Sudden valve tapping

[Raymond]

After nearly 3 years of not running, I replaced the bad fuel injector and changed the spark plugs.  Shortly after starting the test drive, I noticed the misfire and wondered if it was still an injector problem.  After making a U-turn to go home, trying to accelerate to the 45 mph limit, there was a bang and a large plume of black smoke in the rear view mirror.  In the garage, I hear the clacking of what turns out to be the exhaust valve on #6.  I checked the clearance and it is more than a full millimeter. 
So, have I broken a valve spring, or possibly bent a valve?  What are my options?

[Benz Dr.]

 Adjust the valve clearance to where it should be and maybe do a compression test before you restart your engine. If everything checks out start your engine and listen for any odd sounds. If it sounds OK you might be fine.

 Seems odd to me that the clearance would open up that much though.

[Raymond]

I did as Dan suggested.  I adjusted the valve and checked the others.  Compression seems a bit low but the car starts and runs better than I remember.  Still wondering how one valve got that out of whack.  I may find out the hard way one day.  For now, I'm cautiously optimistic and, at least temporarily, delighted to be back in Pagoda world.

[wwheeler]

How hard was it to turn the ball stud when adjusting the valve gap? Was the #6 ball stud looser than the others? The ball stud pins rely on locking threads in the base to keep them from working loose and changing the valve gap unintentionally. Seems like it would be possible for the base to lose it's locking ability. I think the proper locking torque is something like 7 ft.lbs. In either case, it should not move easily. 

Just a thought.