[TR] Replacing gearbox cover type-A overdrive

[Randall]

> although if the smaller pistons
> were fitted the resulting unit could be marginal under full torque.

Then I guess the question becomes "Why implement OD in 2nd if you can't use
it?"  2nd OD is kind of problematic anyway, especially if your engine was
rebuilt with 86 or 87 mm liners.  And while the difference between 1-1/8 and
1-3/8 doesn't sound like much, it means 50% more force applied to the OD
clutch.

Personally, I wouldn't want to make the conversion unless I could confirm
that the unit has the larger pistons.

> > I can buy a TR3A top cover assembly cheaply, and fit it onto my
> gearbox,

If you do, be sure to get an early one with the dipstick.  Otherwise, you'll
have no way to add or check the oil level; as the early gearboxes lack the
plug on the side found on later ones.  Or, use the components from the 3A
top cover to convert your existing cover.

> And, what might "marginal" mean,
> knowing the English love of understatement? Will the gearbox explode,
> or just slip out of OD?

Not "slip out" exactly, just slip.  If you pay attention and always back off
the throttle immediately when it starts to slip, it might be OK.  But while
it is slipping, the clutch surface will wear rapidly and eventually lead to
another expensive rebuild.

Either way, IMO you should check the hydraulic pressure in the OD.  I don't
have the information handy, but ISTR that the early units with the small
pistons ran more pressure than the later ones did; so you'll also need to
research the correct pressure.

-- Randall