Is Oil Purge Required?

Hi experts,

If I understand the discussion correctly, it mainly concerns the hydraulic tappets. Soft hydraulic tappets indicate a lack of oil in the tappet.
What happens, for example, after an oil change, after oil lines have been open for a long time, or after soft hydraulic tappets have leaked, if you turn the engine with the starter until the oil pressure is normal—without ignition?

1. Do the hydraulic tappets then fill themselves?

2. Can soft hydraulic tappets cause damage just by turning the engine with the starter motor?

It is pointed out that engine damage could occur, but it is not explained what exactly happens. The valve springs try to keep the valves closed, so no valves should reach the combustion chamber if the hydraulic tappets are soft - in starter mode only. I am right?

Of course, soft hydraulic tappets cause damage when the engine is running with ignition, but what happens if there is no ignition in the combustion chamber?

Greetings

Hi HKK

The risk is when you start and it is under power, fire in the cylinders with peak pressures.  Cranking with the starter is not a problem.  Under high compression the lifter to cam has to have the lifter full of oil or it may slam into the cam or valve arm as it is not yet pumped up for zero lash.   This can lead to bent pushrods or valves that may float without control.  

Think of it this way, cranking speed of the starter on the engine RPM is only about 250 (about 100 prop speed) When it does start we have a absolute min of 1400 (most run 1600 to 1800 crank) for idle speed.  The vast difference is where you can get into trouble when the engine fires up on its own power if you have any lifters that are not inflated with oil.  

Cheers

Seems like using the starter for an oil purge is overly hard on the starter, sprag and battery. Hand powered doesn't wear anything out and it works.

Roger, yes, excellent point.  It takes very little to do it by hand actually.  I am unsure why some try to avoid it.

Cheers

When doing the first purge on my 914 I turned the prop by hand over and over and over until my arms were ready to fall off, without getting any pressure. I connected to compressed air (as shown in the video) and I got pressure almost immediately.