From my marine experience, I would recommend installing heli-coils in
the aluminum flywheel, then you can use the original diameter and thread
bolt size. You would then have no corrosion issues if you are using
steel bolts of whatever the recommended hardness is. Get a heli-coil
kit for the original bolt size and thread pitch. It should include a
number drill of the correct size for the heli-coil tap supplied in the kit.
In outboard motors, aluminum is used for the cyliinder block and
detachable heads, also the mid-section and lower unit. Stainless steel
bolts are usually used for the fasteners. Hardened steel bolts were
used in some brands of outboards in the past, and were quite a chore to
replace when the service procedure required their removal. Stainless
bolts can eventually get badly corroded in place, requiring careful,
patient drilling to remove, and usually a heli-coil installation is
necessary due to the aluminum corroding away to powder.
Carl Wade
former owner of Kenyon Marine Repair
Williamson, NY (near Lake Ontario)
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