I'm posting this information for anyone who has experienced the same problem (prematurely breaking shear bolts) and may need help finding the cause. I experienced both causes listed below. The second cause (loose drive chain) was a surprise to me. It caused the bolts to fatigue and break prematurely. Once corrected, the blower runs great and strong.
Note: the shear bolts are "grade 5" 1/4-20 bolts and the nuts be locking "nylock" (nylon insert) nuts. Do NOT use stronger bolts (grade 8, 9.8, 10, etc.)
The snow blower shear bolts on the fan shear/break VERY quickly or often, even under normal loads (normal snow, no obstructions in the blower, etc.)
1) The blower "fan" and hub are positioned incorrectly on the shaft, allowing the tips to hit the housing. This can be caused by the hub (attached to the fan by the shear bolt) has moved towards the gear box, moving the fan closer to the housing.
The solution: Loosen the two set screws (Item 5 in the diagram below) in the mating hub (item 3...) and slide the hub and fan back on the shaft, towards the back of the blower, and retighten the set screws. Make sure the unit is free from ice and snow when you do this.
2) The drive chain is loose.
Loosen the 4 nuts (Item B shown in the photo) that hold the carrier (Item A) in position (2 nuts on the top of the housing and 2 on the bottom). Do not remove the nuts. Slide the carrier to one side, tightening the chain, and then tighten the nuts. If the bolts hit the ends of the slots, the chain may be worn or stretched and will need replacing.
I hope this helps.