Different sized engines on the same plane

[Dudley Lang]

I read where a modeler may run say a 35 engine on a plane, then try a 40 engine, then try a 25 engine.  Most of these engines have a different width and different mounting holes.  If I have a Sig Sky Ray how do you mount these different engine sizes in the motor beam mount.  I built this plane for a Fox 35 and now I want to run a LA25.  The LA is narrower than the Fox.

[Andrew Hathaway]

Sometimes you can get away with opening up the holes in the plane to 5/32".  Other times you can reuse two of the existing holes and drill the other pair to match the new engine.  If the width is off, or their isn't wood to be drilled, I use a universal mounting system.  On a profile its pretty easy since there isn't a cowl, or even really a spinner to worry about.  I cut two mounting pads from 1/8" or 3/16" thick aluminum stock, about 1/2-3/4" longer front and back then the engine.  Drill mounting holes for the engine in the middle of the pads, drill mounting holes to mount the plates to the plane at the corners.  Countersink the bottom of the pads for the engine pattern, and bolt the engine in place with tapered seat bolts, with flat washers and nylon locking nuts on top of the lugs.  Then drill the motor mounts to match the holes in the pads. 

[Randy Powell]

I do this sort of thing on most planes I build. They have a removable mount system. So If I decide that I want to try another engine, I just pop out the aluminum mounts and machine another set to fit the new engine. I usually make cowl and such so the bigger engines will fit. The only thing to worry about the position of the needlevalve. That can be tricky.

Edit to add picture: This is just at the roughing out stage, but gives the idea.

[Clint Ormosen]

This model had an OS .35S on it. To put an LA 25 on it, I made this U shaped mount out of 2" X 1/8" aluminum stock I bought from Lowes. The mount bolts to the plane and the engine is bolted to the mount with short 4-40 bolts threaded into the mount.