Quote Originally Posted by keith66 View Post
BSA Airsporter & Mercury share the same cylinder threads as their prewar ancestors so breech blocks interchange. Think the Webley Mk3 is the same size.
I found that the Airsporter plug was the same pitch thread but a slightly smaller OD.

I'll go and double check as I have the cylinders of a BSA underlever and Webley MK3 to hand.




All the best Mick