A few things look unusual to me:
- The break is on the unthreaded part of the bolt. Isn't this supposed to be stronger than the threaded part?
- The break is quite oblique, not like what I typically see when I break off a rusty bolt trying to extract it.
- The fracture surfaces are rusty. I suspect this break occurred long ago, perhaps even during installation of the motor.
The bolts all appeared normal externally before extraction, with 1.5-inch-diameter washers supporting the bolt heads on the inside of the boat. The gel coat wasn't depressed by any of the washers. Note that the break was located 0.5 to 1.25 inches from the bolt head, which places it completely within the 2-inch-thick transom (rather than at the interface between the transom and the motor bracket), so I doubt that the bolt was fractured due to operating in rough seas.
Q1: could this have occurred from the dealer using an air impact wrench to install the bolt?
Q2: is there another explanation?
I will be replacing all four bolts with a jack plate bolt set from Bob's Machine.
Q3: how can I prevent this from happening in the future?