Is Oil Purge Required?

Thank you very much to Rotax Wizzard and Roger Lee for your answers,

but they do not directly address my questions.

That is why I asked ChatGPT:

1. Yes, the hydraulic tappets fill themselves automatically if they have lost some oil, but they cannot empty completely.

2. Damage to the engine, even with soft hydraulic tappets, is virtually impossible when turning the enginge with the starter without ignition.

Background: Due to their design, the hydraulic tappets cannot empty completely. They are mounted above the engine oil level, so partial emptying would be theoretically possible (but a small built-in valve should prevent this). They are supplied by small oil channels. Turning the engine by hand generates a pressure of approx. 1 bar, which is sufficient to supply them.

It is still unclear how long this takes to fill the tappets. It can be assumed that some engine starts only occur with partially filled hydraulic tappets, which then fill completely in the next few seconds or minutes (if they were partially emptied at all, the rattling of the engine then stops or becomes less noticeable).

In this sense, checking the engine oil level by turning the propeller is doubly beneficial: you can check the oil level and ensure minimal oil transport through the oil-carrying lines and bores.

The load on the engine, whether turned by hand or with the starter without ignition, is only slightly higher with the starter; after all, an engine is started with the starter and usually the first turns are not under fire. After all, oil changes or repairs to the oil system are not something you do every day.

It is completely undisputed that engine damage during operation (ignition on) with permanently partially empty hydraulic tappets is caused solely by the inadequate timing of the valves, and the contact surfaces of camshaft and tappets wear out.

The Rotax instructions (turning by hand or compressed air) are correct,good and the first choice, even if they do not explain the background. However, if this does not result in the desired pressure build-up or if the hydraulic tappets cannot fill completely, you can “crank” the starter (without ignition) for a few seconds without any major concerns and without immediately causing engine damage.

Greetings

Oil purge vs venting of the system...

A venting of the system is what we do for engine oil checks when sitting may have allowed oil to accumulate in the crankcase.  This is due to the fact that the oil tank, dry sump system, is mounted above the crankcase and crankshaft.  Due to this in some cases oil may migrate via small clearances past the oil pump and into the crankcase.  As a result the oil volume is not correct in reading a dipstick at the oil tank on a cold engine first start of the day or after sitting for several days or even weeks.  The venting is carried out by simply rotation of the engine, in the correct direction, in a slow manner allowing the piston pressure to push the residual build-up of oil into the oil tank.  This has no effect on lifters, which one full of oil should remain so.  Some lifter bleed-down occurs from the valve train pressure on the internal valve lifter parts but will be insignificant overall.  

Oil purge is different.  When you void some of the oil supply in the supply line, from the oil tank to the oil cooler, to the oil pump, you may then be forcing air into the system and this can create air to be trapped inside the lifter.   Air inside the lifter body can lead to loss of contact of the lifter to the cam face, the results may or may not be catastrophic depending on the duration it takes the lifter to inflate with oil and regain a positive contact to the cam face.  

New engines, engines with new oil lines, new oil cooler, anytime you introduce air into the suction side of the oil system require an oil purge.  i would recommend everyone watch the Rotax video on the subject to understand the methods to carry this out.  

Cheers

https://www.youtube.com/watch?v=O1GPjPZCXd4&t=174s