Inspection authorized mechanic reported during troubleshooting of the EC-130 NR over-speed warning and metal contamination of the upper cyclic and main gearbox; they found the NR sensor shim pack installed incorrectly.

Narrative

On or about Day 0. Aircraft X was in repair under work order. During the Alterations and maintenance performed on the aircraft. The Mast Assembly Inspection was performed. In reference to Airbus EC130 AMM 62-31-00;4-3. Maintenance was performed with Person A under the supervision of Person B the mechanical completion lead here. The inspection was completed and aircraft reassembled and was released for ground runs and maintenance flights by me on Day 17 and returned to service initially on Day 22 by Person C. Work order was closed aircraft later departed on Day 28. Aircraft arrived at the base and later was discovered by the field technician during troubleshooting. That the Mast NR sensor shim stack-up was installed on the incorrect side of the NR sensor bracket. Causing contact with the Phonic wheel. Resulting in the generation of NR overspeed warning and ferrous metal contamination of the upper cyclic and main gearbox assembly. I was made aware of the incident from Person D operations manager on Day 43. In a detailed review of the maintenance it was discovered that the tech Person A signed for the installation of the NR sensor; but also had marked the shim measurement process as N/A. Person E is a repair station QC (Quality Control) inspector supporting Person C. During some of the maintenance activities on that aircraft. Neither QC inspector detected the incorrect installation of the NR sensor shim stack. Thus leading to the damage and subsequent maintenance on the aircraft. During the maintenance buildup Person B and myself with Person A discussed the proper process and informed him that it a few minutes to check the depth and the previously installed shim stack of the NR sensor. Suggestions: It is my recommendation that no one ever marks a shim check procedure from the manufactures maintenance manual as not applicable. Also makes sure we get a second set of eyes on and bought back by QC; by following the repair station critical component procedure. No matter if the shim stack coming off is eventually reused; to verify the shimming is correct.