Rally gauge issue has me puzzled
 

Im in the process of restoring a 1970 GTO. Im working on the dash and noticed the oil pressure gauge needle was gone. I figured it broke off. Just to be sure I took the cluster apart and found the needle intact laying on the bottom of the gauge. It simply presses on so it can go on anywhere. How do I know if Im putting it in the right spot? Thanks in advance.

Leave the ignition off, then put the needle on pointing at zero. Once you do that, turn the ignition on, but don't start the engine. If the gauge is still pointing at zero you are done. If not, move the needle and repoint to zero. The final step is cranking up the engine with a second (mechanical) oil pressure gauge attached to a T where the electric sending unit is. Compare readings. The mechanical gauge is probably right.

Thanks for the info Bill. The gauge is sitting on my bench and the car is all apart. I was wondering if there was a bench test I could do. I have a variable power DC power supply and a volt meter.

I would see if a member could post a pic here for you so that you have a reference point.

The sending unit itself is a variable resistor as are all the gauge type sending units.
Higher oil pressure translates into high resistance.

At 0 PSI - 0 ohms
1/2 gauge reading - 45 ohms
full gauge reading - 90 ohms

Using a 12- 13 volt volt power supply and adding resistance to the circuit should put you in the ballpark.
The only other variable would be the accuracy of the sending unit. To check that you would need to compare against a mechanical gauge

The pointer detached is a sure sign the gauge has lost all of it's internal buffering fluid. That gauge will never last, missing that fluid.

Thanks. That's the info Im looking for.