Oil return line

I have never seen a 7/16" oil hose on a Rotax 912 series. I would question whomever did it and change it.

Fred

The engine uses the piston blow-down pressure (pressure bypass of the rings) to push the oil back to the oil tank.  If the return line from the block to the oil tank is too small, or restricted in any way, it will then build pressure inside the crankcase.  If the pressure is too high it will push the seals.  The test has to be done on the ground and not in flight. 

Cheers

I bought fittings and hose from leading edge . The only hose they offer is 12mm. There is only one fitting size offered by rotax so what size are the fittings 12.12.5 or 13mm?

If you purchased the 840461 Fitting for the Oil Pump, it is listed as " NIPPLE 13.2/9.5-M14x1.5 " 
It is M14x1.5 on the Pump Side and 13.2mm OD and 9.5mm ID on the Barbed end.
12mm hose will give you a good tight fit.
I can only guess that the Oil Tank Fitting will be similar.

If it doesnt work Ill have to look for large ID hose. Maybe the higher HP engine needs it. I also looked into custom  Teflon hose but they said they only have 11 MM.