I have flat spots on some motors and on others I do not. With the 4mm set screw, I don't think it makes a difference, as Mike said. If you don't have the tap and bit, dubro sells them as a set pretty cheap.

The 1515/1Y is one nice motor. I think I'm gonna grab one. I just want to wait and see how my 7XL runs with the Compro I have coming in first.

The motors that don't come with a flat spot you don't have to dremel a flat spot on the shaft. Just tighten the set screw on the round shaft and your good to go. It does help though to have a quality hex driver to get the screw in there good and tight.

Hmm, mine has slipped before with the flat spot and being done up tight.

Yeah, I prefer to dremel a flat spot. It's not difficult and give me peace of mind. These motors can generate LOTS of torque and I don't want to just trust a friction bond.