Universal 25 - flywheel rust

[Fearlessfrog]

I've had 2 sarters break (housing that covers the gear cracked all the way through) on my Universal M25. I was told by 1 mechanic that its due to rust on the flywheel not allowing the starter gear to engage with the flywheel properly. When the teeth don't mesh the upward pressure on the shaft is too much and it cracks the housing. Seems hard to believe that rust would keep the teeth from meshing but curious if anyone has heard of this happening.