motorbuzz said he uses loctite. I'm assuming he means on the nuts - not the nylon mount. Loctite will dissolve some types of plastic and should not be used as a thread-locker for any type of plastic.

I usually drill and tap a nylon mount but I don't tap the last 1/8" or so. That untapped area does a good job of grabbing the bolt. I've never had any type of problem with an engine coming loose or a mount breaking in normal use with this setup.