Q: I'm putting a 2004 3.7 engine into a 2002 jeep liberty and I understand that I have to change the reluctor ring but is it 16 or 12

The two engines you cite use completely different computers (PCM/ECM) and the casting numbers are completely different. You can try it but you are all but guaranteed to have start up and operational problems due to incompatibility with the existing engine computer. As far as the crank trigger wheel, on the 2004 engine, there are 32 teeth, while both versions (two different engines were used in 2002) of the 2002 3.7L engine used a 12 tooth wheel. Inasmuch as the engine is basically run by the computer, the allowed interchanges are only those listed in the OEM catalog. If you use a non interchange engine, you have to change out the vehicle wiring harness and the computer and even that is not guaranteed because all of the other microprocessors on the vehicle, such as the immobilizer system, won’t recognize the new PCM. It is not just a software issue either as the actual hardware in the PCM is different in the two years. If you need a replacement engine, you will find an EXACT interchange, guaranteed to be compatible, if you search at Car-Part.com. If you are interested in an engine installation (OEM to OEM), YourMechanic professionals in certain locales can assist with that. Please simply inquire based on your locale. If you have further questions or concerns, do not hesitate to re-contact YourMechanic.