The International Cessna 170 Association

Google "is an stc a major alteration"

"Any STC is, by definition, a major alteration and also requires a 337 form.Oct 8, 2014"

AOPA https://www.aopa.org/news-and-media/all ... 37%20form.

Several other forums agree.

Tanis STC SE03309CH the second letter is "E" so it is Engine for the 337 and logbook. If it was "A", it would be an airframe STC.

An STC requires that the instructions are followed and a 337 is filed signed by an IA along with the logbook entry.

Then it is treated as any other certified part on the engine subject to SBs, ADs, etc.

If there isn't an STC, then the A&P/IA determines the method of compliance. Example: EZ-Heat Engine Pre-heater claims that it is "FAA Approved" and no STC is required. EZ-Heat is cheaper and they have a model that fits the A-65 so this might be the one Bruce wants.

Ez-Heat has a letter from the FAA stating it does not require an STC and it is not a major alteration. https://www.e-zheat.com/_files/ugd/58bb ... 8f35d3.pdf

An Ez-Heat pad for a small Continental and the one for a C-145 is $189. The C-145 model is 285 watts. I've seen a KATs 250W model for $140 available at NAPA and other places and a lot less for half the watts. Hmmm. Ez-Heat is specifies a 240W for a 4 qt A-65 kidney , KATS specifies only 150w for a tank that size. I'm getting by with a KATs 90w dipstick heater in my A-65 and that requires no log entry at all .