Trevor, what's your take on BSA original pumps stiffening up when unused (all models) and that stiffening being attributed to the Mazak "swelling"?
My initial thoughts would be bollocks.
Oxide growth certainly but swelling of the alloy at room temperature not top on the list of possibilities .
Zinc based alloys do not adsorb much hydrogen from the atmosphere as aluminium alloys will.
And even if they did, energy has to be applied to allow the trapped gas atoms to travel through the lattice structure then bump into each other and for a hydrogen molecule.
Then you need thousands of these events to put enough strain on the lattice to cause the layers of atoms to slide over each other and cause the item to grow.
OTOH it does not take any energy for the surface of the metal to oxadize, in fact it takes energy to prevent this happening.
The corrosion product of zinc based castings can have a variety of structures depending upon how many water molecules get included.
If there is little water the corrosion layer is translucient and as such quite hard to see with the naked eye .
You do not see the oxide layers that have formed on your carb body & slide on concentric carbs but it is sufficient to lock the slide in place then rip lumps of zinc off the surface of either the carb or slide. The process is called stiction ( joining together ) & gauling ( ripping surface atoms off )
Now this happens between a body & slide where the space is around .005".
It will also happen around the hole in a pump body where the running clearance is in the order of 0.0005"
The number 2 culprit would be plain old slump.
Nothing that we think is solid is actually solid and over time nothing greater than gravity will cause everything to go "pear shaped".
It is just some, like glass & lead will do it faster than others like steel & titanium but they all do it
So if stored for long enough not properly supported I would expect pump bodies to distort over time.
Even the Platinum-Irridium bar which used to be the standard meter distorts over time which is why it got replaced with an atomic measurement, and even that has found not to be as stable as we had hoped for so was reciently upgraded again.