momentary relay

Hi All, 1st post, I am restoring an old 1964 fire truck that has a brass bell with a 12 volt Heavy duty acutator, Looking at the relay pict's for momentary switching the capacitor size is not listed. I will be running a 60Amp relay, the orignal capacitor from the truck was a 4uF 400VDC 640W paper capacitor, what should I use as these are no longer available. Thanks AndyAndy

Fire Trader Australia

The capacitor allows the coil of the relay to be energized until the capacitor stores a charge, thus de-energizing the coil. The resistor bleeds off the charge of the capacitor when positive voltage is removed from the other side of the coil. You can increase the output time by simply changing the value of the capacitor. This one will give you about a 1/2 second output. 

How does that 60A relay in your application operate? Is it supposed to be a momentary output controlled with a switch? The 4uF, 400V (not sure what 640W means in relation to a cap though), may be connected across the contacts to prevent arcing and burning of the relay contacts from the big bell, but that doesn't do anything for timing. Are you sure you want a momentary relay?