Should Empty Stator of a Tacho be Covered with Metal?

the stator must not leave open

This makes no sense.

Please use a language translator to get your point across.

I am sorry for my poor English.

"the stator must not leave open"

I want to say that when we open the tacho, the magnet of stator must be covered by some MS pieces.

Your statement was understandable, even though "official grammar" would have been "the stator must not be left open."

Analogous to the magnetic "keeper" already mentioned, solenoid coils that are removed from their armatures/plungers should either be deenergized or have a soft iron bar or bolt inserted to keep from overheating.

We used to call that a "Keeper"

Magnet keeper

From Wikipedia, the free encyclopedia

A magnet keeper is ferromagnetic bar placed across the poles of a permanent magnet that helps preserve the strength of the magnet by completing the magnetic circuit, and are particularly useful for magnets which have a low magnetic coercivity, such as AlNiCo magnets.

Keepers also have a useful safety function as they stop external metal being attracted to the magnet.

For the magnet systems that suffer from this sensitivity (not all do, it depends on magnet type), it's not enough to put some metal after you have emptied the rotor space. The magnet will be already deteriorated. The space must NOT be left empty for any length of time. This applies to some steppers and PM motors also. This can be accomplished ONLY by sliding a metallic ring (of the same diameter) CLOSELY following the rotor so that the magnetic flux is not severely interrupted. That's also how the magnet part is treated before assembling. S.M.