[TR] Transmission Locating Dowels

[MMoore8425 at aol.com]

In a message dated 3/22/2008 4:21:34 P.M. Pacific Daylight Time,  
tr3driver at ca.rr.com writes:

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
_______________________________________________




Randall,
I was just a little bit involved with this issue with Herman when he   
developed his conversion kit. 
He spent a lot of time and money to make sure those dowel pin holes are  
located accurately relative to the center of the crankshaft. The attachment  holes 
for the bellhousing to the engine block are large diameter and relatively  
loose tolerance holes. I say "relative" compared to the dowelpin holes. His  
design, like  the factory design depends on the tight fitting dowel pin  holes to 
properly align the transmission. A likely problem will be clutch  failures if 
it is not accurately aligned.
 
Mike Moore 



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