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Converting to a dual-plug head.

It's a well known fact that the OS140RX engine (and probably others too) runs better with a dual-plug head.  None of these are original OS parts and it can be a challenge to acquire one.  So why not convert the original head to accept two plugs.  Well I had a couple of old heads lying around, and the proper tools so I had to try it. And you know, it's not a big deal.  What you need to do this yourself is a small milling machine or a decent drill press, a machine vise, a 5.5mm drill, a 1/4"-32 UNS tap and a 13-15mm end-mill.  As usual I let the pictures with some comments explain the process.

My mill set at 30 deg angle to the base. My small mill with the tower set to the required 30° tilt.
Alignment of the head with respect to rotation. You need to align the head with respect to rotation (at least the end-results looks much better if you do it :-) ).
Drill hole from inside

Start by drilling a hole from the inside. Start with a center-drill and end up with a 5.5m hole.

 
Alignment of head after turning it around. Turn the head around, and use the drill to verify correct alignment.
Milling down from the outside

Use a end-mill with a diameter big enough for your plug-wrench (13-15mm) and mill down far enough to get a proper seating for the plug.   Then use the 1/42-32 UNS tap to cut the treads.

 

 
Finished head with plugs mounted, from the inside.

Finished head with an OS-F in center and OS-A5 at the rear, seen from the inside.

 
Finised head from the outside, with plugs mounted.

 Finished head with an OS-F in center and OS-A5 at the rear, seen from the outside.