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Welcome. That's quite the motorcycle resume! 

Update!
Much thanks for the help. Followed the advice and drilled/tapped for 5/16×24t all thread. What I had leftover from Norton project..  Found out the outer 10mm steel plug was only 14mm long. The actual oil galley plug is aluminum. Gave me a little surprise when I pocket thru the steel and the bit proceeded to twist the aluminum plug. Was able to pull the steel plug back flush then simply tap the aluminum one until it cleared the oil pathway.  Tonight's task, clean up everything, pin the aluminum plug in place, fill the cavity between the two with silicone and loc-tite seet screw in the 5/16 hole.  Should be good!

Glad to hear it worked out 

Glad you got it. Thats interesting on the plug diameter. The engine I had was a KZ900 I believe maybe a late Z1. The plug was 12mm steel and was the only one. They must have bored it 10mm then 12mm and plugged it. What year were you working on?

Weclome to an awesome forum. Great to have new members. Good to hear your problem was an easy fix. Second time I have heard of this issue, I’m going to put an inspection of this area on my radar. 

Welcome Bob.

Cra-z1 wrote:

Glad you got it. Thats interesting on the plug diameter. The engine I had was a KZ900 I believe maybe a late Z1. The plug was 12mm steel and was the only one. They must have bored it 10mm then 12mm and plugged it. What year were you working on?

Sorry for the delay in responding to your question.  You were correct, the plug is a 12mm diameter.  What I didn't expect was that were two plugs; one steel to plug the outer part of the hole, and an aluminum one towards the interior. The aluminum one is the one you can see in my earlier pictures that has the notch machined from it when they cross-drilled for the passage from the oil pump. When I drilled for the tap in the steel plug, the bit passed through the steel, into a soft material, then bit into the aluminum plug for a few revolutions.  I decided not to attempt further drilling as it was evident I had more than enough aluminum plug material left in the hole to do the job. Super-cleaned everything with brake cleaner after I drew the steel plug back into place. Drifted/tapped the aluminum plug back past the oil passage hole for the pump. Introduced high temp silicone through the threaded hole in the steel plug, then 'loctited' a piece of the threaded rod into the hole.  Fairly confident there will not be a problem going forward.

  All told, it's been an interesting learning experience.  I'm sure that in my very early days of wrenching I may have installed too long of a bolt in the same location while mounting various engine case guards.  It only took 45 years for Karma to catch up with me!