Steve,
I have been using the overdrive units fitted to the TR-3/4 gear boxes for
about 3 1/2 decades. I am not sure if this information applies to GT-6
overdrives. Someone from British Leyland (Michael Barrette?) connected with
Group 44 at the time gave me the following instructions back in the 70's:
1) Remove four of the eight springs (the inner ones).
2) Shim the accumulator spring to increase the operating pressure from 420
psi to 650 psi. They started with an 0.080" thick shop washer and then
worked with shim washers. I bought a 1000 psi gage and started making up an
adapter for testing the pressure, but never got to the stage of testing this
pressure while running the car on jack stands (maybe there are better ways of
testing). I just installed the 0.080" washer and went with it.
3) Drill out the operating valve hole from 0.042" to 1/16" (0.0625").
He also encouraged me to forget about the OD unit and use a 4:11 rear end
ratio with a straight 4-speed gear box. They didn't tell me at the time
about the close ratio gear boxes that they were using.
Periodically, my OD unit would quit working. I would take it apart and find
nothing wrong, reassemble it, and it would start working again. Since I
started using Ams Oil 80W-90 gear lube, I have had no trouble. I am sure
that better synthetic oils are now on the market.
Good luck,
Bill Emery
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