Thanks for the reply. No, it's not a plastic sleeve. It's a cast metal thing like a short thick oval sleeve/coupling on the motor shaft that goes into an oval socket in the gearbox. It's a way of coupling the motor to the gearbox. The couplling thing was evidently supposed to be pressed so tightly onto the motor shaft that they didn't put a holding screw for adjusting or tightening it. Now the motor shaft turns freely inside it, yet it still fits too tightly for me to slip any sleeve inside. I just can't imagine how to get it tight again 