The Twist 40 ARF manual calls for using 3mm socket head screws to mount the engine to the mount. The stated length is 20mm or 0.787 inch (just over 3/4"). Then they call for 3 mm washers & nuts. If this is what you have and it isn't working (for whatever reason) then you need longer screws which should be readliy available at the LHS. 8-32 hardware would do just as well if you have it laying around.