yeah to matters worse with thos exhaust studs it was the 6mm end that screwed into the cylinder so ya swinging on an 8mm nut tightening the exhaust then 'ping'--it breaks off flush with the cylinder. Might've run that up to DT360A as well but remember it on smaller motor exhaust headers, Used to tap the cylinder flange thread to 8mm if we had one break off. Makes you wonder what their plan was with that idea & maybe everyone was on holiday so the apprentice designed it.

Right, just been out to check my AS3 engine again (whole bike was bought dismantled, so engine currently on the bench) and, as luck would have it, the studs are quite loose and so I was able to remove one. Size is definitely 6mm (M6) both ends.

My DT360 exhaust stud broke off, I drilled and tapped to 8mm also like you said. I remember tightening the 8mm nut thinking the whole stud was 8mm, not 6mm going into the cylinder.

Sorry about putting ya off the scent as to 6mm or not. Sometimes i should just shut up haha.

Not at all RT!

Clearly no-one was absolutely sure and as some already knew, Yamaha did fit the occasional M6/M8 stud. And I still reckon that one end of the stud that I posted a picture of looks wider than the other!

Maybe i love stirring lol, but by sheer chance i stumbled on this stud on fleabay when not looking for one. Advertised as 7mm & going by MarkT's theory [which i'm sure is correct] that the second number denotes the size so '07'. Now i wonder if it 'is' or are they measuring the stud shaft which is smaller than the thread due to a rolled thread, bringing the metal up to size from a smaller size [like an 8mm thread on a 7mm shaft]. Although not sure there'd be a full millimeter in it between shaft size & thread size. Sleep well!!.

Just saw the shipping $58 to NZ, that's crazy--not that i need it. What's happened to the world. In saying 'that' i bought a yamaha fuel tap of Trademe [our NZ site] & was an Asian seller i think & showing 31000+ listings. & fuel tap was only $9.78 plus gst $1.50 with free shipping. I was only watchlisting it out of curiosity but when it ended they offered it for $9.78 & they'll pay the gst [& free shipping as i said]. They're pretty keen to make a sale. [hope it arrives haha. Lots of bad feedback but i'm in no hurry for it to arrive & not clothing & color problems so what could go wrong lol. Had the vacuum shut off tap for only a dollar or two more which is crazy when i think of the mucking around i just did with a box full of buggered old taps to try & get my Suzuki Quad to stop leaking. & our dollar has no value on the world scale.

Installed the 6mm helicoil today and testfitted with the old stud, and it's a nice fit. New stud and head bolts on the way in the mail right now, along with a new front sprocket.

Was just a "guess"... Yamaha does often code sizes into the "9XXXX" part numbers but just when you think you've figured it out... one comes along that "disproves the theory"

(My main point was meant to be to check the stud with a known-size nut...)

Congratulations! Hope to see pics or a video when you get it running!