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Can anyone explain how to check the gear box pre-load on the B box, and how to adjust it if needed. It is in the Rotax maintenance schedule but is not covered in my manual as to what it does, and how to do it.

To set the pre load on the gear box, you'll need a press, and a suitable pusher that can straddle the shaft and press on the outer portion equally on both sides of the gear.  Compress the springs in the press with the pusher, and remove the keeper ring halves.  Release the press, and then remove all the components, inspect, replace worn spring washers, and lube them up for reassemble.  If the pre load was extremely bad, you may need to use a fine file around where the keepers pounded out the grove before the gear will come off the shaft. When re-assembling, make sure you stack the washers properly, and add a thin shim to the bottom of the stack-up.  Try to compress everything and  install the keepers.  If you can get the keepers in, then take it back apart and replace the thin shim with a medium.  If you can still get the keepers in, replace it with thick one.  Continue to incrementally increase the shims until you can not get the keepers back into their slot, then remove one increment of shimming, and install the keepers.
That's about it.

If it seems like a little more than you want to do, I can do it for you  for $30 plus any shipping or parts needed.  This check should be done every 100 hours, but is often delayed until the 150 hour service when the entire engine is inspected.

Tom Olenik
Olenik Aviation
Authorized Repair Station for Rotax Engines
www.buyitsellitfixit.com/rotax.htm