You could be right RT, and your observation is partly why I caveated my reply.

CMSNL in Holland stock the item (and I suspect that it is still available from Yamaha as well) and therefore have a photo of it. This is attached and it did occur to me that one end might have an ever so slightly wider diameter than the other. What do others reckon?

I'll ask my dealers who did Yamahas back then if they can remember.

No, both ends will be the same size thread [7mm if i'm right--but could be wrong] except the end that goes into the case will be threaded so its fractionally tighter into the case as it doesn't need to come out again. Also the top end will be [whats the word] radius'd & the bottom end will be straight cut. Also the bottom thread will be a bit shorter. They must roll the thread rather than die cut it as the stud is fractionally smaller than the thread on all those sort of studs .

Going by the part number... my guess is 6mm... can you take a stud down to the store and see what nut threads on? That's what I'd do.

Ok, i give in & think MarkT & Gr8uncleal are probably correct. Found this by googling so copy & pasted below.

Default How to find Yamaha bolt sizes...
Every now and then someone asks what size a particular bolt is. Just thought it would be helpful to post this from another site. FYI, you can get the part# from most Yamaha dealer websites or here.

"How to get sizes of bolts from the part numbers...
The Yamaha microfiche shows the bolt size, you just have to know how to read it. Here is how it works...

The part number is broken down into 3 parts 99999-06012-00 is an example. The second part, which is 06012 tells you the size. That bolt is 6mm in diameter and 12mm in length. If the second part was 08024 the bolt would be 8mm in diameter and 24mm in length. The first part of the part number tell what type of part it is such as bolt, screw, washer etc. and last 2 digits generally determines the finish such as black, cad, etc"

Just an FYI.... in case anyone might think that all of the studs used on Yamahas were the same thread size on both ends.....here is one that I remember fighting on the exhaust flange....

Thanks DEET.

I recall reading about this part, either here or on the UK based aircooledrd forum. At least the one that you show is obviously different (M6 and M8 if I recall correctly)!