IBM Model F and Beamspring keyboards are some of the longest-lasting keyboards ever made. However, after 40+ years, there is one part that has almost always failed: the foam.
The role of the foam is to distribute the pressure of the metal plate onto the switches/barrels, enabling it to hold them in place on the PCB. When the foam has disintegrated, the barrels/switches can flop around and feel noticeably worse. In Model Fs, this can even be enough to cause keys to sometimes not click/register.
Therefore, the most important part of a Model F / Beamspring restoration is usually putting in a new foam.
Foam variants:
F XT - fits 5150/5160 PC (Personal Computer) XT, 5322 System/23 "Datamaster", 5291/5292 Display Station "Bigfoot", 5324, 5155 Portable
F AT - fits 5170 PC AT
F122 - fits 122-key Model F "F122", 104-key Model F "F104" "Unsaver", 4980, 3205, 4704 Display Station Model 200 "F62" "Kishsaver", 4704 Model 300 "F77", 4704 Model 400 "F107"
5251 - fits 5251/5252 Display Station, 5281/5282/5285/5286 Data Station
3278 - fits 3278/3276/3279/3251/8775 Display Station, 3101 ASCII Display Terminal, 3727/3725 Operator Console
3277 - fits 3277/3275 Display Station
Displaywriter - fits 6580 Displaywriter, 5253/5254 Display Station
Some foams are made to support multiple keyboards versions and thus may require cutting a section off with scissors (very easy). Namely:
3278 and 3277: if your keyboard does not have the numpad, simply cut it off.
F122:
For Unsaver: cut the numpad off.
For 4704: For F107, cut off the function key cluster (24 F keys). For F77, additionally cut off the left 10-key cluster and the numpad. For F62, additionally cut off the navigation key cluster.
Some very uncommon language variants have different physical layouts; these are not supported.
Your foam may have some folds in it from shipping; this does not affect the function and will not be visible once installed anyway.
