Masterclass 3

Servicing The Onoto Pen by Arthur Twydle

To the untrained student, the repair of an Onoto pen must look daunting, but you can actually learn to restore this pen to full working order in less than an hour. 

Onoto pens will not fill or function correctly unless both the cork seal at the back end and the plunger washers at the front end make good inktight and airtight seals.

To fill an Onoto pen, unscrew the shank and withdraw the rod to its fullest extent. Insert the nib and the end of the nib carrier ( section) into the ink. Press the rod home and screw up the shank, keeping the carrier in the ink until the filling operation is complete. The pen fills at the completion of the DOWNWARD stroke, the opposite action to a syringe. To use the pen, unscrew the shank about half a turn, take off the cap and place it on the other end. When you have finished, replace the cap, screw the shank right home and the pen is sealed and cannot leak. In other words, the Onoto has a shut-off valve. The pen has to be 'switched on' by unscrewing the knob half a turn, and screwing it back tight when not in use.

Disassembly and assembly of the front end

1. Remove the nib section. This unscrews with a normal right hand thread. The cone assembly of the plunger is now exposed. This is secured to the rod by a LEFT HAND thread.

2.  Grip the exposed part of the cone between the finger and thumb using a rubber pad. Unscrew the whole plunger from the rod. (Remember - this is a left hand thread). Do not force it. If it will not move, soak it in cold water for about half an hour. Do not use pliers or you could break the rod. There are special pliers to grip the cone which you use as a last resort, but in most circumstances the plunger unit should unscrew without such extreme action.

3. The cone assembly and plunger are now detached from the rod and comprise two parts held together with a hard rubber pin. This is usually a friction fit but sometimes threaded (right hand thread). Knock out the pin with a fine steel punch and separate. A punch can be made from a hat pin cut down to about 1½" and filed flat at the end.

4. Remove the old gasket which will be hard and fossilised and will probably disintegrate. Also, remove the old backing washer if there is one fitted.

5. Fit a new HARD washer onto the peg, followed by a SOFT washer. Replace the cone to make a tight fit. Line up the hole and fit the pin. You may have to reduce the thickness of the backing hard washer to get the pin holes to line up. It can be easier to line up the holes and lock the parts together with a simple straight pin such a paper clip. Snip off both ends and lightly file off any protrusions. Smear the soft washer liberally with silicone grease (see Spares). This will also grease the internal barrel wall. When secured, the soft washer will become curved.

Disassembly and assembly of the back end

This method involves the replacement of the back seal without removing the rod.

The purpose of the back seal is to ensure that ink will not escape from the knob or blind cap joint, and also make an airtight seal, thereby creating a vacuum on the downstroke of the piston. This seal is a compressed cork seal.

1. The cork seal is held in its recess by a threaded retainer ring usually made of hard rubber. This must be unscrewed (right hand thread). Carefully remove the old cork seal with a pin. You can now fit the new cork seals. The corks we use now are half the length of the originals, so you will need to fit two. Make an oblique cut in each cork with a razor blade.

2. Spread the split cork around around the rod and tamp down hard into the recess. Repeat with the second cork until it is below the surface, then replace the retaining ring. This must be tight to make a good airtight and inkproof seal.

3. Smear the rod with a touch of silicone grease to ensure it moves freely through the new corks.

The Factory Method

The alternative to this is the method I learned at the Onoto factory over 40 years ago. This method is still necessary if you find it impossible to carry out instruction 2 of the front end disassembly sequence.

The pin is removed from the knob or blind cap and the knob unscrewed from the rod (reverse thread). The whole rod and plunger can then be removed through the open end of the barrel. With the rod and plunger removed, you can exert more pressure to separate - even using two sets of pliers close to each other (but remember the reverse thread) or you will break the rod. This method of course eliminates instruction 2 of the back end sequence as the uncut corks can now be pushed directly onto the rod.

Without the correct Onoto tool you will need to improvise a tool to grab and unscrew the cork seal retaining clip. You can do this by making a simple spanner from a piece of hard material. Drill or file a half circle to fit the rod diameter and insert 2 sharp spikes or needles.


The notes above refer to the standard Onoto but there are variants. For example, there are different size (length) rods and different plunger diameters as in The Magna, The Mammoth and The Minor. But in general all the rod diameters are the same and if a rod is broken it can be exchanged. If a rod is exchanged, it is critical to ensure that the plunger washer clears the recess, otherwise the pen will not fill. Sometimes this may be caused by the shut off not allowing the washer to clear.

A rare variant involves a removable barrel seal unit, and in this case not only the cork seals must be tight but also the threaded barrel end.

You will need larger diameter flexible washers and backing washers for the Magna (see Spares).

Some explanatory notes

There are two places where left handed (or reverse) threads appear on practically all Onoto and DeLaRue pens. This is top and bottom of the rod and of course all parts which attach.

CONE - The common cone used on all the old pens had a pointed end which matched the feed shape. This is a flow control and shut off valve, so the pen had to be switched on to ensure constant flow by unscrewing the knob half a turn, and for safety screwing it tight when not in use.

ROD - This is a hard rubber rod with a reinforcing wire down the centre. It has two left hand threads and one hole in the top end threads. Great care should be taken because this hole makes the rod very fragile out of the end cap.

PIN -  This is hard rubber (vulcanite) and goes through the cone and plunger peg to prevent separation of the 2-part assembly. The size is approximately 5mm x 1mm and sometimes requires an eyeglass to find. It is removable from either side. The same thickness pin secures the knob but is 9mm long. This one can be difficult to find as it is about 2mm from the flat end of the knob, and one end of it is usually lost under the stamped model numbers. Some adapted cradle, jig or vice is often necessary to hold the part, so leaving both hands free to use a light hammer and knocking out tool. I use a straight dental pick and, as explained earlier, replace with a snipped off metal pin.


© Peter Twydle 2012