if we're talking about mounting the engine to the mount, i have a hard time believing the sheet metal screws are enough to hold against all the vibration. i know many say they hold fine, but i never realy liked the idea much.  i drilled my mount arms for 8-32 cap screws and used nylock nuts. milled a small flat on the taper of the arm for the nut to sit on. i just don't like screws there and the additional weight of bolts and nuts was worth the piece of mind.
     for the mount to firewall i "invented" what i think is a neat trick.....
     my plane has a spinner so alignment is critical.  with my fire wall epoxied to the fuse at the offset angles, i can move my mount around slightly to get the spinner lined up without sacrificing  the offset angles by using shims to get the spinner lined up. if i do need to change the offset a bit with a shim, there's enough play in the holes to "trig" the mount over to gain spinner alignment again.
     i made a 1/4 inch thick, H-shaped plate no bigger than the diameter of the engine mount base that is drilled and tapped for the mounting bolts in the base on the mount. this H-plate clamps the mount to the firewall with the mounting bolts. the H-plate sits against the back side of the firewall and the holes in the firewall are large enought to allow about 3/32" movement in all directions. it allows me to put it all together on the firewall, then, with the bolts just loose enough to be able to move the mount around, install the cowl and get the best alignment at the spinner / cowl junction. then i tighten the bolts to clamp the mount in place and lock the mount in that location with a small fillet of 5 minute epoxy around the base of the mount where it sits on the firewall. if i have to take the engine off the plane for any reason, it automaticly goes back in the exact same place. the aluminum H-plate doesn't weigh any more than 4 nyloc nuts and the bearing/clamping area is many times greater than 4 nuts.