O Ring Question From A Know-Nothing

[Lloyd]

I have an Ebonite fountain pen that I would like to convert to an eyedropper. I would like to put an o-ring on it and, when I measured the diameter of the threads on the section, it was 9.9 mm. What size o-ring do you think I should use to make it into an eyedropper? I already have silicone lubricant.

[Ben Looijesteijn]

I suppose you want to use the blind cap to close the reservoir? In that case the O-ring should not only fit around the threads, but inside the blind cap as well. You will have to make a 'slot' at the top of the thread on the barrel and a receiving fit at the end of the thread inside the blind cap. The O-ring is best fitted inside the blind cap.

[Lloyd]

This pen (not to name names) is a simple cap, section, screw-in nib/feed assembly, body. I "can" just use silicone on the section threads but I want to add an o-ring to reduce the risk of over-tightening as it wasn't specifically designed for eyedropper filling.

[jalbert]

I can't say for sure, since I don't know what pen you have, but more than likely you will need to modify the section and barrel to accept an O ring, since there will need to be some space for the o ring to squeeze in to when you screw in the section. Here's an example of a piston filler of mine that has an o ring seal on the section. It has a recess cut behind the threads, as well as a lip cut out in the barrel. As far as the size of the o ring goes, it's largely going to depend on the diameter of the recesses you cut.

[jalbert]

Oh, and here is a handy chart regarding o ring sizes.

 

http://www.oringsandmore.com/oring-size-chart/

[Astronymus]

There are also O-rings in modern metric measurements. Just saying.

 

There are also different ways to place an O-ring to get a seal. In case of a pen you might need to make a groove that will hold the ring.