912 fails in French Alps • VL3 Evolution • 2020

[Garfly]

 

 

Some great video story-telling, including air-to-air shots from friends flying in company with the crippled VL3.

 

 

 

 

The culprit (on a 750 hour engine):  

 

[facthunter]

Exhaust valve stem failure. The head of the valve must have floated around in there for a while. It's made it's impression.. ALL engines can fail. Particularly Piston engines.  Nev

[johnm]

the old boy (father) in the camera plane had great eyesight ................ never went to specsavers

[danny_galaga]

😲

[Garfly]

Speaking of exhaust valve failures, Peter Anson, writes here about his Sonex's Jab self-destructing.

The outcome, luckily was very similar to that of the VL3's and its Rotax fail.

 

The demise of my engine | Peter Anson – Engineering

WWW.ANSONENG.COM

 

 

[RFguy]

quite alot of recession visible, this head has been overheated for some time.

I wonder if it was a hydraulic lifter that ran out of adjustment length (due to recession and jabiru factory supplying wrong pushrods from the factory and not telling anyone) , and thus the valve never closes properly, hangs open and overheats .......

 

[danny_galaga]

1 hour ago, RFguy said:

quite alot of recession visible, this head has been overheated for some time.

I wonder if it was a hydraulic lifter that ran out of adjustment length (due to recession and jabiru factory supplying wrong pushrods from the factory and not telling anyone) , and thus the valve never closes properly, hangs open and overheats .......

 

But it never closed properly,surely that would be picked when doing compression leak tests, or even trying to tune it?

[RFguy]

yeah recession - that would have been picked up  if done every 25 hours but maybe the loss of compression was marginal so was not investigated I don't know.

It takes very little pressure loss of the valve against the seat to completely kill the heat transfer....

the older exhaust  valves have a definite life, also. chuck them at 500 hours I say. newer jab valves are much more heat hardy - different material..... (and they're also 3x the price), so its possible the valve just saw too many  tension x temperature cycles.

 

I am guessing for a 912 to fail, with valves < 2000h,  that indicates an abused engine.

Edited July 16 by RFguy

[onetrack]

Valve stems normally fail due to carbon buildup on the stem above the head. The carbon buildup holds heat and causes hot spots in the valve stem, resulting in stem failure. Another factor can be that the carbon buildup on the stem can stop the valve from closing properly and hot gas escapes and causes hot spots on the stem. I'd be checking the other valves for carbon buildup and trying to figure out why it was happening if it's present. Reasons for carbon buildup can be too much cool engine operation, poor quality lube oil, and dodgy fuels.

 

Edited July 17 by onetrack

[facthunter]

The later engines run a more retarded spark. and tend to run hotter. That is just as likely to be part of the stem failure and poor metal. It stretches the stem and forms cracks in it. IF the head overheats the (insert) seat may move into the aluminium and lose the adjustment in the hydraulic lifter Nev