Heat sending unit

[54manhattan13]

 I feel I can't trust the temperature gauge on my 54 Manhattan (it never moves).
 I see a temperature sending unit for sale on eBay but it says in the discription that it fits 1951-early 1954 Kaisers and Darin's.
I checked with my local NAPA store and he can't tell me why.
Does anyone know about a change in temperature sending units from the early 54 Kaisers and the 54 Manhattan and later 54's?

[joefrazer]

It's because the dash gauge on the 54-55 cars has a different calibration than the 51-3 cars.