I'm installing an eme 60 in a h9 extra 300 the problem is they have already drilled and installed 1/4-20 t-nuts in the motor mount for the da 60. The eme 60 uses 10-32 t-nuts and i really don't want drill the motor holes bigger for the 1/4-20 screws as it may not be safe to so. Here is what i plan on doing ........ use the 10-32 screws and put those through the existing 1/4-20 t-nuts in the motor mount then use lock nuts and tighten them down. Also the 10-32 screws does not fit snug inside the 1/4-20 t-nuts so they do move round inside them a little bit but i think that once i get them tightened down it should be ok. Does anybody out there think this would be ok.

If the mounting dimensions are the same, try this. Get a set of ¼ inch bolts and have a machine shop turn down the threads to 10/32. Leave enough of the ¼ inch threads to engage the T nuts from the rear. Use a washer, lock washer and nut to mount the engine. Dan.

To complicate your problem, the EME standoffs are probably threaded 5mm .8 rather than the 10-32 you suggest they are. 10-32 bolts thread into 5mm threads just fine but they are loose by several thousands of an inch. Loose enough to cause problems in many cases. Please check to see whether they are 5mm or not.

Why don't you just punch the blind nuts out of the firewall and in their place use the correct size socket heat cap screws threaded in from the back. Use adequate large fender washers on the front and back side of the firewall to distribute the load and you're good to go.

This is fairly standard practice. In fact, it is rare to have blind nuts in a firewall and I wouldn't use them if they were there. Not strong enough in my opinion.

Agreed. Knock the blind nuts out and thread them in from the back. Use fender washers on both sides of the firewall and loctite on the bolts. You'll be good to go.

Plus 3 on the method just described.