Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
Max bore size for a unlinered barrel

I have a couple of +060 barrels for a M20 one of which is ideal for oversize bore to fit a B33 piston for my M21.My question is how far can I bore out a barrel fitted with a liner ? The B33 piston I have is a +020 , I cant seem to find a unlinered barrel... Dave

Re: Max bore size for a unlinered barrel

Dave I'm planning to do the same thing one day (just for fun). My M20 already has an M21 crank and I've acquired a new B33 standard piston.

I can't remember if my barrel has a liner or not till I take it apart, so I was wondering the same thing and been doing some calculations in anticipation.

I have a new liner on the shelf which measures 3.5" (88.9mm) OD. My new piston measures 3.343" at the widest part of the skirt plus say 005" for skirt clearance = 3.348". Deduct this from the OD of my liner =.152" now halve this to give remaining wall thickness = .076" (1.93mm)

Adding your extra 020", I calculate that you should have .066" (1.67mm) wall thickness of a standard liner......Maybe you could double check my maths?

Ian says he's seen liners bored out successfully down to 1mm wall thickness. Ron

email (option):

Re: Max bore size for a unlinered barrel

Thanks for reply Ron,

My WM20 has a 85mm bore running a unknown hepolite piston in a linered barrel been like that for last 8000 miles so guess another 20 thou would not hurt.Never seen a unlinered barrel must be fairly rare? Dave

Re: Max bore size for a unlinered barrel

Haven't seen one either.
Is it possible to bore the liner out completely and use the cilinder as such ?

Re: Max bore size for a unlinered barrel

It would be a real gamble...BSA used a relatively poor quality iron casting with a liner fitted...

Post 51 (from memory) they improved the barrel casting quality and ran without the liner...An unlinered barrel will run a piston of 90mm without any problems so if you have a linered barrel just get an oversize liner fitted to run an 86/87mm piston...

Westwood liners will manufacture non standard liners if required but they also carry a huge stock for different engines...It's worth giving them a ring once you have worked out the dimensions required and they will try to match it from stock...Ian

email (option):

Nieuwe pagina 1