Quote Originally Posted by T 20 View Post
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.
The Airsporter threads are definitely the same pitch but the plug is about 0.22 mm smaller in diameter than the pre War underlever threads.

The Airsporter back block wouldn't screw into the back block of my Webley MK3 cylinder as the threads didn't match --- the Webley threads being finer are probably some weird German thread form.




All the best Mick