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I just ordered a supply of Torrington style roller bearings with the idea of converting some stovebolt original equipment rocker arms to roller bearing operation. The rocker shafts will need to be ground down to 18MM diameter, which will also eliminate any wear on the bottom of the shafts, while still retaining the original flow of oil through the center of the shaft. Reducer sleeves made from seamless steel tubing will keep the shafts centered in the original support brackets on the cylinder head. Here's an Ebay listing for the roller bearings, which are 18 MM inside diameter, 24 MM OD, and 12MM wide.

www.ebay.com/itm/287123015654

The rocker arms will be drilled and reamed for a slight press fit of the bearings, two per rocker, with a narrow lubrication groove between them. The rocker shafts are hardened all the way through, so there won't be any concerns about the reduced diameter breaking through any surface hardening. They're so hard that a regular tungsten carbide lathe tool bit has a difficult time cutting them- - - - -the finish diameter will need to be done with a tool post grinder to get a good finish and an accurate diameter. I've also got some Rube Goldberg ideas down at the lifter end of the valve train- - - -substituting a newer, less expensive lifter and pushrod setup for those increasingly scarce and e$$$pensive OEM parts!
Jerry

What is the OD of an original rocker shaft?
The reason that is ask is that if there isn't much metal removed to get to 18MM, I found a rocker arm tower in my voluminous stash which has a cut in the side of it midway of the bore. This cut appears to be factory issue. I have no idea what engine it came from and I only have one.
This cut in the tower would add the ability to clamp down on a slightly undersized shaft like what you are creating if the reduction in shaft OD isn't excessive.

The ID of the hole in the rocker arm is approximately 13/16" (0.8125")- - - - -18MM is 0.709". Approximately .100" diameter reduction. That's going to require a thin wall reducer bushing, either steel or brass. I'll probably machine a piece of seamless steel tubing to make the adapter sleeve and press fit it into the rocker shaft standoffs, then drill it for the 3/8" attaching bolts. The rocker shaft can handle that amount of diameter change without any ill effects, I believe.
Jerry

Yep .100 is a bunch.

please keep us updated on your progress, pics also.

One of the "high tech/high dollar" You Tube videos of a twin turbo stovebolt engine also includes using a roller lifter camshaft with needle bearings in the block. Also a Wayne type crossflow aluminum cylinder head and computerized port fuel injection. I think that's a step or three too far on an engine design that's approaching its 100th. birthday!
Jerry