[TR] Overdrive gearbox help

Randall tr3driver at ca.rr.com
Mon Feb 1 16:00:06 MST 2010


> What year gearboxes fit; do TR6 ones work or are they 
> different?

Later TR6 boxes used a J-type OD, which will involve a certain amount of
hassle to fit to a TR4A.  ISTR someone wrote they had to notch the frame
rails (as well as fabricate the rear mount) but YMMV.  But the early TR6
boxes with A-type OD should drop right in.  Be sure your locating dowels are
in place (TR6 used special locating bolts instead of dowels).  There might
possibly be an issue with the length of the 3 studs in the back of the
engine block, as the gearbox flange got thicker at some point; but ISTR that
was after the switch to J-type OD.

> What other pieces are needed to make the swap?

You'll need the angle drive for the speedo cable, and the operating switch,
relay & wiring for the OD.  Might need to cut an access panel in the side of
your transmission tunnel, if it's not already there.  The TR6 shift lever is
a little different, so you'll probably want to swap in your original lever.

> And what are the things to look for in a gearbox as far as 
> likely problems?

It's unfortunately hard to inspect them very well, without either driving it
or disassembling it.  But I'd pull the drain plug from the OD and look to
see how much crud is caught in the screens & magnets, plus pull the top
cover for a general inspection.  You can also try sliding the synchro rings
against their mating gear and check the clearance, but again it's hard to do
this accurately without disassembling further.  Can't seem to find my notes
on a minimum clearance, but maybe someone else knows.

> I got an email from a fellow who has one 
> for sale for $1,200.  I don't know if that is a good price or not.

Depends on how much of a guarantee it comes with, IMO.  If it's guaranteed
good, that's a great price.  But if it's strictly "cash & carry", I'd be
leery.  It's easy to drop over $1000 on repair parts to overhaul one; and
you can get a fully overhauled, guaranteed box from Quantum Mechanics for
$2500 or so.

-- Randall  


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