air in the hydraulic tappets and engine damage

Watching.  I'm also curious to know what evidence of an imperfect purge is observable to the owner without turning a wrench.  If a purge isn't quite 100%, will it remain that way?  Or will it get better/worse with time?  How will observable behavior change?

Was suction rated hose used? I’ve seen it before with the hoses getting sucked flat

Please read everything below and see the attached photos.

In ROTAX Service Instruction SI-912 i-004R1 / SI-915 i-003 SI-912-018R2 / SI-914-020R2 https://legacy.rotaxowner.com/si_tb_info/serviceinfo/si-912-018r2.pdf, It states the following: 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. These inspections have to be performed - before the first engine run, - after re-installation (after overhaul), - after the lubrication system is opened and drained during maintenance work (removal of oil pump, oil cooler, or suction line). NOTE: Not affected are the removal and replacement of components that do not drain the oil pressure galleries. These maintenance instructions shall be considered at any maintenance events, retrofitting, repair and overhaul.

Air can also enter the oil system if the oil tank pickup tube becomes unported. The oil tank pickup tube can become unported if the aircraft is operated at zero gravity and in negative “g“ condition for more than 5 seconds at max. -0.5g. The oil tank must also be mounted in the correct position and within the set limitations of the Z & X axes on the engine (See Rotax installation manual Section 79-00-00 Page 20. https://www.rotax-owner.com/pdf/IM_912_Series_Ed3_R0.pdf.

If the Oil cooler/radiator is upside down (connections on the bottom), it can trap air in the oil system OR if the oil cooler is not mounted in the incorrect postion within the Z axes on the engine (See Rotax installation manual Section 79-00-00 Page 22 https://www.rotax-owner.com/pdf/IM_912_Series_Ed3_R0.pdf

With ALL that said, is it possible the oil lines were incorrectly connected or internally blocked in some way? Were the oil lines properly routed and not up against the exhaust system resulting in a burned hole? Did you have any oil leaks? Were the pushrod tube O-Rings changed recently? 

If so, it is possible that one of the oil pushrod tube O-Rings was left behind in the lifter bore when the cylidner was removed. If new O-rings were installed on the pushrod tubes and an O-ring was left in the lifter bore, it would double up the O-rings and cause blockage of some oil galleries (see photos). A pinched pushrod O-ring can also cause a similar issue.

Please keep us informed on what you discover. 

Rotax Owner - I am very interested in the Rotax instruction " The oil tank must also be mounted in the correct position and within the set limitations of the Z & X axes on the engine (See Rotax installation manual Section 79-00-00 Page 20. "

My interest is related to my oil cooler installation, which is at 90 degrees to the sketch/advice.

My questions are:

# Why is this important?

and from the above

# What are the potential impacts from such a deviation?

Thanks a lot for your quick and detailed asnswer. Before the failure there was no cyclinders detach or overhaul. Engine has only around 120 hrs from new  and had only routine maintenace (oil , filter, plugs, carb sync etc). engine ran perfect.

Problem arise immediately after mandatory changing tubes of oil circuit (5 years interval).

Immediately after tubes change engine showed very low oil pree (before was normal). After this low press oil problem rotax tech checked oil tubes and they were free of flow restrictions. Oil tank was ok. Connection and routing of tubes was right. No oil leakage. oil radiator  has no obstrucions to flow. The only mistake of rotax tech was to not pay attention in the purging phase with no check of hydraulic lifter/tappets 

i know that rotax doc says there is explicit risk of engine damage in cause of incorrect purging of hydraulic lifter but i'd like to ask to forum expert how can a collapsed hydraulic lifter destroy an engine without other signs other than low oil pressure.. 

in my limited knowdledge a damaged valvetrain do not open valves in the right amount with a big power loss. in our case engine power was ok and only symptom of engine damage was low engine oil press (near low operating limit with warm oil).