I'm not sure at all what size the threads in end of AT crank are... could be 7mm too.

 If its 7mm its likely a finer pitch like 1mm pitch. Ebay pic sort of looks 8mm but might be a pic trick. Does look fine too in that pic . tried to link the ebay thing but went 'error' then took all i wrote as well lol. So ebay number is eBay item number:185482632003 Ok now i'm on the right page i see a flywheel. So could you buy this ebay bolt & cut it short the thread & 'nut it', or thread & weld a nut once wound on then call it a short bolt. Writing style changes looking from my end, not sure why.

I expanded your pic up of the crank & it sure don't look good down in there. Probably need a new good quality tap. I think if it were me & the thread is dodgy i'd re-thread to one size bigger even if it meant going imperial although 8mm & 5/16 are the same sizewize. But if it "is" 7mm then go to 8mm, or 8--go to 9 but need a tap & die to make a bolt. Ok, enough of me & silly ideas but i've done them all lol. But wait, there's more. If it 'is' a fine thread 8, it might match to something like 5/16 unf thread. Still, depends how good it is down there.

I’m currently working on redoing my AT2 I measured my bolt and it’s 7mm 

CMSL says its a 7mm but I don't think that is right.
 www.cmsnl.com/yamaha-at3-1972-1973-usa_m...06.html#.ZA9IXnbMK9I

Might just order one off ebay and check for 10 bucks.
www.ebay.com/p/1325431702?iid=323785144532

 

Thank you all very much! M7 it is.. I'll need to find a fine thread m7 bolt and check if it'll go although RT is right I will most likely need to go up a size to m8 as my threads probably arent in the best shape. The only saving grace potentially is that the bolt in before was only threaded in half the threads so the rear half should be okay... Who knows. Either way we'll make her work. always appreciate the ideas RT!

So is it an electric start motor converted to magneto & does the stator plate locate & have screw mount holes in the right place. I see the heading so is it an LT2 mag rather than a 125 mag [we had mostly magneto 125's where i'm from]. If it's an LT flywheel & plate i hope the crank taper matches the flywheel. Best of luck & i love mix n matching things--till it doesn't work lol.

Yeah,, its something of a setup. I bought the LT2 thinking it was stock for $200 last year and didnt look at it upon purchase because it was raining and it was $200 there isnt a reason to not buy it. Get it home, change the oil, new spark plug, and gas, and see if something will happen. Well, the thing actually started untill it abruptly sheared the woodruff key which is how I found out it was a 125 AT engine. The guy i bought it from didnt know anything about it and never messed with it but he got it from a farmer and the bike is in great shape other than the tank has major sun fading on the top. 

Also... if anyone is in the SD / MN / IA area I would have no problem selling the bike. Need to downsize some as I am moving and my garage will be shrinking slightly. Dont have any pictures but if anyone is interested I will.

Be interesting if bikes could talk. There's been some radical things going on in its life up till now.