Withholding all laughter for someone new to purchasing airplanes I came across the following situation.

1956 PA22-135 converted to a O320 150hp. I noticed when reviewing the paper work and logs there was no STC paperwork for this modification.
The seller said "that's the way I bought it"...
Just wanting to cross my Ts and dot my lowercase Js.
Does the 1956 PA22 require an STC for that modification since the 0320 was one of many "engine options" under the PA22 type certificate?
If so how do I check that?
If it's not under the same type certificate do I need to buy new STC paperwork from WagAero to make the modification legit?

I'm feeling bold this afternoon. I'm gonna say...

That engine is on the type certificate (same TC for all PA-22 models and configs). Nothing more than a log entry.

Last century STC was not required. I had a PA-22 ramp checked and grounded at Coronado Airport in Albuquerque by ABQ GADO. About six annuals prior when the mechanic installed an O-320 he scratched through the 135 after PA-22 on the original airworthiness and pen and ink entered 150. The original airworthiness must stay the same and it is an error/violation to change it. Who knew?

OK, the back story. When I left Gallup at sunup on a Saturday, GUP Flight Service Station asked where I was going. I think my nemesis Mr. Runyon at GADO thought I was in a different Tri-Pacer. How many of you have been ramp checked at 0730 on Saturday? My wife had to come from Tohatchi, six miles from Arizona, to pick me up. Margaret Lamb, my lawyer, made Mr. Runyon give me a new original airworthiness on Monday. She is one tough lady pilot, author, and lawyer.

Anyway, no mention of needing any STC.

Check the "serial numbers eligible" on the PA22-150 portion of the TC. As long as it falls in that range, ( Serial Numbers Eligible 22-2378, 22-2425 and up (Normal Category). See NOTE 3 for Utility Category.) then you can (with the proper equipment, according to the TC) convert to that configuration. If it's NOT in that serial number range, you need an STC.
John

Post question on BCP and become in enlightened
As always thanks!

p.s. the serial number does not fit in any of the PA-22-150 or 160 configurations. :/