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

"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!