Engine Overspeed Recording Devices

[Helifan]

Hi

Does anyone know of any device out there that records engine overspeed occurances for piston engined helis like the Schwitzer or Robby?

I am looking for a way for aircraft owners - who have aircraft leased out to schools or charter operations - to know when their aircraft have been subjected to a overspeed that borderlines on the possibility of serioius engine damage and yet goes unreported.

Any clues???

Thanks