I used a D7 for some years as my main combat 15.  A really great engine - started well and ran like Jack the Bear by the standards of the day.  Also stayed together, although it needed a new rod once in a while. More recently, the engine got banned under our area vintage diesel combat rules (it would simply bury any PAW or Oliver), but I still have an example in an own-design wing which I fly once in a while just to fly for fun.  It still goes as well as ever and runs great!