RE: Motor Mount for Flexibility - 4*60 & Hog Bipe
I ran into this problem a while back, I built a plane and installed a Mag 52 four stroke on it. It turned out to need about 8 oz of weight up front, so I decided to use a Mag 70 instead. The 70 was a little wider than the 52, so I caculated the difference and milled that amount off the inside of each lug of a new set of beams. I redrilled them for the distance from the firewall for the 70 and all was well. I use the Dubro Viberation mounts. The 52 to 70 width difference is max for those mounts. There was enough beam width left, but not if I had to go wider.
The beam will always be on the vertical centerline of the engine, looking at the engine with the head vertical. If the mount had the horziontal center OK, then milling them equally would keep that center with the larger engine.
The other option is to use an aluminum plate on the firewall and mount your engine mounts to it, and the make the plate to fire wall mount pattern the same on all planes. In other words, there are a number of ways to accomplish what you want. My first suggestion eliminates any weight gain while the second does add weight up front.
Don
PS, My main computer is down because of a power supply issue, so I can't show photos of the mods I did on the Dubro mounts.