Took my 65 coupe on a cruise last weekend and the engine suddenly sounded like it began running on 7 cylinders, which I later determined was the case. I inspected everything visually under the hood, and nothing appeared to be wrong. However, the left exhaust began emitting noticeable smoke which smelled like burning oil. Until about 20 minutes later, the smoke ended.
I was watching the oil pressure gauge closely and it never dropped, but checked the oil level when the cruise reached its destination. It had dropped, but was not quite down to the add mark on the dipstick. I bought a couple of quarts of oil at a gas station, in case I needed any on the way back home, but did not. Engine ran pretty rough at idle, but smoothed out a lot running at 55-60 on open roads. No other signs of engine problems. No noises, oil pressure held steady and coolant temp stayed normal.
Back home, I let it cool down a couple of hours and started to investigate. Pulled plugs 5-8 and found #7 oily, but the other 3 plugs normal. Removed the valve cover and found #7 intake rocker lying beside its broken stud. It looked like a fatigue fracture starting from the bottom thread root of the 3/8-24 thread that the prevailing-torque lash-adjusting nut threads onto. Pushrod was straight and I could see no evidence of the rocker bottoming out on the stud.
My big surprise was that when I removed the lower part of the stud that is threaded into the head, anti-freeze came out, indicating that the tapped hole enters the head's coolant passage. I had not expected this, but from searching on the internet, it appears that this is normal and sealant should be used on studs to seal coolant from leaking past the threads.
I found several recommendations for the "best" sealant. Some of you on this site must have experience with this and have a recommendation for sealant, and can explain why you think your choice is the best.
Thanks in advance.