I have an Mpjet brushless I'm going to mount in Sig Rascal. I purchased clam type mount for speed400 and it fits rascal great, but it is too big to hold the mpjet (diameter about 1/16" greater than the motor). What can I do to make it fit? Can you suggest a lightweight solution that will not detrimentally affect cooling?
Mounting those boxes in any model designed for beam mount is tricky.
I assume you have the MPjet clam shell mounts that work OK on 400 sized motors.
I have no magic solution - every model I have made with these gearboxes I have been forced to make the front of the model a removable cowl pulling off forwards, and install a bulkhead made to fit the box, with a hole drilled both in the front of the cowl and in the bulkhead to allow air to be rammed into the compartment where the motor and ESC sits.
If you are good with metal or maybe paxolin, you could make up a radial mount for the gearbox and epoxy that to a U shaped beam mount that comes across behind the motor and then forward each side of t to connect to the mounting plate, but you will need some substantial bits to make ot = heavy.
What I would do, is work out where the box sits, and cut the fuselage off with a razor saw, then make up two identical bulkheads and pre drill them to take locating pegs.
Open up the front one for clearance round the box and the rear one to fit the box, and glue one into each half of the fuselage.
Then make good with filler and a new bit of covering or paint.
Let me make sure that I understand. I would like to attach the motor/gearbox assembly to the airplane via a clamp holding the motor. Are you saying that I have to attach the motor/gearbox assembly using the mounting "brackets" moulded into the gearbox? Via the motor would be simpler and I thought that there isn't enough thrust acting on the gearbox assembly to deform it (I'll pull 12A@11V tops).
I think your approach should work - simply that I never tried it, always assuming the 'ears' were there for good reason!
Perhaps snipping a coke can into some aluminum sheet and fabricating your own clamps would be possible.
Or a couple of hose clips around the motor, and e.g., a GWS type motor 'stick' would work. I've seen tie wraps used, but the MPjet motor runs HOT even on no load - I'd be hesitant to use a thermoplastic strap frankly.
Perhaps the answer is to use the MPjet clam shell, and wind a strip of the coke can around the motor to make an aluminum spacer that will achieve a sensible fit.