eyes: ....Ian

email (option): ian@wright52.plus.com

Hi Ian

You're probably the most suitable person to answer my questions since you started the whole 720cc conversion.

Would boring out the barrel as much as possible without breaking through (say to 94/96mm diameter) and then fitting a liner and then boring out the liner to the required 90mm be a good idea ?

Looking on westwood cylinder liners website the wall thickness of their liners ranges from 2mm to 5mm, so I'm thinking my idea of ending with only 2mm of liner wall remaining might not be so daft.

I'd rather refit a liner to a previously linered barrel than run it without if I can, failing that the search for a post war unlinered barrel continues....

email (option): bsa@wooky.plus.com

I'd have to run some measurements on a linered barrel to confirm whether it is possible...I'll do it when I get home after my impending trip...Ian

email (option): ian@wright52.plus.com

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..

email (option): petercomley@web.de

Just looked up the spec of that liner...

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.

email (option): bsa@wooky.plus.com

Have you changed your mind about coming to Normandy?