With Kent's custom wrenches and a set of feeler guages and instructions adjusting the valve clearances on your old 1965 to 1985 diesel is not a difficult job - and it should be done every 12,000 to 15,000 miles for peak starting and power performance.
Problem & Solution
Common among these chassis:
But it is possible to run into a couple problems when adjusting these valves.
1. The large hex cap on the top of the springs may want to keep turning while you are trying to adjust the locking nuts. If that turns, then you will not be able to adjust the clearance properly. Kent explains how to lock that big cap up in his instructions on adjusting diesel valves clearances.
2. You have two threaded nuts that you have to loosen and tighten on the stem of each valve. The nut on the bottom is quite narrow (not many threads) and if it has been over tightened in the past it is possilbe for the threads to strip or deform. Once that happens, you will find it very difficult to acheive the right valve clearance. You may try and try and never be able to get it "just right."
The only solution is to replace the locking nut with new. Don't worry, the threads on the valve stem never strip as they are a much harder metal.
NOTE: Instructions for replacing the locking nuts is included in Kent's manual on replacing diesel valve stem seals. He recommends if you have never replaced the valve seals and you are going to replace one or more of the locking nuts - best to do the seals at the same time.