*We have a customer who has a hydraulic press for testing ball bearings, and they require a replacement pressure gauge to suit, however the model is now obsolete. We think we can work out what pressure gauge is needed based on the bore size of the cylinder, is this correct? The gauge will need to read in metric tonnes for a 20 metric tonnes press, but I am being told we need to know the effective area so we can calculate the psi required to achieve a 20 metric tonne force for the press.*

Yes, if you can find out the effective area or diameter of the bore, then you can calculate the maximum required pressure, so the range of of the pressure gauge can be selected and markings added on the scale for metric tonnes.

If the effective area of the cylinder bore is known then use this calculator:

Force & Area to Pressure Calculator

If the effective diameter of the cylinder bore use this calculator:

Piston Cylinder Force & Diameter to Pressure Calculator