Mike I used allen head bolts in the Ryobi. Im pretty sure they were 10-24. I changed them because I didnt think there was enough length after the mount was installed. I would guess that the factory bolts are probably the cheapest available, plus on the weedeater they are not designed to hold the entire engine.

I remember you were having problems with the bolts loosening up and what probably happened was the bolts loosened a little and the vibration sheared them off.
Adam