Oil tank filter nut

Year of bike ?

A 71 Roadster takes the later oil tank. Two M26 Dowty washers will replace the copper or aluminium washers and provide an oil tight seal.

A plain 1 & 1/8" AF socket.

Download this PDF, get a cheap digital caliper and you will never need to ask for the spanner size of a fitting again, plus be able to see the close enough sizes that could work if the exact spanner size is not avaiable.

https://www.crawford-space.co.uk/old_psc/spanner-jaws.pdf

Combine that with a Thread gauge for £8 to £10 and you even get to see what abominations the Dump previous owner has inflicted on your bike with retapping to different threads.