Nope.We only use what works...I have seen exactly 3 Air Hobbies engines, 2 were 4.2 and one was a twin..the first 4.2 was assembled too tightly to run.I had to bore the rear bearing bore deeper in the case to free up the crank..The second 4.2 was made with the back bearing bore .015 lower in the case than the front one, then someone ground the rear part of the crank smaller so the case would fit together..It ran, but the rear part of the crank was loose in the inner race of the bearing..It's still in the drawer in my shop, never to run again..I'll use the only good parts, the Sachs cylinder/piston and the Walbro carb, on something with a useable crankcase and a crank that hasn't been destroyed by grinding the rear shaft too small to fit the whole bearing race..
The twin did run after I changed the ignition to one that worked properly and refit some of the clearances in the engine....None of these engines were sent back to Air Hobbies..I figure if they can't get it right the first time there's no way they could do it later..There's a twin here in Flagstaff that doesn't run either, I haven't seen that one yet...