There are three potential possibilities. Either you didn't make sure the air gap between magnet and magneto coil arms was correct; allowing the spinning magnet to hit the arms, thereby placing stress on the crank, or you didn't tighten up the magneto coil screws sufficiently, allowing the magneto arms to come into contract with the spinning magnet or you left something metallic (like a washer) in the magneto housing; allowing it to get trapped between the magnet and magneto arms.So Could something I didn't check prevented this?
From the severity of the damage in the photos, i'm suspecting that it was either a loose washer in the magneto housing or it was the woodruff key that keeps the magnet in alignment with crankshaft indexing slot.
I have had this situation happen to me before, when the woodruff key slips out behind the magnet as it's being installed it on the crankshaft. If you are aware that this can happen, you are paying attention that the woodruff key can slip out.
If the woodruff key did indeed slip out, there's a change it could become trapped in between the spinning magnet and the magneto arms. Considering that there is only 0.5mm air gap between magneto arms and magnet, the problem will become quite serious when a 4mm woodruff key tries to pass through a 0.5mm gap.
The slant plug design is the manufactures solution to allowing fitment of these engines into bicycle frames with restricted space. Because of the packaging requirements and henceforth the altered physical dimensions, the modified cylinder head design gives the engine higher compression than conventionally designed OEM cylinder heads.the replacement is going to be a slant plug if I go that way. Will it be better quality?
Higher compression = higher engine torque, with some people also reporting that these engines produce increased power due to the slant cylinder head.