What grade bolt to use as shear pin on a old MF 12 square baler?

[Birch Hill Farm]

I have an old MF 12 square baler and it keeps eating through grade 5 bolts even when there is almost no load on it they will shear. Is something out of adjustment or do I need a higher grade bolt? I can spin everything by hand when empty so I dont think anything is binding. Maybe dull knife?

 

[SPYDERLK]

What size bolt does that baler use? Are you tightening or just snugging it? Does the unthreaded shank of the bolt extend a safe distance beyond the shear zone? Shearings slowly egg the holes degrading the predictability of torque to shear. Gr 8 may actually do damage, but at least, would egg the holes more readily. I ask about tightening because without it you may have the bolt rocking a little which gives an impact.
larry

 

[Birch Hill Farm]

Its a 3/8 bolt and it is shearing in the threaded portion. I just snug the nut down when installing. When the bolts shear I have to hammer out the remaining part do to the shear point of the bolt bending over slightly. Almost like the hole is egg shaped.

 

[SPYDERLK]

Its a 3/8 bolt and it is shearing in the threaded portion. I just snug the nut down when installing. When the bolts shear I have to hammer out the remaining part do to the shear point of the bolt bending over slightly. Almost like the hole is egg shaped.

Threads smear ... and the bolt is smaller and weaker there. -- Get a bolt long enuf that the unthreaded shank extends thru the shear zone as much as possible - at least 1/4". Tighten it fairly firmly ~ 20 or 25 ftlb. Use locktite on the theads so it wont back off.