When I re-powered my SR, the new Briggs engine's mounting holes were threaded. Consequently, I didn't have any trouble using the mounting bolts that the mower came with (i.e., the ones used to mount the original engine).

I'm not sure why your new engine's mounting holes aren't threaded. You might want to call Briggs customer service and ask if it's OK to tap them yourself.

I buy new Brigg Quantum engines every year. None of these are threaded. The mounting bolts will thread these, but I like to run a tap through them before installing on a deck.

Thank you for the feedback on that! I contacted B&S directly as well, and they confirmed that the bolts are self-tapping, and designed to cut the threads in the new engine base. I was able to do that yesterday, so the new engine is installed now ... added some oil, some gas, started right up on the 2nd pull. Runs like a champ !

Thanks for the update---glad to hear that everything went well for you.