The nuts and bolts of the matter.

[Huntsman]

I have some odd bolts on the Keeble. They attached a Daimler Dart caliper to the Keeble made De Dion tube.

Definitely 20 TPI. (So not 7/16 Whitworth, not 7/16 BSF)

Appear to be just a bit smaller diameter than 7/16UNF. About .410".

A 7/16UNF bolt won't go in.

A 5/16W socket is a very tight fit on the heads.

A 5/8AF socket is a loose fit.

What are they?

[Huntsman]

Major Fallout said:

A pain in the ass!

Quite right!

Just been discussing in the office, we think maybe they are 7/16UNF class 1 - so undersized.

[Huntsman]

Thanks all. I had bought new 7/16 unf bolts thinks that what they are, but the originals have been zinc plated so I'm going to refit them.