I have recently had two stress fractures in my 912 / Classic exhaust system.
The
first occurred on the outside of the last bend into the silencer from No 2
cylinder (stb front) and the second occurred on the outside of the first bend
in the down-pipe from No 4 cylinder (stb rear). The first was in the form of
a letter "H" with the verticals circumferential and the horizontal longitudinal.
After speaking to Andy, I had it gas welded and it has remained intact. 20
flying hours later the second fracture occurred (fortunately less than 10 miles
---From home), which blew apart and caused me to make a semi emergency down-wind
landing, because the vibration with more than 1/4 throttle was unacceptable.
The aircraft has flown just short of 500 hours and operates from a fairly rough
farm strip. The exhaust pipes are mild steel and the box is stainless. I would
like to hear from any other owners details of any exhaust fractures, hours
flown and type of field used (i.e. hard or grass runways etc) I know of at
least
one other, but there may be more.
The whole weight of the exhaust system is suspended off the down pipes and the
stub tubes off the box. The inertia when the aircraft is bumping along rough
ground is therefore considerable and I believe is the cause of the stress
fractures.
For piece of mind, I have decided to replace the whole exhaust system
and having discussed the problem with Andy, he is very kindly arranging to weld
some supporting lugs to the new silencer box so that I can provide metal straps
to the crankcase and engine mount (cross section member rigid with the
crankcase)
to reduce differential movement and resulting stresses. If premature
exhaust failure due to mechanical stress is going to happen, the factory might
consider offering a mod to help support the weight of the system. I have
offered
to provide details of the support system I set up for mine.
Best wishes,
William
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