Home Forums Tech Talk Unknown hour rec engine, what to look for to know when to rebuild.

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    • #9083

      What should I look for to help determine when a unknown hour FR125 Rotax engine should be rebuilt?

      The previous owner had a rebuild schedule, but did/could not disclose the hours on the engine.


      Are there any indicators other than hours and gallons of fuel consumed to have some sort of an idea when one of these engines are due for a rebuild on merit alone?

      I have looked at what I can through the exhaust port and power valve, and all looks quite well from what little one can gather from this vantage point.

      No scoring, very little in the way of carbon build up, very small amount on piston dome.

      Rotating the crank gives no discernible play in the piston rod crank interface.

      Is there any technique or tests that can be done externally to help determine life expectancy, assuming it is a well running engine and seems to go very well over enough hours to know it is currently running correctly?

      How are recreational engines typically ran for rebuild cycles?

      What is the typical mode of failure?


    • #9130
      Kirk Deason

      Hi David,

      A compression test would be a good start.  I cannot comment with any authority on what you should expect from a Rotax, but when I test my stock Hondas, I expect to see about 120 PSI. Get a baseline and test again after running for a couple days. If the pressure is dropping below your baseline then the ring may be due for replacement and it is probably ready for a new top-end.

      You may also want to post in the Rotax specific forum further down the page.

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