I said somethings in Confirmed Grade 8 thread about this, but thought this should be a new topic.
Shear bolts which are ductile medium carbon steels (Grade 8 is an alloy) are under normal conditions the weakest point in a new or in a normal/standart quality system (gearbox+pto system+engine+attachment.) Therefore, under normal conditions, even Grade 8 bolt is expected to fail first. But is this really so? No. Because either gearbox/pto/engine/etc is low quality or one of them components is old/ill. So, what to do? You need to make it the shear bolt the weaker than any component in this combined system. But, not much weaker as you will not want the bolt broken frequently even when it hits a small stone. Since your old/ill system is special it will require a certain amount of shear load which Grade 2 or 5 or 8 will not match exactly. Then, what to do? I say Start with Grade 8 bolt and adjust its shear load capability by adding a certain size notch in your bolt. You can make a few experiements by changing the size of that notch in the bolt until you find exact shear load for your special system (it's special because your system isn't new anymore.) Your this special bolt with a certain notch will be somewhere between, say, Grade 2 and Grade 3 or between Grade 5 and 8. Happy shearing.
Shear bolts which are ductile medium carbon steels (Grade 8 is an alloy) are under normal conditions the weakest point in a new or in a normal/standart quality system (gearbox+pto system+engine+attachment.) Therefore, under normal conditions, even Grade 8 bolt is expected to fail first. But is this really so? No. Because either gearbox/pto/engine/etc is low quality or one of them components is old/ill. So, what to do? You need to make it the shear bolt the weaker than any component in this combined system. But, not much weaker as you will not want the bolt broken frequently even when it hits a small stone. Since your old/ill system is special it will require a certain amount of shear load which Grade 2 or 5 or 8 will not match exactly. Then, what to do? I say Start with Grade 8 bolt and adjust its shear load capability by adding a certain size notch in your bolt. You can make a few experiements by changing the size of that notch in the bolt until you find exact shear load for your special system (it's special because your system isn't new anymore.) Your this special bolt with a certain notch will be somewhere between, say, Grade 2 and Grade 3 or between Grade 5 and 8. Happy shearing.