400 Engine rocker nut backed off

[winger4647]

The 400 engine had been running fine up until a day ago and I heard a loud clacking coming from the passenger side bank of lifters. Pulled the cover and found the nut on #1 had almost backed itself completely off the stud. I put it together the way I found when I changed the head gaskets (took pictures.) There is a spacer between the nut and the rocker. The nut itself is chamfered on one side and flat on the other. The flat side was/is installed against the spacer. I could not find anything in my manuals to tell me if this is correct. Any ideas why the nut would back off?
Thanks for any feedback!

 

[Tiha]

You had it assembled correctly.

I have had that happen before. In my case I often used heads from who knows where with unknown mileage and use. Just assumed the nut was worn out and not locking anymore.

New nut and all was well.

The chamfer on the nut is the locking portion.