Narrative:

In performing a replacement of all six cylinders on the engine due to each cylinder having low compression; the cylinders were removed and sent for overhaul/repair to X/X aircraft repairs. The cylinders were returned after rework and also supplied by X/X aircraft were new pistons; piston pins; and a gasket/O-ring set for each cylinder. The gasket set included two o-rings for each fuel injector; however I made the mistake that both o-rings were to be used on this engine. The teledyne-continental io-550 engine (non turbocharged) only requires one o-ring; the exact same cylinder is on a tsio-550 (turbocharged) engine and it requires two o-rings. However; the same fuel injector is run on both turbocharged and non-turbocharged engines. There is a groove for two o-rings and I mistakenly installed two o-rings on the non-turbocharged engine. The fact that the same cylinder and fuel injectors run on two different engines of this nature and the fact that the gasket kit for this cylinder overhaul included two o-rings; I fit both the grooves with an o-ring.I now know that the book does only call for one o-ring on the non turbocharged engine and two for the turbocharged. This mistake was done completely unintentional. The combination of the above factors caused the mistake noted in this report. The owner called to notify me that the engine was running rough when he departed from ZZZ and he knew there was a miss in the engine but he continued to fly to ZZZ1. He stated that there was lower than normal apparent power on takeoff from ZZZ. He also said the same thing about departing ZZZ1 and he also said it was still running a little rough; but he continued to fly to ZZZ2. While in ZZZ2; he had a technician look over the plane and he [technician] found the two o-rings installed on the fuel injectors. He also stated that the spark plug had also quit working. The aircraft is being worked on now in ZZZ2; there was no accident or incident involved with the situation. This is the only situation I have ever been in like this in over 20-years of aircraft maintenance and the only reporting form I have filled out.

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Original NASA ASRS Text

Title: An Aircraft Maintenance Technician (AMT) Inspector was informed he had installed two O-Rings on each Fuel Injector on a Teledyne-Continental IO-550-N; Non-Turbocharged Engine where only one was required. The Cessna-350 aircraft engine had run rough on three flight legs. Two Injector O-rings had come with a Gasket Kit for each of the six cylinders he replaced.

Narrative: In performing a replacement of all six cylinders on the engine due to each cylinder having low compression; the cylinders were removed and sent for overhaul/repair to X/X Aircraft repairs. The cylinders were returned after rework and also supplied by X/X Aircraft were new pistons; piston pins; and a Gasket/O-Ring set for each cylinder. The Gasket Set included two O-rings for each fuel injector; however I made the mistake that both O-rings were to be used on this engine. The Teledyne-Continental IO-550 engine (non turbocharged) only requires one O-ring; the exact same cylinder is on a TSIO-550 (Turbocharged) engine and it requires two O-rings. However; the same fuel injector is run on both Turbocharged and Non-turbocharged engines. There is a groove for two O-rings and I mistakenly installed two O-rings on the Non-turbocharged engine. The fact that the same cylinder and fuel injectors run on two different engines of this nature and the fact that the Gasket Kit for this cylinder overhaul included two O-rings; I fit both the grooves with an O-ring.I now know that the book does only call for one O-ring on the Non turbocharged engine and two for the Turbocharged. This mistake was done completely unintentional. The combination of the above factors caused the mistake noted in this report. The owner called to notify me that the engine was running rough when he departed from ZZZ and he knew there was a miss in the engine but he continued to fly to ZZZ1. He stated that there was lower than normal apparent power on takeoff from ZZZ. He also said the same thing about departing ZZZ1 and he also said it was still running a little rough; but he continued to fly to ZZZ2. While in ZZZ2; he had a Technician look over the plane and he [Technician] found the two O-rings installed on the fuel injectors. He also stated that the spark plug had also quit working. The aircraft is being worked on now in ZZZ2; there was no accident or incident involved with the situation. This is the only situation I have ever been in like this in over 20-years of Aircraft Maintenance and the only reporting form I have filled out.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.