Mechanic with Inspection Authority (IA); reports on the confusion surrounding a Supplemental Type Certificate (STC) that allowed the use of a Continental IO 470 Non-Var crankshaft in a IO-520 series engine.

Narrative

Built a Supplemental Type Certificate (STC) motor for individual; repair shop; which converted an IO-520 series motor to an O-470-50. FAA just informed me I put an unairworthy crankshaft in this aircraft; back in April 2004. Aircraft was crashed (not due to engine problem); hit a tree on approach. Salvage sold aircraft; person who purchased it brought complaint to FAA. The crankshaft was sent to repair shop who did not certify due to it being a non-VAR crank. The Airworthiness Directive on non-VAR crankshafts does not apply to O-470 series engines. I purchased a 4 counter weighted crank; same as a VO-470 or O-470-U crank and had crankshaft repair facility certify it was approved; except not in a 520 series engine. I did not sign Return to Service in a fraudulent manner. I complied with the intent of the STC. Also; other builders have done this before; to include the original STC developer. No previous problems in the maintenance field. Note: Unqualified people obtained this engine through salvage; and disassembled a destroyed engine. Callback conversation with reporter revealed the following information: Reporter stated the STC allowed the modifying of a Continental IO-520 engine that involved changing the IO-520 pistons to lower compression pistons; removing the fuel injection system and installing a carburetor with a larger venturi and an IO-470 Non-Var crankshaft. Reporter stated the term VAR; refers to a Vacuum Air Remelt process that was believed to be a stronger and cleaner process; with less impurities in the metal; which is then machined into a VAR crankshaft. An Airworthiness Directive (AD) did not allow for Non-Var crankshafts to be installed in a Standard IO-520 with fuel injection; but the STC did; when the modifications were accomplished. The IO-520 VAR and IO-470 Non-VAR crankshafts; applicable to the STC; appear identical and have the same part numbers. The VAR processed crankshaft should have a welded VAR marking on the crankshaft. Reporter stated by derating a higher horsepower IO-520; using the approved STC modifications; allowed a longer operating life for the engine. Each crankshaft costs approximately $4500 dollars. Reporter also stated the VAR type crankshafts have just as many failures as the Non-VAR type.