[TR] Transmission Locating Dowels

[Randall]

> As to your drill rod suggestion, how does a 3/8" drill rod 
> differ from a 3/8" bolts shank? Is drill rod fractionally 
> larger or is a bolt shank thinner?

Yes, exactly.  If you take a micrometer to a 3/8" bolt, you will find that
the shank is always somewhat smaller than .375".  It actually varies from
bolt to bolt, but .368" is probably typical.  Frequently they are even
several thousandths out of round.  There are various reasons for this, but
basically it doesn't hurt anything (in 99.9% of all applications), and makes
the bolts much easier to manufacture (meaning cheaper) as well as to
install.  But, being .007" undersize means the gearbox can be out of line by
as much as .014" (plus whatever other tolerances stack up), which is
definitely enough to be a problem.

However, the drill rod is ground to a precise diameter.  The rod I used is
specified as being between .3745" and .3755".  This isn't quite as good as
an actual alignment dowel (3/8" nominal is .3751" - .3753"), but I believe
it's "good enough".

I actually tried to start with a long alignment dowel, but they are
delivered already hardened and my tooling just wasn't up to cutting threads
in a RC40 surface.  The drill rod comes annealed, so is much easier to cut
(and IMO strong/hard enough to use as-is, without further hardening).

And since that Stag had already broken it's equivalent of the rear plate
(it's an alloy casting on a Stag) without the dowel bolts, I feel that
Triumph/BL had a good reason for switching from a standard alignment dowel
to a custom (and expensive) "dowel bolt".  Perhaps it's less critical on a
TR6 with a steel rear plate, I don't know.  But I've also had the
bellhousing flange break (on a TR3A) and I know the factory took a lot of
other steps to strengthen this area, so ...

Randall