what's this thing in my engine?

[catspaw]

I've had the car, '74 911 stock, for just about a year and it has always ran well. I took it out yesterday to run a couple of errands and she feels like she's skipping on occasion. I figure maybe it needs a tune-up not having gotten any paperwork with the car. So i decide to pull a plug and see what condition they're in. I decide, by chance, to pull #2 plug. I pull the spark plug wire and boot off but for the life of me I can't get the spark plug socket on the plug. So now i decide to pull the valve cover off, no problem. With the valve cover off I still can't get the socket on the plug. I stick my finger way down to the base of the plug and there's something in there. I finally get it out (see attached photos) but I can see nowhere under the valve cover where anything is missing. One side of this "thing" is female threaded and the other is female allen wrench 10 mm. Any ideas? I may as well put in new plugs and wires although the plugs looked fine. Also what plugs and wires do you recommend, it's a Sunday driver.

[erobinson]

umm That thing is not good. It is a head nut.

[crashmy911]

Looks like it didn't break off. See if you can put it back on the threads and retorque it!! I think 24ft pounds

[rs6er]

Head stud nut

[Nostril Cheese]

This happened on my SC. I went ahead and re-torqued all the head stud nuts.

Use some anti-sieze and thread it back on.

[Jim Sims]

Pulling (not broken) stud - nut and stud loose tension - nut vibrates off stud.

Or the mechanic just forgot to tighten it correctly.

[catspaw]

Thanks. I'm guessing this can be done with engine in car? Yes -no?

[rs6er]

Yes

[Gunter]

It's a barrel-nut from a head stud. 10mm long, long Allen key needed. I cut off the shorter end of a long Allen and stick it into a regular 10mm socket 3/8 drive.
Becareful when re-torqueing!
Use anti-seize lube and look for the special washer that should sit under the barrel nut!
Not more than 24 ft-lb with a CALIBRATED torque wrench. Check them all. If you are lucky, the nut just came loose but......................
it is possible that the stud was pulled out of the case slowly, thereby loosening the nut which finally fell off.
This is quite common with the Magnesium cases up to '77.
You'll be able to feel if that's the case when you torque.
Set the wrench to about 20 ft-lb and try. If it feels "mushy" it means that the stud is pulled out of the threads.
Now you have a choice to make: Rebuilding the engine involves "time-certing" the case. The proper rebuild of a 2.7 liter is costly; I wouldn't do it because it makes more sense to spend the money on finding a 3.0 or 3.2liter engine.
Here is hoping!