Tiller shear bolt.

[Pete Judd]

I was taught that with slip clutches, you want to take the tension out of them, lock the tiller, and make the clutch smoke, to burn off the varnish if new, and then if older, it will polish the rust of the plates. Won't hurt anything, and then tighten the bolts until the attachment works, but the clutch will still turn under a dangerous load.

 

[Villengineer]

If a 5/16 bolt is close then most likely it was originally a M8 bolt. Do you still have the original bolt's head? It may still have the markings to tell it's grade.