Push in Fitting with M5 thread on one end and 4mm O.D. push in connector on the other. Used on compressor covers to connect all the fuel, oil, pressure and starting gas lines to the turbine. The Push in Connector makes it easy to quickly disconnect the line from the fitting.
This size is mostly used on the larger size homebuilt turbines in the 18-30 lbs thrust class.
The through hole is an internal hex so it’s easy to bolt on or remove.