I'm slowly getting my '72 164E back on its feet after the head gasket failed back this winter. I was putting the rocker shaft back on today when I found that no bolt will torque in one of the holes. It looks like it's time for helicoil.
I do not have any experience using helicoil. I have seen it done a few times and am pretty good at following directions. I mainly need to know what size to buy. I don't have a caliper to measure the diameter of the bolt with.
Thanks in advance for your help.

I'd try a longer bolt first.

I'm using the ones that came with the car. All of them will torque, except in this hole.

Is it the bolt hole with the oil gallery? Have a good look down the hole, there's bound to be plenty of good threads in deeper.

The rocker shaft hold-down bolts are 5/16 UNC, which is a very common size for helicoils. It sounds like the head is on the block, which is too bad because it is a job best done (and most easily done) in a drill press. That will keep the hole perpendicular and perfectly round, which is tougher with a hand drill. Nonetheless, you should be successful first time. If you buy the kit, try one first on some other small item and get the hang of it.

Perhaps I'd be better served to tap it to the next size up. I wonder if a 9/16 bolt would fit through the hole in the rocker shaft.

You can coil this on the block but try to borow a large drill moter for low speed and hi torq & stabel control. Dont try this with your ryobi.

NO!!
Use the 5/16" kit. It is easy.
You will have to get the tap for it also and if they offer you an installer
tool you would be well-advised to get it also.

One of the rocker shaft bolt holes also carries an oil passage.
By all means you don't want to use a 3/8" bolt in that one and it
would definitely be better to use the same size in all of them if
possible.

Normally unless you had a bolt that was too short, you would break the
bolt before you would strip the threads in the head. It is really important
to use the right length bolt to get a proper length of thread engagement.
--
George Downs Bartlesville, Heart of the USA!