I have 6 conversion motors, all hard mounted to the firewall. Like they say above, if you need the stand off space use posts. Most of mine are WWI planes so I just mount the backplate directly to the firewall. Here is an example of one of mine, I needed to clear the mounting bolts and cooling fins on the back. Briggs and Stratton four stroke gas engine. Flies my Fokker DVII great, no shaking.