Ola Fremming's homepage


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.