> shock is transmitted by the magnetic > field and knocks the PMs loose causing > motor destruction
Well, not necessarily. While it's true that mowing up on large rocks, bricks, firm stumps buried in the grass or (particularly) fence wire will cause the permanent magnets in the motor casing to de-bond from the assembly, repair is possible if the magnet hasn't actually fractured into pieces from the impact. I've salvaged multiple motors from this damage by re-bonding the magnets with gorilla-kill epoxy. It's not a pleasant process, as trying to align the magnets inside the motor casing covered with quickly setting epoxy is a struggle, and the added fun of fighting the magnet attraction/repulsion makes it all the more physical.
In 2020, I fractured my last mower motor and had no replacement casing to build a motor from loose parts. At that time, myself and another EVDL List member put in an order for seven NOS casings from an online supplier. Between the two of us, there was a big pile of armatures, end plates, brush holders, bearing retainers etc. We managed to get six functional mower motors and a motor for his lawn sweeper attachment out of the project. It was a big job, as most everything we had been squirreling away was rusted solid from years of use and neglect. The red wrench was used liberally.
Which reminds me of an important tip to pass on - Every time you use your mowing deck, ~immediately afterwards~ use a putty knife to scrape the grass clippings out of the underside of the upturned deck, then hose it out and let it dry before putting it away. This will prevent the deck from rusting through, and preserve the blades and motor bottoms as well. I replaced all of the fasteners on my mowing deck with stainless steel bolts, nuts, and washers too. Use plenty of anti-seize on things, and don't overtighten if you go this route.
Last gem of wisdom on mower motors: The mower blade is attached to the motor armature by using a through-bolt and serrated steel washer. As part of this assembly, top and bottom fiber washers are fitted against the mower blade. This, when properly tightened, forms a torque-limiting clutch, so that some of the force of an impact with the blade will be lessened, perhaps preventing motor damage.
When I need to mow in unfamiliar territory where there may be hazards lurking in the grass, I just use my gasoline tractor, and keep the ET in the carport until I know it's safe.
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