Hi Bob,
40 tpi is a typical model engineering thread used world-wide.
Example - in live steam engines for any gauge up to 12 or 15 inches, it is used for olive/nut/flange pipe fittings and can be applied to any spanner size that suits.
Take a look at Reeves of near Birmingham https://www.ajreeves.com/6525.html
Descriptions like this abound - BLOWER VALVE 1/4" x 40 where 1/4" is the stud or bolt diameter and 40 is the thread pitch.

Also I wonder if it might be one of those weird hybrid fastenings that proliferated at that time.
Example - you could get British Association (BA) fasteners with metric threads and standard BSW hex bolt heads.
Example - BA 4 and BA 5 are marginally either side of 40 threads per inch.

I purchased a British 1948 model MG sports car, and it was completely BSF (British Standard Fine) bolt heads with metric threads.
I cant remember the pitch, the bolts were probably manufactured in Germany.

Kimball

Edited by user 08 December 2023 02:22:12(UTC)  | Reason: Corrected dimensional description

Thanks a lot for your comments and thoughts, guys. What I really wanted to know is where one can find those Märklin nuts, with internal thread of 40 TPI. I tried both BSW1/8 and UNF Nr6, both with 40TPI internal gauge, but none of the fit the diameter of the bolt (is true for the valve shown below, but also for the fixing screws in the machine).



Another question is how to dismantle the water level inspection glass safely. It is tightly fixed apparently... cannot move it out.

Bob

Hi Colin
Thank you so much for your correction.
I have edited the text in my post.

Regards
Kimball