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 machin 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 small mill with the tower set to the recquired 30° tilt.
You
need to align the head with respect to rotation (at least the end-results looks
much better if you do it :-) ).
Start
by drilling a hole from the insde. Start with a centre-drill and end up with a
5.5m hole.
Turn the head around, and use the drill to verify correct alignment.
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 an OS-F in centre and OS-A5 at the rear, seen from the inside.
Finished head with an OS-F in centre and OS-A5 at the rear, seen from the
outside.