Howdy Toby:
1st do a little more investigating. I'm assuming that your engine has non-adjustible rocker arms. Remove the rocker cover and start the engine. Watch to see if oil is oozing out of all of the rockers, clear to the front of the engine. If it is good. Next item. See #2.
IF it isn't, It's likely due to a plugged rocker shaft or plugged oil holes on one or more rocker arms. IF all are dry, you must disassemble and clean the inside of the rocker shaft. If only some are dry It's those rockers that need attention. These rockers will have two oil holes. One on the valve side at about 10:00 and the other on the pushrod side at 3:00. The easiest way to clean the holes is to run an 1/8" drill through.
Aother possibility is that the oil transfer slot machined into the mating surface of the head got plugged on assembly.
If you run with dry rockers it won't take long to wear the ball end off of the pushrod and the cup out of the rocker arm.
#2-Next check each rockers adjustment, by checking for cleanance between the rocker and the valve tip with the valves closed. This should be done with the engine warmed up, but not too hot to touch. If they have clearance, you either have a stuck lifter, or macnining has caused clearances to be out of the tolerance range.
If the slop is only on one or two rockers, it is likely due to stuck hydraulic lifters. You can try running a can of one of the commercial products engineered to unstick sticky lifters. High detergent oils can sometimes help. Or remove and replace.
If all lifters are out of tolerance and sloppy, it is likely due to a mismatch of parts or machining. The lifters are designed to work in an 1/8" (.125") range. Usually machining tightens lifter clearances. So, I'd look for an extra thick headgasket, short pushrods or gaskets or shims under the rocker arm mounting staunchions.
Or get a set of adjustible rockers and pushrods.
In either case, this should not happen on a rebuilt engine.
Adios, David
