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