Youngmic asked
But what's your point?
Mic I'm sorry if I caused you to jump through hoops but the question was posed to 'start the conversation' ( in pollie speak). I also apologise for my tardy reply . This working till we're 70 is the pits. WORK- the curse of the drinking class.
You quoted valid possibilities from the Jabiru Service Bulletin and Sunny mentioned shortfalls in alloy and steel strengths. I'd heard all of these reasons in various pilot BS sessions and generally in more colourful language about Jabiru.
I couldn't understand why the bolts were breaking. They should have been plenty strong enough given that 3/8 bolts will yield at 2.5 to 5 tons. Nor could I believe that in this day and age Jabiru would choose some inferior alloy for their engines. Detonation, severe operation and over torquing didn't explain it either as the bolt problem was fairly widespread and those scenarios aren't all that common.
I never did guess the real problem. I recently found out while visiting Bundaberg and got talking to people involved with solving the mystery.
It was that old nemesis of aviation - harmonics. The natural frequency of the long 3/8 through bolts was found to be excited at certain high power settings.
Once excited, the bolt makes like a sine wave vibrating the bolt and of course the ends with the nuts. Eventually the vibrating nuts & washers chew into the alloy they're up against and the bolt becomes loose. Mr pilot tightens the nuts and tells anyone who will listen that the alloy is s#*t. With enough repeats of the above the bolt would break. Then Mr Pilot tells everyone the bolts are weak.
Harmonics, flutter, divergent resonance are part of the dark arts in engineering engines. It's a science in itself and is complicated. Even now some GA engines have no go rev ranges. A Grumman I flew had (from memory ) 2,000 - 2,250 no cruise area. Many normally aspirated Continentals have a recommendation to not cruise below 2,300 rpm.
Back to Jabiru. The bolt only resonated at high power settings. Hence Flying Schools going to full power more often, or a bigger % of their flight time, were more likely to experience failure at lower hours than the rest of the fleet.
The cure is to install the 7/16 bolts which don't flutter in the rev and load range ever likely to be seen by a Jabiru.