I gave up long ago with commercial blind nuts, much happier with home made dural ones. Thread o.d + 1/16" is enough as outside diameter. And lenght same as thickness of engine bearer, so they touch the engine mounting pads.
I knurl them, allready wet of epoxy, by rolling between 2 flat files and press in holes that are pre-impregnated with epoxy. Then curing in oven, at 70..80C.
Maybe an overkill but much less work than when you have to fix one that's gotten loose and oily
I wouldn't trust threads in wood in engine mounting, but in secondary places like tank mount and lead-out guide they're more than strong enough if done well.
Normally I just wax the screw and force it into a slightly small hole (something like 2,7..2,8 mm hole for M3 bolt, or 2,6..2,7mm for 4-40 bolt) impregnated with epoxy.
By the way, those square machined Du-Bro blind nuts WERE plated brass at some point. That's when I stopped using them.
L