Hi!
The flexible header pipe (from Krumscheid) I first used on my MVVS 45cc engine began to leak black oil the first time I ran the engine.
Instead I built a rigid header with straight and 90 degree steel tubes from Krumscheid. This has worked now for 5 years without any trouble what so ever.
Header lenght is aproximately 30cm but it was built not with any specific lenght in mind, just to have a nice curvature and fit the airplane and engine cowl.

It turned out to work very good. My 45cc MVVS engine with the small MVVS cannister silencer, turns a 24x8 Mentz prop at 6400-6600rpm on 95 oktane fuel using 2,5% all synthetic oil.