Oil purge after gearbox install?

Do a purge!

With the exception of a standard oil/filter change, any opening of the oil system (full drain, hose replacement, etc)  should be followed by a purged  - for peace of mind if nothing else.

Sooo easy and quick to do - why would you not?

Keep in mind that the engine crankcase is exposed to normal atmospheric pressure whenever the engine is not running. Air will enter the induction system and then slip past the piston rings. Sure, this is a bit of a slow process, but the pressure inside the engine (and gearbox) is equalized with outside air within moments of shutting off the engine. With this in mind, you can see how removing the gearbox does not do anything to void oil from the system. There are no pressure changes, and no oil is drained from anywhere that would not normally drain while the engine is sitting unused.

So that's the reason it's not required in the manual.  However, there may be other reasons to purge the oil after replacing the gearbox.  For instance, if you suspect the gearbox was making some wear metal, then you should flush the oil system as per the MML, and then purge once complete.  I personally think swapping the gearbox is an excellent time to flush the oil system. This assures that any metal found on the magnetic plug after the new gearbox is installed is not from a previous problem.  So, Sean and I end up at the same answer.  

Typically, purging is required when you open the system on the suction side of the pump, up to and including the oil tank.  And of course, if the motor is fully drained of oil during a maintenance procedure. One of the most common reasons to purge the oil system is if the motor was turned over by hand with the oil tank drained, which can suck air into the lines running from the oil tank to oil pump. At that point, the pump can essentially lose its prime. 

In 23 years I've never done an oil purge when ONLY removing or installing the gearbox and I have never in all those years heard it or seen it taught in a Rotax class. The amount of oil that spills out isn't that much. No different than putting on an oil filter without pre-filling it. Other than the small amount in the gearbox no oil has drained from any hose, lifter, pump, oil filer, ect... The only oil that comes out of what's left over in the gearbox. The gearbox is a huge hole space when it comes to oil quantity. There is usually only a small amount in the bottom of the gearbox so other than that amount no other oil comes pour out. Once the gearbox is back on then you can do a vent process just like when doing a normal oil change.

Pulling the gearbox off only dumps the oil from it and not from anywhere else where it's important to make sure that there some oil volume.

Hi All

As Jeff and Roger have pointed out there is no call out for purge of the oil system when a gearbox assembly is removed and replaced.  The suction side, from the oil tank to the oil pump, is not breached in a gearbox R and R.  A gearbox gets its oil via the crankshaft center feed to the gearbox bushing.  When removed on a stopped engine there is no oil pressure and therefore no oil flow or oil bleed in the system.  There is only a small amount of residual oil from the running prior to shutdown.  The oil galleys to the lifters do not drain and that is the critical concern when we look at what would be cause for adding an oil purge to any work.  Remember that the prop shaft is not getting positive oil feed, it is only feed oil by spray from the gears (from the oil weeping by the nose bushing and on gearboxes newer than 2018 from an oil spray off that same said bushing that is directed to the prop shaft spline area.  When the engine stops there is no longer any pressure so the oil stops flow and I have never seen one leak oil even with the gearbox off in the shop.  

The main point to look at is keeping the oil pump supply line in prime, full of oil, to not have any air lock on the suction side of the supply.  The return line is always full of air and has no bearing on the supply as this air is removed in the oil tank by the air/oil screen inside the tank by design.

Cheers