See AC 43-210 about determining whether or not you need a 337.
y. Major Alteration. An alteration that is not listed in the aircraft,
engine, or propeller specifications that also fits one or more of the
following: An alteration that might appreciably affect airworthiness by
changing weight, balance, structural strength, performance, powerplant
operation, flight characteristics, or other qualities affecting
airworthiness; is not done by accepted practices; or cannot be done by
elementary operations.

z. Major Repair. A repair that, if improperly done, might affect weight,
balance, structural strength, performance, powerplant operation, flight
characteristics, or other qualities affecting airworthiness, or is not done
by accepted practices or cannot be done by elementary operations.

Jim

"Robert M. Gary" wrote in message
ups.com...
http://www.planeplastics.com is telling me I need to get a 337 to
replace my plastic air vent covers. Is that true? What would an
inspector be looking for? Too much cooling for the backseat and too hot
up front?

-Robert