Please let us know if you find anything cast in. Many others have
looked and not found anything.
However, one of the 1958 service bulletins that Michael Porter, Dan
Buettner and others have conspired to make readily available has the
answer, which is :
It is unlikely that the brake disc will ever require attention, but
should it ever suffer damage or be excessively scored, Messrs. Girling
Limited and ourselves recommend that it be replaced with a new disc.
If, however, replacement of the disc is not possible and regrinding is
contemplated, great care should be exercised, as incorrect grinding can
seriously mar the efficiency of the brake. It is a skilled operation and
should only be undertaken by competent engineers.
The machining limits shown below should not be exceeded under any
circumstances. The maximum that can be removed from either side of the
disc is .025” (.63 mm) and, as illustrated overleaf, measurement “A”
should not be LESS than 1.475” (37.46 mm) and measurement “B” should not
be more than 1.025” (26 mm).
This gives 0.450" as the minimum thickness (assuming both the grinding
and the wear are perfectly symmetrical). I measured a disc in the parts
bin that I thought was nearly new, it only miked at 0.463".
See ftp://ftp.thelittlemacshop.com/1958bul.pdf for the full text and a
diagram showing not only the relevant measurements, but a caution to
radius the grinding into the 'top hat' portion of the disc.
I've watched the local auto parts house turn rotors. They sure don't
impress me as "competent engineers" <g>
> I've never looked on a Triumph rotor but I'm going to look.