Scooterotica

OK, my apologies, I thought you'd made a typo. Yes that is M18 by the look of it a bit odd to say the least. If you want to use it then I'd suggest cleaning the threads up, check the thread pitch by offering up the M16 to the M18 threads (if they fit together then the M18 thread pitch is 1.5mm), buy a new nut BUT be careful M16 x 1.5 is the original nut and this is a "fine pitch". M18 comes in three different pitches standard is 2.5mm and there are two types of fine pitch 2mm and 1.5mm pitches. Needless to say get the right one to suit your axle. Then fabricate a locking washer.

OR, pull it to pieces and fit the right type and save the M18 version for emergencies (once you have matching nut and washer etc!)

Thanks Rich & Yanker. Just been in garage measuring. Now i suppose i could get a locking washer fabricated, but would have to drill and tap for the locking screw. Just tried searcing for a m18 nut that has same external size as m16, but cant find one, unless anyone knows better. Looks like i may replace shaft. Trouble is, its getting tedious as this will be 3rd time ive been into the depths of this engine whilst its sat on the bench.