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The following statements, though based on over Eighteen years of direct experience should still be considered "unscientific" opinion. Any statements or general conclusions are solely the opinion of the writer.

When considering major service on a Rotax UL series engine, the single most expensive question, is:
"Must the crankshaft be replaced?" 

At Green Sky, we have found little reason to depart from the "Recommended Maintenance" plan published in the Rotax operator's manual. 
That plan recommends a crankshaft replacement interval of "300 hours or 5 years, whichever comes first." Those who choose to push hours beyond the "factory recommended guidelines" do so, at increased risk of failure. The increased risk may be a reasonable bet, in certain circumstances and unreasonable in others. Historically, it is not rare to find air-cooled 2-stroke cranks which have run 2 to 3 times the 300 hour recommendation. Conversely, it is not rare to find circumstances where the crankshafts of 532 or 582 have failed very near that 300 hour mark. ...some have gone past a little, some might not have made 300 hours. The cause for this discrepancy may be open for debate, but the fact remains, there is a significant risk of failure, running beyond published recommendation with the liquid cooled 532, or 582.   These observations are the basis of two policies adopted by Green Sky Engine Services

  • If a Liquid Cooled Rotax  2-stroke is at or near 300 hours time on the crankshaft, we will not perform any service or operation on that engine which does not include crankshaft replacement.
  • If the owner of an Air Cooled Rotax 2-stroke wishes to run a crankshaft that has total time in excess of 300 hours, Green Sky will service the engine without crankshaft replacement, providing the owner has knowledge of the increased risk, and no anomalies are found with the original part.
 What kind of failure?
Although any of the component parts of the crankshaft assembly can fail, rod bearing failure is very destructive, and can occur with little warning. ..sometimes only seconds. 
Another crankshaft anomaly which is cause for rejection of the part, though rare, is worn center main bearings.
Extremely rare, is actual breakage of center axis components. 		 Can the crankshaft be inspected to reliably predict remaining life? 
Green Sky routinely makes crankshaft checks during scheduled maintenance. These checks include visual inspection, plus measurements of runnout, both in and out of the crankcase. In the event runnout is beyond limit, sometimes a crankshaft can be adjusted. If runnout is excessive, and/or unusual occurrences exist in the history of the engine, running beyond 300 hours, even in air cooled application is not recommended. In spite of these checks, a predictable outcome is elusive, at best. Those uncomfortable with this fact should select the "factory new" replacement part.
 
What can be done to prolong crankshaft life?

That is the thousand dollar question. 

  • Generally, the engine that runs more frequently, with the fewest hours of storage, has a better chance of extending the TBO. 
The total engine installation also has a bearing on the outcome.
  • Vibration is an enemy of all components. Design and construction of the motor mount and the quality and condition of vibration isolators, therefore are critical.
  • Select a propeller which is within the inertia limits of the reduction drive. (3,000 kgcm² for Type A and B drives, 6,000 kgcm²)
  • Additives and special "friction reducers"?  Other than some testimonials, there is insufficient concentrated scientific data to draw a conclusion.
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