How close is the modern replacement material to the original stuff?

I'm no expert.

My understanding is - not close at all. I might have bad information, but my understanding is that the modern replacement does not equal the many positive attributes of the pli-glass (balance of strength, longevity, corrosion- and stain-resistance).

The original pliglass sacs look like amazing things. In my (still unrestored) latest '51', the sac is still very-much pliable and ready to be used. I had no idea these were the original kind of sacs used. I always saw '51's with standard straight black rubber sacs.

You didn't say which 51, and since people refer to the diaphragm as a sac, the question is even more confusing.

If you are talking about the 51 vac filler diaphragm, they are identical. You have to trim them to length before installation, but they are the same.

If you're talking about the aerometric filler, there is no modern equivalent. The plyglass sac was unique. They're lasting longer than I think Parker expected them to. They stain, on occasion they get a puncture, but in the significant majority of pens, they work just fine and will for the rest of my lifetime!

The most common replacement sacs used are the silicone sacs made sold by Woodbin in Canada. These give the desired transparent sac, and resistance to ink. I don't know though that they will outlive the plyglass sac. We just don't have the 50 years of data yet....

Ron, sorry to hijack the thread, but as you mentioned, pliglass sacs get stained. I've got an aerometric P51, and for a long time I thought that someone has once replaced the pliglas sac with a black latex one. Yesterday I noticed that the previous owner did not replace the sac, he only used black ink...
Is there a usual method to clean the sac? It would be the best if there was one without the need to disassemble the pen. Would filling the pen with ammonia solution help?

So far as I've found, nothing I'm willing to put in the pen removes the ink stain.

I think once stained you are stuck unless you want to replace the sac.

Peter

But a stain doesn't necessarily damage the sac, does it? It just makes it look weird. It won't like, affect the ink that you put into the pen, right?

Right. I've had people suggest replacing the sac because they want a clear one, and I tell them that I don' have anything that will last longer than the one that's there, and that a silicone sac can stain too.

I don't know of anything that will remove the stain from the plyglass. When I've cut one of these sacs, the material is stained all of the way through. A solution of ammonia will clean a sac, as will 409, without damage. It is possible to run a Q-tip, with some of the swab removed to make it smaller, down the sac. But there's always the risk of pulling the sac off of the sac nipple. I don't do it with all pens. The use of a syringe to squirt water into the sac to flush the sac works.

I've encountered primary failure modes in the aerometric pens. One is a puncture in the sac, caused by a burr on the pressure bar. The other is where the sac has come loose on the nipple. I occasionally find that the nipple that the sac attaches to is soft, and the joint leaks. I honestly don't know if it was made this way, or if the material has failed in some way. There are opinions on what has happened, but nobody that I've talked to knows for sure. Speculation is that the adhesive used damaged the plastic.

Ron,
so your last statement implies that Parker did not use shellac to affix the ply glass sacs onto the nipples, but rather some type of glue?

That's a question to which I don't have an answer. That's why I say "speculation is..." I've discussed the soft nipple on the sac guard assembly with a number of people, and nobody is sure why it happens, and it's been suggested that the adhesive might be the reason - implying that something other than shellac was used, but no confirmation. I don't even remember seeing anything about it in the Parker repair manuals.

Perhaps David Nishimura or Richard will have an answer or comment.