Exhaust Valves, F head

[bmorgil]

Nemo, a classic worn valve, eventually burning through. I would think since all others were "good" that cylinder was a little lean. Maybe a little vacuum leak on that cylinder. That seat will also be ruined. Replacing the valves with SS is always a good idea. They are stronger. However the wear caused by UN-leaded fuels effects the seats the most, the exhaust seat primarily. To combat this harden seats are installed. There is a discussion about this in the "Well the darn thing is out" thread. The Willys engine (Basically a Continental Industrial Engine) may have had harden seats from the start. The reason being the exhaust valve gets hot and pounds on the hot exhaust seat. Hardening and/or lubricating the valve and seat, and in some cases the use of valve rotaters, makes it last much longer. The lead in leaded fuels provided great lubrication. When unleaded appeared, many auto engines without hardened seats, had valve seat recession. The exhaust valve would pound its way right down into the seat. I would not put a motor together without hardened exhaust seats. There is no draw back only positive, contrary to popular belief they do not fall out when installed correctly. All aluminum heads have inserted seats. There are many who don't do it. They are not wrong for three reasons. If you don't put the motor under loads for extended periods it probably makes no difference. If the motor is loaded hard for extended periods it will make a difference. There may not be enough material to install them. And it is quite possible the seat area has been "hardened" already. If you clean up the exhaust seat area, you may see an insert or a "Hardening Ring" where the metal looks a little different, where the seat may already have been induction hardened. If the guides are bad so are the seats. The wobbling valve ruins the seat seal.

The valve guide wear can be done by feel. The valve should fit smoothly with no perceivable side to side play.The valve should feel smooth with no side to side play. A VERY small perceivable amount of "play" indicates they are starting to wear. When they are "new" with a little oil on the valve stem, they drop slowly by there own weight. It is a .001" to .003" fit.

The exhaust valve and seat probably needs to be ground or replaced on that cylinder. That means you are going to have to take it to the Machine shop anyway. You sure can see big time valve trouble in #4. the rest can't be far behind. On the exhaust guides in the block, I would be surprised if they are worn. The fact there is no rocker arm to impart side load to the guide helps here. In an installed engine, they could be done. Using the correct driver they are driven out and then driven in to the correct depth. They must then be reamed to fit. If you are clean and careful, why not! Go to the Jeep Universal Service Manual Section D paragraph D-58 for instruction on how to make drivers. Or you can go here for drivers and reamers https://goodson.com/collections/valve-guide-drivers . However I do think you are going to have to have that #4 exhaust seat reconditioned.