Socket.

It will be around the 22mm mark, get yourself a good socket set and you will never regret it. If you plan to only ever own Jap make it all metric. These plugs are a -----, very shallow so easy to slip off. You will need a extension bar as these plugs stick fast at times. One "good" slip and you may well stuff the plug making it hard to get out.
Good luck
Lyndon

Ray
As Lyndon said, but go into Supercheap and get one of those cheap sets in the red container, they will crack under a bit of stress, and you don't need an expensive set like Sidchrome , Kinchrome is a good start,

.

Hi Ray. I have found if you use a "single" hex socket, ie similar to an impact socket it will not slip off or damage the plug so easily. You can get these type of sockets from any Kinchrome tool seller. Repco is one. Bob.

Remember when reifitting the plug it only needs to be tight enough to seal, you don't need to lean on it!!!
If having trouble with it leaking replace the sealing washer, don't overtighten!!!
If it has a copper sealing washer heat the washer on the stove till it's red and let cool slowly in the air this will retemper it and allow it to seal better.

The nut requiers a 15/16 socket.

Steve

Single Hex 24mm or 15/16.

For the fill plugs on the diffs sometimes a ring spanner is better as the socket can foul on the handbrake cable causing it to come slightly off and strip the nut. Some of the nuts are alloy.

Cheers
Dave

as said 15/16
best tip is to make sure you can undo the fill plug before you drain

biggest problem you will have doing it the first time is garanteed they are done up to 50,00000000 lb/ft and there easy to round off. they dont need to be done up tightly

24mm ....do yourself a favour and get a single hex socket. Same as the impact sockets. These plugs usually bind up and round off real easy. With the above socket, it'll be a walk in the park.

Remember to remove the filler plug first. Lets the oil drain easier and nothing worse than draining the oil and finding you can't get the fill plug off to refill it.
Fab.

Like Steve says, 15/16 is a better fit than 24mm. I hold it on the plug with one hand and a light tap with a rubber mallet loosens it.

"are there alternative plugs available"
Yes!
Can I suggest you replace the non-magnetic drain plug on the gearbox with a magnetic plug. Just buy a rear diff drain plug for the same vehicle. Same thread. And buy some new aluminium washers (get them from Toyota for a dollar or two each). The magnetic plugs are more solid and I think less likely to round off.
If you wanted a different size head, the old 55series had a taller 17mm head on the same thread plug. It was worse! Rounded off very easily.

Cheers
phil

as said before, invest in a single hex socket for diffs.
You will use it for the rest of your life and curse all previous owners who havent.
If its too butchered, weld a nut on the head of the plug.

15/16 fits diffs and gearbox in a few different vehicles.

24mm and 15/16 are so close that they are essentially the same.

anyone explain to me what's meant by a 'single hex socket' Does that mean a 15/16" socket or some other thing?