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   Post on Broken Rubber Donut on C Box in Rotax and other engines

   By nlappos, posted 3 Nov 2018

   Here is something we all should know. My rubber donut behind my C Box gave up the ghost 2 flight hours ago, it broke and separated in one place and cracked in another. No real vibrations, but a screetching noise was heard for a second or two, then everything was normal. I still had drive to the prop, and all seemed normal, but I made a landing to be sure.
   On post flight, I saw that the donut was cracked and broken. It was probably the original donut, perhaps hundreds of hours old, and is clearly dried out. On further inspection, I noted that the starter gear had separated so the starter would not spin the engine. The three bolts that hold the starter gear to the damper and the magneto flywheel in the back of the engine had sheared.
   Here's what I think happened: The donut was probably cracked and old, and I can't say that I inspected it. It finally cracked through in one place, and set up a torsional vibration. The back of the engine had the damper plate, it started vibrating and sheared the three bolts, almost immediately after the donut broke. This made the brief screeching noise, and also made the starter no longer spin the engine.
   I pulled apart the C box, (this web site was great: http://www.aerofixaviation.co.uk/rotax-type-c-gearbox-service.shtml   and replaced the donut with a new one. I dropped the engine, and removed the back plate to expose the starter gear, extracted the sheared bolt ends (left hand drill and an extractor) and then replaced the bolts with new ones. Put it all together, mounted the negine, and have now flown about 2 hours with everything copacetic.
   Lesson learned - LOOK IN THE INSPECTION HOLES AT THE SIDES OF THE C BOX OFTEN, YOU CAN SEE THE ENTIRE DONUT. REPLACE THE DONUT EVERY 5 YEARS, PER MAINTENANCE MANUAL (PARAGRAPH 10.1)
   For those wondering if the 670 caused the broken donut - I doubt it. At 6000 feet and higher where I am, I "only" develop 65 horsepower on the 670, so the donut sees what a 582 gives it at sea level. As proof, the max fuel flow I have ever seen up here is 7.4 gal per hour, the same as what I got on my 582 at full throttle.