Jerry wrote:

As far as boring the housing is concerned, I don't think  you have to

worry about it.  Just take the housing to a good machinist.    I  have
done it once on a practice 12A housing, and it was not a problem.  If
you check my insert drawings  you will note that  the ports are located
in inches from the housing center line.  Makes it easy for the
machinist.  I will be doing a practice 13B housing next week.  I will
report back on how it goes.

Jerry  (not a machinist or engineer sadly)

Fred O. wrote:

Thanks for the posts from both you and Rolf.

I'm certainly not a machinist either (proof follows (;-) ) but as a crazy
experiment I clamped a junk rotor housing to the table of my drill press
which was set 90 degrees to its normal position. Then I drilled a 1/4"
inch hole in the center of the circular recess above the exhaust port,
approximately parallel to the housing base. Then I used a bimetal hole
saw with a 1/4 drill rod guide to saw the hole in the housing. A little
nerve racking towards the end, I was concerned that just before the
hole saw finished the cut things might jam up but it broke out clean.

I would never have tried this if I had access to a decent mill but the
only problem I see is that the hole came out oversize with a slightly
rough finish because of some wobble during the sawing. It's certainly
possible to begin a p-port this way.

 
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