Gearbox drain plug

Correct.

Easier to remove than the drain plug. Inspect and if necessary polish sloping faces of plunger. Water gathers in old standing gearbox oil and the plunger housing picks it all up. Gear action massively improved if plunger not corroded or scored. 

Hi Peter, Sounds like a good preventative maintenance idea. Not done it before, so can you indicate where the Index Plunger is on Ian's photo please, is it the larger nut to the right of the gearbox drain plug? Anything to beware of when removing? Regards, Nick

Hi Ian, as you say, the Index Plunger is on the right of the small drain plug narrowed in Ian's photo.
ONLY on the later post 1963 lightweight gearboxes. The index plunger on earlier gearboxes is in a different position.
Nothing to beware of when removing except spring can propel plunger across the garage. Don't worry unduly about aligning the chisel head plunger on reassembly. It settles in the selector cam. Bit of goo on the thread of index plunger housing keeps the oil in the gearbox. 

Thank you Peter, mine's an early one, where's the index plunger located? Regards, Nick

Getting a bit off subject heading here. On earlier lightweight gearbox the indexing plunger housing is located behind the gearbox sprocket in the face where the mainshaft and sleeve gear emerge. Not something you would want to disturb. P

Agreed! Back to gearbox drain plug then, whereabouts is it on early models? I've had a look under mine and can't see anything obvious, mind you my knees were giving me some jip so I wasn't down there long...

Its round the side - facing the Primary Chaincase
See attached picture

Ian, 
The head of the drain plug is quite short. It sits in a counterbore very close to the crankcase joining flange. You need a box spanner or thin wall socket to remove the drain plug (if it is secure). Very little clearance between hex head and crankcase flange. 
Your photo shows the damage from previous attacks and illustrates my point well. 
Peter