This photo shows a broken jack; the jack allows the hammer to "escape" from the piano key. Well, it's complicated, but without a jack when you press a key, the hammer just wacks against the string and stays there--"THUD"! I have two broken jacks--one came broken when I got it, and I broke one when I roughly removed the keys for refinishing.
Shown here is my Lie-Nelson dowel press, along with a jack component that I made with the dowel press. The dimension of the tiny dowel that stick off of the end of the dowel jack is 1/8 of an inch. Very tiny, very hard to see and very hard to work with.
Here is the other half of the jack into which the dowelled portion is glued and inserted.
And here is the dry-fit jack reassembled next to an original jack.
Glued and drying...
The reassembled Whippen, with the repaired jack installed. 
