Gearbox drain plug

[Phil C]

The gearbox drain plug on my 1965 B40F seems to have a very thin hexagon, doesn't it?  (please see photo.)  Or are they like that?   Phil.

[AWJDThumper]

It doesn't need to be any thicker especially since it's screwing into aluminium. The same with the actual drain plug bolt.

[Phil C]

Fair enough, thanks AWJDT.   Phil.

[Phil C]

I'm having difficulty getting a good purchase on the hexagon to loosen it.  A socket is difficult to get on because the casing casting is in the way on one side. So I'm thinking an open-ended spanner, but am a bit worried about damaging the hex, which is not in great condition anyway, and, as I say, is very slim. Does anyone know what size it is (assuming it's original)? It appears to be something like 13/16" a/f. And what is the best tool for undoing it?  Thanks.  Phil.

[AWJDThumper]

Quick answer which might be wrong but 1/2" BS (F or W) and not AF.

[Phil C]

Measuring across the flats it might be 7/16 BSW / 1/2" BSF, so I'll need to get a suitable spanner.  But getting a purchase will still be difficult, because of a) the casing casting getting in the way of a socket on one side, and not being able to get an open-ender on square. This is made worse of course by the slimnness of the hexagon, which means a spanner easily slips off. Maybe a box spanner (its wall being a bit thinner than a socket's)? Has anyone removed this drain plug recently, and if so could you tell me what you used please?   Phil.

[JulianS]

3/8 inch drive sockets can be quite useful, 1/2 inch drive often too thick for some of the spaces on our bikes.

[Phil C]

My socket ratchet is 3/8" drive, but I don't think even the right size socket, if I get one, will go on the hexagon properly at one side, because of the casing casting being in the way. I am basing this on offering up a socket (13/16" af) that nearly fits the hex. I guess I could buy the right size (BSW/F) socket and try it, but don't really want to waste the money if it won't do the job.  There doesn't appear to be room to get an open-ended spanner on squarely either, and with such a slim hex an open ender is very likely to slip off. I don't want to mess up the hex too much. Has anyone actually unscrewed the plug recently, and if so what did you use?  Thanks everyone so far.   Phil.

[AWJDThumper]

Just undone the drain plug on a C15 which should be the same. Requires a 7/16 BSW socket or spanner.

[AWJDThumper]

ps. Didn't have any difficulty fitting the socket to the drain plug but it's best to hold it firmly on the thin nut with one hand (to avoid it twisting off) while apply forced to the end of the wrench with the other. The nut is actually 1/2" BSF 16 tpi and the same size as 7/16" BSW as noted.

[Phil C]

Thanks very much AWJDT.  Regarding getting the socket on, please could you tell me: do you reckon the casing on your C15 is the same shape as on my 1965 B40F?   Phil.

[AWJDThumper]

Give me 30 mins and I'll try the socket on my B40F but I don't expect any difficulties with the socket fitting. I might even find a mirror and check the crankcases around the drain plug to see if they are identical.

[AWJDThumper]

My '67 C15 is slightly different to my '65 B40. In the case of the C15, the bottom of the drain plug casting is proud of the bottom of casting for the gearbox sump which runs horizontally from the drain plug to the right across the bottom of the gearbox. In the case of my B40, the bottom of the gearbox sump projects 1 or 2 mm below the bottom of the drain plug casting. However, the sump casting is milled away to the same level as the drain plug casting to give a clearance of the order of 5mm for the drain plug nut allowing you to get a socket on it.

[Phil C]

In that case I should be okay. Thanks very much indeed, AWJDT. I'll go ahead and buy a socket.  Phil.

[JulianS]

Looks like access to the plug could be difficult - this extract from factory issued parts bulletin G31 of March 1965 shows that  the spot facing was increased in diameter to enable a box spanner to be used to remove it.