Which one should I use?
 

My daughter has a 56 bird that needs a solenoid. I have 3 old ones. Questions are: How can I tell if a solenoid is for 6 Volts or 12 volts? How can I test a solenoid to see if it is good or not?
Thanks in advance for the help.

Re: Which one should I use?
 

Most 12V solenoids I have seen have 12V stamped on the cover. 6V solenoids are typically not identified.
You can also test with a 6V battery as well. Ground the case and apply power to each wire. One will cause the solenoid to engage, somewhat violently. A 12V solenoid on 6V is a lot less vigorous, hopefully you have one of each to compare.
Don't forget to twist the solenoid when installing it to line the shaft up with the pawl inside the transmission.

Re: Which one should I use?
 

perhaps I missed something. Are we talking about a starter solenoid or an overdrive solenoid?

Re: Which one should I use?
 

That is my question too.