[linearB]

I looked down at a stop light this afternoon and saw this on my '09 Bullet 500:
http://i.imgur.com/u39a0ON.jpg

Not inspiring. The nut on the other end must have shaken itself off, and the bolt had shaken itself nearly out of the socket by the time I noticed. (Both things must have happened over the space of ~8 miles).

I knocked the bolt back in and got it through the hole on the right front peg on the other side, but I don't have the nut anymore (obviously). Stopped at my usual auto garage, but they didn't have a nut on hand to give me and told me to go to a hardware store. I'd rather not ride it there to have it on hand to size the bolt; does anyone know what size nut I need to lock that sucker back in place?

Thanks.

[Arizoni]

If the bolt sticks out of the far side of the frame at least 10mm all you need is the nut.
The nut must be a high strength type because of the high torque that is specified for the rear engine mount location so if you can find a M10-1mm high strength nut your in business.

The correct torque value for the bolt is 50N/M or 37 Lb/Ft.

If your bolt sheared off its threads I can tell you what I ended up doing.
The M10-1 X 260 high strength bolt was totally unavailable here and I didn't want to wait for NFG to ship one to me (along with the lost high strength nut), so I went to my local hardware store.

They have several different sizes of threaded steel, both in regular strength and heat treated high strength materials. 
I bought a length of 3/8-24UNF heat treated high strength threaded stock, two grade 5, 3/8-24UNF nuts and a brand new hack saw blade.
Using the new hack saw blade, I cut the threaded stock 1 inch longer than the old, sheared off bolt.
(Buy the new hack saw blade.  This heat treated steel will be almost impossible to cut with a old dull blade.  In fact, after cutting off the length of threaded stock, the new blade is now dull.)
Threading a nut on one end I inserted it into the frame and thru the engine mount and tightened the nut on the other end to 37 Lb/Ft of torque.

That was over 9000 miles ago and it's still working great.

(added)  If you noticed a lot of engine vibration recently, you'll be amazed at how much of it goes away after getting the rear engine mount bolt fixed. 

[linearB]

Thanks for all the information!

The bolt's treads were still fine, so the old nut must have just unscrewed itself over time and fallen off. I put on a new 10mm nut with a washer and tightened the other nuts up as well, and everything seems good.

[bluesdaddy2]

Thanks for all the information!

The bolt's treads were still fine, so the old nut must have just unscrewed itself over time and fallen off. I put on a new 10mm nut with a washer and tightened the other nuts up as well, and everything seems good.

Maybe this is too late but a little Loctite would help keep that from happening again.