In article ,
Werehatrack wrote:
On Sat, 23 Sep 2006 10:51:51 GMT, wrote:

Presta main - m6-0.75
Presta cap - m5-1.0

Both of those are remarkably uncommon sizes. Good luck finding a tap
or die in those thread and diameter combinations outside of a
specialty tool supplier.

I don't believe these threads are correct. The presta valves
I've measured have inch-based thread pitches - 32 and 24 tpi
respectively.
But in this case, I think the OP was looking for neither of
these, but rather the internal thread in a removable-core valve stem.
That, I haven't measured.

However, I think it's a moot point: there's more going on inside
the valve stem than just that thread - the seating area that the valve
core seals against needs to be shaped appropriately too, and it seems
like more trouble than its worth to try to reproduce it all.

Were it me, I'd probably just slather some epoxy in the stem
where the crack is, and permanently glue an extension in place, letting
the epoxy seal whatever crack is there.

-Luns