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

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!