Here's a good thread on the subject >
https://www.fordbarn.com/forum/showt...denser+reading
"Almost all automotive distributor coils use a 0.25-0.29 microfarad capacitor. The operating voltage rating should be no less than 600 volts DC. There are now capacitors available in the 0.25 microfarad range rated at 630 volts AC. Since the AC peak voltage is 1.4 times the nominal value these capacitors are actually 880 volt DC capacitors making them ideal for ignition use."