Thread Size

[DBDBrian]

Thanks all for the input, it has turned into a very interesting "Thread".
Rechecked the footrest that I am fairly sure is original, and still pretty certain it's 11 X 1.25 mm. The retaining lock nut also has a 17 mm hex, which would seem to indicate a metric thread.
The thread major o/d is 0,421", 0.012" down on 11 mm, and 0.0165 down on 7/16".

I also have some new pattern rests, which are definitely 7/16" x 20 UNF . with the lock nut being a 5/8" hex . Interestingly a standard 7/16" UNF bolt has a 5/8" hex head , but the nut is 11/16" hex.
The picture below compares the original metric rest, with a 7/16"X 20 UNF bolt, as can be seen they do not match up. I used the bolt as it has a greater length of thread than my 20 tpi pitch gauge.

11 mm thread.JPG

P1080603.JPG