They are probably British Whitworth thread (BSF). There are no metric or SAE equivalents, so you will have to find a donor stem or replace both nut and bolt.
Steve Barner, Bolton, Vermont
> Date: Tue, 6 Jul 2004 16:32:21 -0700 (PDT)
> From: r cielec <teaat4p@yahoo.com>
> To: classicrendezvous@bikelist.org
> Subject: [CR]GB Hardware: Metric vs. BSF?
>
> Ahoy !
>
> Again bent low to the ground by weight of ignorance.
> RE: 1970's era GB stems; nut & bolt for clamp
>
> Metric fine pitch 1.00 did not fully thread in nor on. So, don't know if
threads as bad as they look or, GB hardware is not this metric size.
>
> Any one know for certain?
>
> Purpose of this is so I can go hunting for replacement nuts and bolts.
>
> You betcha - if I find SAE pair to fit, I'll use it.
>
> Regards,
>
> Richard Cielec
> Chicago, Illinois