Help on Reverse Box

[tkeagle]

With all the updates doing on buggy. Had the reverse box off the engine and decided to clean it up a little, and add fresh grease in it.
Now when I connect the box to the shaft and tighten down. I'll spin the tires, and the lock nut, that holds the gear box on the shaft, loosens and spins off.
I'm not sure what I'm missing. Is there some sort of bearing on the outside that keeps it from spinning off?
At first was working fine. Had to take off and on multi times, adjusting the location of sprocket, to the new engine. After running it on jack stands for little bit, must have worked it self all in, and used same amount of shims as the old one. But when first hooked up, was clearly offset.

When took out old engine, found that my nut that keeps the gear box on, had came off. Must have happened on that last ride. No dmg. Chewed up sprocket a little, but still on in good shape.

Couldn't find another nut that size, but did find a Lock Nut made for the shaft from Buggy Depot. It's a tight fit, but got it in. Could it be it's to tight of a fit, and it's locking it self to the box and spinning off? Or is there some kind of bearing on outside that I'm missing?