Oil gauge restoration/calibration?

I have an original oil gauge from an R5 that needs I think some work? It provides a maximum reading (needle slams to right of dial and stays there) regardless of the actual pressure. I get a reading of ~25 psi (oil is hot) with other gauges that I have hooked to the R5 which I assume is about correct.

Any recommendations on a place to restore and calibrate this style of gauge (assuming ~25 psi is a good pressure for an R5)??

Thanks for any help

Ross

Should be some information on gauge restoration if you search the "Archives". Was always to "pricey" for me so I didn't capture the source.