> Specifically for the gearbox WITH overdrive, Triumph
> originally called for 90wt gear oil, the same as in the
> non-OD gearbox and diff.
Actually, until about 1960, Laycock and Triumph specifically warned AGAINST
using 'hypoid' oils, and recommended either 30 weight or 50 weight motor oil
(depending on ambient temperature). This was documented in the owner's
manuals, the A-type service manual, and even a factory service bulletin.
Ken Gillanders once reported contacting someone that was involved at the
time to find out why the recommendation changed; and was told that Triumph
changed without the blessing of Laycock, because they were having too many
warranty thrust washer failures in the gearbox. Using gear oil didn't seem
to increase _warranty_ failures in the OD. In other words, getting the cars
past warranty without expensive repairs was more important to them than long
OD life.
> The ideal thing would be to pressure test your specific
> overdrive unit with different weights of oil.
BTDT. Everything from ATF to straight 90 weight made no appreciable
difference to the pressure, in either an A-type or J-type.
See, the thing is, they both have relief valves that regulate the pressure.
The opening from the relief valve is as large as the intake opening to the
pump, so no matter how thick the oil, the relief valve always stays in
control. (The oil flowing into the pump has only 15 psi or so to push it
in; while the relief valve has some 400 psi pushing it out.) The only
possible exception to this would be if an errant rebuilder has tried to shim
up the pressure so much that the springs run into coil bind before the
relief port is fully uncovered (also BTDT). Or if dirt/grit gets caught in
the relief valve (ditto).
> The A-type runs
> somewhere between 450 and 500 psi, if memory serves.
Depends on the application, and which style of A-type. The later non-IRS TR
ones ran lower pressure.
> Whatever
> you use, just avoid any GL5 formulas. Those have sulfates in
> them that can attack and eventually damage the brass or
> bronze parts in the gearbox.
More correctly, _some_ of them have sulfur-based additives that can attack
"yellow metal" parts. Even many GL4 oils use that additive; it's what gives
used gear oil that distinctive "rotten eggs" odor. The warning against GL5
is because _some_ GL5 oils use more of it in order to meet the GL5
specification.
Randall
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