Let's think about this tank removal and cleaning and purge. When you do a normal oil change you drain the tank. That also means that any oil in the fittings on top of the tank are drained.
So if you disconnect both fittings and hold them up or put a plug in them then you don't loose any oil. You have drained no more oil than a regular oil change from the tank only. I always prefill my oil filter and when everything is back ready to go with oil then you can always vent the system.
When you put the fittings back on then why would you need to do a purge? You never lost anymore oil than the normal everyday oil change.
Purging should always be done after a hose change or any procedure that causes a large loss of oil or air was introduced into the system. When I purge I'm parinoid enough I do it twice. :) I do it pretty much like Rotax describes in the SB including removal of one plug per cylinder. I do rotate the prop about 70 times in a minute than get some oxygen LOL and then do it again. I can generate about 55 psi on the instrument panel, always have oil come out the oil retun hose and have never had a soft valve.
p.s.
During a purge I do add that EXTRA .5 liter to the oil tank to fill the air voids and not to get too low on the oil in the tank that may inject air further into the system.
p.s.s.
When in doubt it never hurts to do a purge.
If there's another side of the story I'd like to hear it.
SB quote.
Reason
This document should provide further guidance for the lubrication system with respect to:
- purging
- venting
- to avoid air in the lubrication system
ROTAX® was informed of a limited number of engine failures in the field resulting from a lack of
proper oil purging after the engine had been first installed and /or the engine had been re-worked.
This Service Instruction should help to make sure that the engines do not suffer such engine failures
in the field. As air can be trapped in the valve tappets and cause valve train failure it is very
important to complete these instructions in their entirety.
