Honda GX25 Engine Disassembly/Assembly

The Honda GX25 is one of the smallest four-stroke engines in production. It has an output of 1 horsepower, and is typically used in low-power applications like lawn or agricultural equipment.

Disassembly instructions

Most of the following steps will require a 4 mm Allen wrench. We will note if this is not the case.

1. Remove the red plastic outer casing (a single 18.6 mm screw)
2. Remove the ripcord (three 22.6 mm screws)
3. Unclip the white plastic fuel tank from the body, keeping it attached via a cord to the carburetor.

4. Unscrew three 22.5 mm screws holding carburetor to body.

5. Disconnect shorting wire from magneto (just pull) and remove carburetor/fuel tank from engine body.

6. Remove metal cap on top of engine (two 16.9 mm screws)

7. Use an 8 mm wrench to remove nuts on the muffler.

8. Remove single 16.5 mm screw on heat shield directly below muffler. Remove muffler and heat shield.

9. Remove spark plug wire (black rubber wire), and use a spark plug wrench to remove the spark plug.

10. Unscrew three 26.6 mm screws to remove clutch cover.

11. Use 14mm wrench to remove clutch nuts.

12. Use specially-designed tool (see Mike) to remove the flywheel directly below the clutch.

13. Go back to the 'top' of the engine. The timing belt is wrapped around a gear. Use a 2 mm pin to push the axel that this gear sits on out of the engine. Push from the end by the tappets (components with springs).

14. Remove the belt from its gear, letting it hang loose.

15. Remove the oil tube connection at the top of the engine, using tweezers to slide the metal clamp down the tube.

16. Remove four black screws (33.8 mm) and two silver screws (28.9 mm) from the bottom of the engine body.

17. Remove the engine bottom. You should now be able to slide the piston, axel, and timing belt out of the top part of the engine.

Reassembly Instructions

Reassembly is mostly just reversing the steps you took to disassemble the engine. The most important aspect by far is getting the orientation of the timing system correct. If your timing is off, the engine will not start. Please try to keep track of which screws correspond to each part. If you are ever unsure, refer back to the screw measurements in the disassembly instructions.

1. Slide piston back into engine, with belt attached to its gear on the axel. Slide belt through so that you can access it from the top of the engine.
2. Re-secure the engine bottom by reattaching four black and two silver screws.
3. Reinstall the flywheel on the main axel, noting the notch that forces it into a specific orientation.
4. (VERY IMPORTANT) Align small mark on flywheel to the bottom of three bolt holes.

5. (VERY IMPORTANT) Insert timing gear into top of engine, aligning two markings with the horizontal surface. Make sure the two tappets (metal flaps behind the timing gear) are facing down, and the gear is resting on top of them.

6. Re-fit the timing belt onto the gear (making sure to keep the gear and flywheel aligned) and hammer the timing gear axel that you removed earlier back in.

7. Try rotating the engine to check your timing. The flywheel should rotate with only occasional resistance from the piston.

8. Re-attach the clutch (see above).

9. Re-attach the clutch cover.

10. Screw in the spark plug and attach spark plug cover/wire.

11. Replace muffler and heat shield (see above).

12. Replace engine cap (see above).

13. Plug shorting wire into magneto (see above).

14. Reattach the carburetor and clip the fuel tank into place.

15. Replace the rip cord and red plastic engine cover.

16. Say a quick prayer, and attempt to start your engine.