Draining/Refilling O/D Gearbox

[USSprinter]

As I understand it, the total capacity (when dry) of gearbox plus overdrive on my Sprint is 1.9 litres, and that the O/D shares the gear oil with the gearbox. I have questions on actual volumes when draining and refilling as a service operation.

1. Upon removing the gearbox drain plug, how much of the total oil capacity finally drains out? (assuming the car is level.)
2. Is there much of a lag as the O/D oil moves towards the exits, prolonging the time to drain?
3. Is there a significant amount of oil remaining undrained in the O/D unit?
4. In refilling the gearbox, does it take any significant time for the new oil introduced into the gearbox to also refill the O/D unit?

I guess my fundamental question is when do I know the I have totally filled both the gearbox and the O/D? My concern is being "fooled" into thinking it is filled by having oil seep out the fill point, while the O/D unit is still taking on fresh oil.

It should be obvious that this will be my first time undertaking this task.....

[cliftyhanger]

Can't help with the volumes, but would suggest removing the filters from the overdrive. I think one is "difficult" but often quite clogged. Item 37 in this diagram
https://mossmotors.com/j-type-overdrive-1974-80

That should help drain the overdrive too.

[Carledo]

You'll probably get "most" of the oil out by draining the main plug, a bit more by dropping the overdrive filters. Cleaning these in the process is worthwhile in it's own right.

On filling, I always fill full at whatever speed I can manage (I have an old Castrol rotary hand pump to speed the process) then leave it to stand for half an hour and top up again. To be fair I seldom get more than a couple of teaspoonfuls extra in on the top-up but it's sort of a "better safe than sorry" habit. If I didn't do it, i'd have that little nagging voice in my head.

If you've left the tunnel in and are reliant on squirty bottles to fill the box from below, it's probably safe to assume that you can't fill the plughole as fast as the oil can "flow out" inside the overdrive.

I too have no idea of the actual capacity in the "real world", when it starts coming out of the hole, it's full!

Don't forget this box ABSOLUTELY requires a GL4 rated oil.

Steve

[xvivalve]

...and DON'T be tempted to put any friction reducing additives into the oil, the overdrive relies on friction to work.

[USSprinter]

Gentlemen,
Many thanks for all your most helpful responses!
Mike