Are these Lycoming engines interchangeable between piper cherokees?

Question

O-320-E3D

O-320-E2A

Answer 1

The answer is going to be found by referring to the Airplane Type Certificate Data Sheet (TCDS), which lists all the approved engines for the PA-28 series of aircraft. You can download that here:

http://www.airweb.faa.gov/Regulatory_and...

Click on the red Adobe PDF symbol.

Answer 2

Some are some are not.  Depends what engine was on the Type Certificate of the airplane.

The reason for the different dashes after the 320 is of course a model number, but what it usually meant is how the accessories where mounted and little changes to the engines (often required to fit in a particular manufacturers cowling).

Now in the case of your O-320-E2A which was in the Cherokees until appx 1974 when they switched to the O-320E2D you can switch the engines.  This used to be a simply logbook entry - but the FAA is becoming a pain and they may require a 337 modification form be done.  But it is no big deal, check with your IA/A&P some FSDO office will just say do it and other will be a pain.  Your mechanic should know if you have an intelligent (easy to work with FSDO) or a pain in the @@@ FSDO.  

If you have a difficult one you can do what I had to do once and have the work done in a different FSDO's area (one that understands airplanes).  In my case it was simply flying the airplane 25 miles away to get out of Oaklands FSDO and into Sacramento's FSDO.