OMAS CS Piston Repair

 

It was immediately clear that this OMAS CS was in need of repair since the filler knob came out of the barrel, instead of stopping when it reached the bottom part of its travel. After disassembly of the pen, the cause of the malfunction was visible: the piston track peg was loose. This caused the piston to rotate together with the knob which, without an end stop, was able to unscrew itself to the point of coming out of the barrel.

Fig. 1

This filler mechanism is far from ideal for its inherent weakness: in this pen there was no trace left of the weak peg that was originally present in the sleeve shown in fig. 2 and 3.

 

 

Fig. 2

Fig. 3

I decided to manufacture a new peg by drilling a hole in the sleeve and inserting a new peg designed to protrude as needed inside the sleeve. In many instances, I would have made the peg of hard rubber, but in this pen the groove in the piston is very small and it requires a very thin peg.  I decided against using hard rubber, fearing that it might not be strong enough for this application because of the small diameter of the peg.

 

I decided to use brass, using a small brass nail as a starting point.

Fig. 4

 

After drilling the appropriate hole, I used a mini drill to rotate the brass nail while shaping it with a fine file, almost as if it were on a lathe. I made sure that the end was rounded, to avoid excessive friction that may cause undue wear of the piston.

 

 

Fig. 5

I secured the new peg with a drop of CA adhesive, for extra safety. The peg, in fact, is positioned between the barrel and the piston and cannot really move in any significant amount.

Fig. 6

Fig. 7

A few finishing touches with a fine file to smoothen the area between the peg and the sleeve, a dab of silicone grease to reduce friction and after a careful test, the pen could be reassembled.

Fig. 8

 

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