I have a few O.S. Max-s 35's and the mufflers have plugs where the exhaust pressure fitting would be.  The threads and hole size are quite a bit larger than the later model exhaust pressure fittings.  I'd like to pressurize the fuel system on my Nobler in which resides one of my 35 Max-s'.  I figure the fuel pressure fitting for this muffler is long out of production.  I was going through my hardware drawers in the garage looking for some way to solve this problem.  I came across the drawer that has assorted automotive grease fittings.  Hmmmm.  I took a 1/16" pin punch and put a 1/4" grease fitting in the bench vise and I punched the ball, spring and plug out of the fitting.  I was left with an 1/8" hole through most of the fitting with the exception of where the ball seats at the end of the fitting.  I simply drilled the fitting straight through with a 1/8" drill starting at the threaded end.  Then I took a piece of 1/8" by about 5/8" long brass tubing and pushed it through the hole.  I then set the assembled fitting on the anvil part of the vise and tapped lightly on the brass tubing trying to slightly enlarge the diameter of the tubing for a tighter fit.  I then silver soldered the tubing in place.  I tapped the muffler with a 1/4-28 tap and screwed the fitting into the muffler.  I'll probably refine the process a bit on the next one but this worked out just fine.

It's crude yes, but it is on the bottom of the plane anyhow.  I am hoping the silver solder does not melt from the exhaust temperature, that was why I swaged it for extra insurance.  When I make another one I'll probably do a bit better of a job.  I want to make sure this one doesn't come apart first.  Of course if this one fails it's back to the drawing board.