After dealing with an intermittent no start issue that eventually lead to a complete no spark issue, I decided to replace the distributor on my 1991 F-150 with a 5.0 and E4OD. After weeks of sitting, it fired up, I timed it to 10* BTDC, and everything sounded great...then the problems started.
I began tightening down the distributor and it begins turning counter-clockwise. No biggie. I'll just hold it in place with 1 hand while I tighten the lock down bolt with the other. No matter how much pressure I held on the distributor, it still wanted to turn. I figured that maybe since the engine was still running, it may have been too much energy for the shaft so I turned it off. Tightened the distributor to where I couldn't turn it by hand, and went to fire the truck back up. The timing is WAY off. Even though I couldn't turn the distro by hand, cranking the motor still turned it counter-clockwise. I had to loosen the hold down bolt to re-time.
I tried tightening the hold down bolt even more, which lead to pulling 3 threads out of the block. Luckily I had a slightly longer bolt on hand and a few washers which I used to rig up a makeshift lock down clamp, but I can still turn the distributor by hand. Oddly enough, firing up the motor doesn't seem to make it move, but I can't say that I feel comfortable taking it out on the road like this.
The distributor is fully seated.
This is the replacement that I bought:

I was using the factory hold down clamp and bolt.

Any ideas on what could have gone wrong?

 

Maybe the hold down plate is upside down?

 

I wish, but nope, at least I don't think so. I put it back on the same way I took it off...flat side facing up, beveled edge on the bottom and the bevel toward the distributor.

 

It doesn't make sense. There is nothing that sould be rotating against the dizzy while tightening the "clamp".

 

I've heard of some distributor shafts and their gears being just a little too far downhole.
All I can recall, sadly.
I'd pull it out and compare to one at a parts store.