I've been reading this thread with interest, learning what I can. But I admit I don't understand this idea of the feed "sagging" and how that would cause the 51's nib to press more tightly against the hood (above it) leading to the reduced flow. This is what I'm understanding the experts are discussing here. I presume I'm completely misunderstanding something.
Now maybe if the feed "sags" and presses against the hood (below) that somehow restricts the flow in the feed which restricts the flow to the nib.
Hopefully someone can explain this so I understand better. Thanks for sharing your knowledge.
Sorry about misunderstanding and I fear I have participated in it...
In vintages pens with hard rubber feeds (NOT "51"s) the feed tends to sag and the nib tends to follow. This happens over a fairly long period of time, however. Modern pen makers look at the vintage pens, trying to copy them. Not realizing the "sag factor" of the feed, they make nibs with the tines too close together. Why? When a feed sags and the nib follows, the tines get closer together at the tip. So, modern pen makers think the tines have to be jammed together...
Now, fast forward to the "51". Feeds can sag (the large majority of them are hard rubber). The nib on a "51", though, usually won't. On the "51" the nib isn't held in place by the feed, per se. The nib is held in the collector (and so, for that matter, is the feed). The tubular design of the "51" nib means that it will usually not sag with the feed, because the tubular design is typically stronger and more support is given to the tines by the very design of the nib.
But, in "51" feeds, there are some variations. The early feeds do not have slits in them. So, it is more important for the non-slitted feeds to be firmly against the nib. The slitted nibs are, I think, a bit more forgiving. So, in this case the feeds sag and withdraw from the nibs. It's important to have the feeds re-engage with the nibs to have the flow work properly.
I doubt that a sagging feed will push the nib into the shell and I'm sorry that my comments (and others) gave that impression.