Flying in the face of a commonly held belief about hand hole sizes I want to bring to the table a little bit of information about the ETA 28xx and Seiko 7Sxx mechanical movements.
There is a basic tenet that the hand hole size of 1.50 for the hour hand & 0.90 for the minute hand are common between the Seiko 7Sxx and ETA 28xx series movements, this is a close generalization but the actual sizes are:
Seiko: 1.50 hour, 0.89 minute
ETA: 1.51 hour, 0.91 minute
On the grand scheme of things these differences seem small and in some cases they are, however as we will see, it can cause problems.
Take for example, the hour hand made for a Seiko. It will fit on the ETA because you are taking a slightly smaller hole and installing it on a slightly larger post. 0.01mm is not enough to prevent the hand from seating properly when going from smaller to larger.
The problems can start when you want to take a ETA specific minute hand and install it on the Seiko. If the hand hole tolerance fell on the small side and Seiko post tolerance built on the large side the hand will fit. I have put ETA hands on Seiko movements and have had no problems, this is rare though. Most of the time the ETA minute hand will freely lift off the Seiko post.
To remedy this situation you can turn the hand over and bend the return flange a little for tighter fit or, if you have many of the same hands, you can try different ones to test for a better fit. If you are a hobbyist and don't mind if the hand doesn't fit perfectly square on the post then the first option is viable, personally I won't do this because I can't have customers with watches where the minute hand can come loose at any given moment.
The gist of this post can be summed up that ETA minute hands have a very high propensity of fitting loose on Seiko movements, while the hour hands will fit fine for either movement and the Seiko minute will fit the ETA, albeit somewhat tight.
YMMV, the hands I make are held to very tight tolerances and this entire size issue may be affecting me more than others, lately I've had to have hands re-made and not be able to finish watch mods, figured I would share my findings with you guys.
(FYI, it is very easy to prove this out by taking a quality set of calipers and measuring the post sizes of the movements mentioned.)