Posted this request under repair and while it had a # of views, no answers followed so I thought I'd try the Parker posting site. Does anyone have a source for the O rings that fit the shells of P 51's? I have a few pens where the rings are broken/messed up and in need of replacement. Does anyone know their measurements? Help!

I don't know any direct for pens. However, measure up the O ring, and go to an industrial O ring supplier, see if they have any.
If given a choice of material, choose 'Viton' first and 'Neoprene' as the second choice. Reason, Viton is resistant to almost anything. Neoprene is attacked by extreme acids. 'EPDM' would do, but may not last more than 10 years.
As for hardness, get one in the 30-70 range of the 'Shore A' hardness scale, preferably the lower end. These numbers represent how squishy the rubber is. 70 is hard, but can be dented by a fingernail. 30 is quite soft, and won't load up the pen's section too much when you tighten it, and will deform nicely to seal it. Parker, by the feel of the one in my P51, probably used a 50 hardness rubber.

Regards

Richard.

The black snorkel 0 rings will work in a pinch. I recommend putting a little silicone grease on the 0 ring before you put the pen back together.

Richard,
Thanks for the very specific information!
John

As I can't seem to find any O rings and snorkel rings are an "in a pinch" solution, perhaps the best seal may be had by forgetting about the O ring and using the section sealer goop on the threads of the connector/shell? Is there anything that would argue against that solution/strategy? Thanks for your consideration.

There's nothing that says that you have to use an 0 ring in the section. I do think that it gives some support to the edge of the shell, but it's not crucial. I would recommend shellac rather than section sealant though..... and don't over tighten. You could introduce stress cracks if you do. Just snug will do it. Let the shellac set, and it'll hold the shell in position for you.

I use shellac not only on pens that I repair for clients, but for my own pens too. I just think it's the better material for sealing 51 threads.