[Thomas]

You are right, Steve, a six sided socket does the job. It's a 11/16 inch and I just got the nut loose (was pretty tight). I also found the description of a wall drive ring spanner in the internet. Very interesting stuff! Thank you very much for sharing your expertise and experience! 
Cherio, Thomas

[Greybeard]

Thats the one, Thomas   
Its got quite a long thread on it, which I think will be a whitworth thread so easy to reproduce if necessary. I cant recall the nut size just now but if it proves troublesome, try a six sided socket or a wall drive ring spanner - they are the sort with the rounded lobes that exert force on the flat of the nut rather than on the corners like a 12 point ring spanner does.
Once its out, with a bit of patience and a file you can clean up/ reduce the head size to the next smaller size of spanner.

Steve

[Thomas]

Hi Steve, than it must certainly be this one. I already realised it. It is somewhat worn (brass?) and I hesitated to force it too much.
Cheers, Thomas

[Greybeard]

I know that mine is a side valve but I would be surprised if there was much difference. My drain plug faces the rear of the bike, almost horizontal right at the back of the crankcase at its base. Its quite a large chromed nut but easy to miss unless you get right down to look underneath. It doesnt have to be taken all the way out to drain as it has a hole drilled down the centre of it and one at 90 degrees.

Steve

[Thomas]

Yes, Steve, never believe in work previously done. But what plug under the crankcase is it? I have none. I thought about the one at the crankcase side (see pic) but it is difficult to open and I wouldn't expect a slotted screw.

[Greybeard]

Just the one under the crankcase. There'll not be a huge amount of oil in there (hopefully) but it may as well be drained off rather than mixing with the clean oil. If you don't know when it was last done it could be worth taking the oil tank off altogether and cleaning it thoroughly to remove any sediment in the bottom of it.
I'm a great believer in not trusting that a job has been done properly by a previous owner

Steve

[Thomas]

Hi all,

is there any other drain plug besides that at the oil tank which has to be opened for the oil change?

Greetings, Thomas