Hi there,
I have a brand new os 50 engine that I tried to break in today.
The drive shaft came lose while the engine was running and fell off several feet away. now i think I am missing a part. there is a notch in the crank shaft and a notch in the drive shaft there is supposed to be a peice that goes in to lock the two together? what part do i need to order? would a new drive shaft have that piece? I hope I explained it good enough , thanks for your responces.

It's called the Woodruff Key. look up the part number from Os site and order.

Or bring the engine to your local hardware or auto parts store and see if they have one.

why did if fly off? did you not tighten the prop?