Unless running a high rpm motor and concerned with ‘valve float’, adding locknuts is likely the best long term solution to worn hex screws. Be advised though, that tightening the nut always alters the gap, so you must allow for that when adjusting the screw.

6 VOLTS/POS. GRD. NW INDIANA

the engine in question has the lock nut style adjustors,so some of the previous posts did not  give correct advise.
I noticed a engine that has not had regular oil changes and the top end is in trouble,a short turn cure would be remove rocker asembleys you will find scoring on the shafts especially the 1s farther from oil entry.new rocker shafts and refaced rockers would help. even with some scoring of rockers new shafts and removal of spill tubes,and blocking the out flow.sadley the rockers do not get enough lube due to flow spilling reduces flow and sludge builds up mostly at far end.when rockers off check valve seals,they can be changed with out removing heads. there are several cheap tools that make it easier.  good luck

The O.P. wrote “some ‘nuts’ were easier to turn than others”, thus initiating confusion. Also, it likely isn’t uncommon to find that some mechanic may have swapped out the original adjusters over the years for the interference type, since setting the lash is so much faster without locknuts.
Though he claimed the engine looked clean, I assume 65 yrs. of sludge/corrosion may eventually cause some of the screws to bind a bit, or maybe overtightening the locknuts could have damaged the threads (?).

6 VOLTS/POS. GRD. NW INDIANA

I pulled the valve covers to set lash and found some of the rocker arms are not centered on the valve stems. They are off as much as 1/8" to 3/16". The ones that are off are too close to the mounting pedestal. Is this normal? ]

I was hoping TED would comment on this.. If the rocker arms are allowed to walk, it may be a reason that they throw push-rods.

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It does look a little strange at first glance, it is normal, this way Ford didn't need to make left and right hand rockers.

All the Best
The other Ted.

The rocker arms not being centered on some of the valve stems is normal for the Ford Y.  With the Ford Y rockers having an extra wide stem contacting pad, one rocker fits all.  This was part of the original design and was likely the most expedient way of putting the engine into production.  If Ford would have gone to the trouble of centering the rockers over the valve stems without modifying the valve layout accordingly, three different rockers would have been required and that would have created a nightmare scenario both in production and for the later rebuilds.  When I do the roller rocker arm setups using Harland Sharp or Dove Manufacturing rockers, I end up using centered, left, and right adjusters to make everything work out while centering the roller tips on the stems.  I’ve seen the Rocker Arm Specialists rockers having five different rockers in which to make that all work.  It can get overly complicated very quickly.  Ford simplified the rocker situation by just having a wide pad to fit all the various valve stem locations.

Lorena, Texas (South of Waco)

The rocker arms not being centered on some of the valve stems is normal for the Ford Y.  With the Ford Y rockers having an extra wide stem contacting pad, one rocker fits all. 

THANX TED.

It makes perfect sense now that you have explained it.

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You're Welcome

Ted

Sorry I haven't looked in for a few days. This is the type of adjusters I have. I would call them an interference fit adjuster nut. Is this a later style adjuster?  I think this engine was a replacement because it was painted green, not red when I got it. All the nuts except 2 take about 25 ft-lb to turn,the other 2 take about 5 ft-lb. I was going to squeeze those two in the vise or use blue locktite as suggested earlier. If I can get replacement nuts for those two, would that fix it, or do the threads on rockers wear?. Thanks for all the replies, i've learned a lot.