Generic German Piston Filler Replacement Piston Seal

[Jeph]

Yesterday my girlfried returned from shopping with her sister and handed me an old German fountain pen that she got at an antique store.
This is the first pen that she has bought me, so it has sentimental value.
There are no identifying marks on the pen, and the nib is a generic Krupp stamped steel one with no tipping material. It writes beautifully by the way, a broad italic with good flex and butter smooth as long as you stay in the sweet spot.

This is the pen, but do not worry, it will be a good looking pen when I am finished.

I was able to remove the piston filling mechanism and it needs a new piston seal. Since it is a no-name pen, I am not sure how to go about finding a replacement seal. I have added a picture of the piston and seal.

If anyone knows what type of seal this even looks similar to so that I can start my search for a replacement I would very much appreciate it.

German Grey Initial sm.jpgPiston Seal.jpg

[tandaina]

I replaced a dead solid plastic seal that looked similar with two o-rings. Finding the right size takes some trial and error but it works.

[piscov]

As tandaina says the best option is using orings. They are cheap, easy to install and work very well. Go to a oring shop buy several sizes and try them out untill you get the correct size. Use a bit of silicone grease to lub it.

Another option is using a cork seal made from a cork from a wine botlle. If you buy a good wine you can use the cork and enjoy the wine as well!!!
Try a good Portuguese wine, the pen will work much better and so do you ;-)

Sent from my GT-I9300 using Tapatalk 4 Beta

[Jeph]

I am a little embarrassed that I did not think about the double o-rings. Especially since I have seen examples of cork being replaced by a pair of o-rings on several repair sites. Last week I was looking through the bargain tool bin in a store looking for things that would help for working on fountain pens. I had a box of mixed o-rings in my hand for $5 and I talked myself into putting it down. That might have been a poor choice. But, o-rings are something I do know well. I can take the measurements and find exactly the right o-rings to fit, or at least get as close as possible. I expect that I will even be able to choose the amount of compression. It will be funny when I walk into the machine shop and they watch me sticking a bore mic into the barrel of a fountain pen.

My current favorite wine is Spätburgunder Blanc de Noir Kabinett feinherb made in the Pfalz region of Germany. It is a red wine (red grapes) that looks like a white wine and is perfect for drinking ice cold (“American cold”) on summer days. Unfortunately it comes with a screw top. I asked the owner why such a good wine came with a cheap screw top. He said that he has the taste exactly right and he does not want the flavor to change with time. He said that he only uses cork when he intends the wine to age. The wine is superb so I see no room for argument.

Plus, that entire process of cork shaving, cutting, and wax impregnating really sounds like too much work.

[Jeph]

I have now finished all that I can do to restore this pen and it is back writing. The blind cap (hard rubber) is not salvageable, nor is the clip. I will be on the lookout for donor pens to grab those from. The discoloration of the barrel is permanent, but it has actually started to grow on me. I like the red tint better than the green near the top of the pen. The nib is very badly corroded but has such nice flex that I am going to try and tune it as a project for another day. It has no tipping and I removed the gold wash. The imprint reads:
KRUPP
V2A
STAHL
25
<Mountain Imprint>
I thought that the mountain imprint was actually just corrosion until I had cleaned it up enough to see the imprint from the back side of the nib.

The real point of this post is the replacement of the old cork seal. I thought that it was rubber until I removed it. It was simply ink saturated cork. From what I saw on this pen, using a replacement cork seal is the better solution over using o-rings. There are many reasons that this is true. First, the portion of the piston where the seal is installed in threaded on this pen. That is a sure way to get a cut o-ring and eventual seal failure. The second reason is the outside diameter of the rod seal surface compared to the inside diameter of the barrel where the seal rides. There is not a standard, or even a standard special size (there is such a thing, specification AS 568A)) o-ring that will fit both of those dimensions simultaneously. In addition, the edges of the seal lands were sharp with burrs, and the interior land faces were very rough. These qualities do not play well with rubber o-rings but would not make a difference to a cork seal. Alas, I have no idea how to go about making a cork seal from scratch, so I keep pursuing the o-ring solution since this is apparently a popular repair method.

For o-ring seals, the surfaces of both the seal groove and in our case the barrel is important for a proper seal and seal longevity. The OD of the piston is .182-.185 inch. The ID of the barrel in the sealing area is .328 inch. <I had a ton of seal design crap here that bored even me so I took it out.>

My solution was to use a pair of standard Butadine Nitile (Buna-N, N0304-75) MS28775-009 (7/32” ID) o-rings. Uninstalled, they are .208 ID (+/- .005) and .348 (+/- .011) OD based on .070 (+/- .003) thickness.
I chamfered the edges of the seal lands and smoothed the interior land surfaces as well as I could to minimize seal abrasion during installation and while installed. To get the o-ring ID to work and protect the ID of the seals from the cut threads, I installed some 3/8” standard polyolefin heat shrink tubing (found at any hardware or electrical repair store for environmentally sealing wire splices) between the seal lands. That brought the OD of the piston to .215-.220 inch. The width of the lands was only aprox .125 inch (they were not close to true) wide, yet my pair of seals were .136-.138 inch wide. Because the o-ring(s) need to squat in the groove to work properly, they need at least 10% free space in the seal groove to prevent seal extrusion. So I used a piece of sandpaper and shaved the mating faces between the seals between .010 and .015 each. That gave me about .015 inch of clearance. I lubricated the o-rings with silicon grease and they installed with no issues. Once installed, their OD was .342 inch. That provides a near perfect .014 inch interference fit with the barrel wall, or .007 inch per side, the preferred 10% compression for both effective sealing and longevity. So far it works great. We will see how it holds up over time.

I have provided some pictures of the seal installation as well as the finished pen.

Shaft Seal.jpgGerman Krupp Parts.jpgGerman Krupp Final.jpg

[whych]

The Krupp nibs were made either just before and/or during the war. They are pretty scarce, so look after it.
Since it is a German pen, it's easier to use metric O-rings.
If you have access to a machine shop, you should be able to turn a replacement blind cap using black acrylic/plastic. If you do, turn the cap for the pen's cap as well so they match. (If you soak it to clean out any dried ink, it should unscrew.) Keep the old parts.

Most of the 'everyday' wines now have screw caps and are a meant to be drunk immediately.
To get a corked bottle, like @piscov says, you need to buy an expensive bottle of wine. Of course, being Portuguese, he is biased toward Portuguese wine, so try one of their Roses.

[Buntoro]

Awesome pics! Thanks for sharing and posting this up! I am going to attempt to repair a few piston fillers myself.

Thanks!

[earthdawn]

@ Jeph that was a GREAT and very informative post !!!

Excellent pics and description.

[Jeph]

Apparently Robert Burns worked on fountain pens. In the process of trying to tune the replacement nib I filled and emptied this pen several times. I left it nib up for a little over an hour and noticed some ink leaking out of the filler knob. I decided to take it back apart and see what the problem was.

One fundamental premise of the o-ring seal design is that the seal lands are fixed. As I have now found out, the plastic disks that were installed on either side of the cork seal were simply press fit onto the piston shaft. The dragging of the deformed o-ring against the barrel wall exerted enough force on the lower ring to move it. It moved enough to compromise the seal. A perfectly sized cork seal would probably have exerted less force. I cannot say that it would not have also moved the disk, however.

The upper disk is still somewhat firmly retained, although I did not put much force on it. Visual inspection shows no features not also on the lower, loose disk. I take that to mean that it is press fit on also. The good news is that it will now be easier to install a cork seal as I do not need to split it and then glue it back together somehow.

The only idea that I have had to make the o-ring version work would be to drill through the shaft and install a drift pin below the lower disk to hold it in place. Of course, that might just make the upper disk give way. But even the cork seal will require that the lower disk be fixed into place somehow.

I have put it away for a while to think about things before I make a decision. I had promised to update how it worked after some use. So, I have successfully learned another way not to repair a cork piston seal, at least on this pen. More as it develops.

Seal Oops.jpg

[whych]

The o-rings need to be just tight enough aginst the barrel to create a seal. Usually the press fit of the bottom disk should be enough to hold it all in place.
Most other pens use a screw into the shaft.
Try a bit of shellac around the bottom of the shaft. This should keep it in place.
I used two 3mm x 1mm thick orings on my pens which provides an adequate seal.