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How to Rebuild a Rotax 503 Ultra Light Engine

Soaring along at 3,000 feet in an ultralight aircraft is no place to have an engine malfunction. If the engine stalls, the pilot has a very dicey situation on his hands. As buoyant as ultralight aircraft are, they still require thrust to fly efficiently, and they do not glide as well as hang gliders. If it's time to rebuild your Rotax 503 ultralight engine, it's important to do it right.

Things You'll Need

Specially-ground screwdriver for removing piston pin circlips
Compression tracer
Cleaning agents and solvents
Stroboscope
Two-stroke oil
Lithium grease
2 pressure gauges
Dial gauge
Micrometer
Multipurpose grease LZ
Feeler gauge
Stud extractor
Grinding wheel
Engine parts (see parts catalog)
Vernier calipers

Instructions

- 1
  Download the Rotax maintenance manual from the link provided in the Resources section.
- 2
  Conduct a tool and part inventory. See chapters 8 and 9 of the maintenance manual for necessary specialized tools and materials, as well as the parts list for all the parts you plan to replace with new ones. For a complete rebuild, you need a comprehensive parts list.
- 3
  Drain any fuel and oil from the engine. Unscrew the oil plug at the bottom of the engine and let the oil drain. Pour out any unused fuel from the fuel tank.
- 4
  Disassemble the engine and clean each part you plan to retain, using carburetor cleaner and solvent. As you disassemble the engine, look at each part for signs of friction, chipping or wear and tear. Lubricate again, using an authorized lubricant from the Rotax 503 maintenance manual.
- 5
  Replace each part you discard with an authorized Rotax replacement part, in accordance with the maintenance manual. If you are unsure, you can also purchase a Rotax engine rebuild instructional video for visual guidance and coaching, from the link in Resources. Carefully reassemble the engine. Ensure you have no parts left over at the completion of the process. Check each part for loose fits or poor connections. Check tension on the V-belt, and adjust if too loose. You should be able to flex the belt between 9 and 10 millimeters, not more.
- 6
  Lubricate each part in accordance with the instructions in the maintenance manual. Some parts require thicker lubricants than others, so it is important to follow the directions precisely and use the correct lubricant for the part.
- 7
  Add two-stroke motor oil and fuel. Start the engine by pulling firmly on the starter handle. This is the handle attached to the cable wrapped around the spool in the side of the engine. Allow the engine to run for a while, and listen carefully for problems. If there are no problems, you have a successfully rebuilt Rotax 503 ultralight engine.

Aviation