Tools and Materials:
- Complete Honda 300EX Engine Rebuild kit
- Honda 300EX Service manual
- Dial calipers
- Piston ring compressor
- Small straight punches & screwdrivers
- Air compressor
- Rags
- Safety glasses
Step 1: Preparing the engine
1. Remove the engine from the quad according to the service manual.
2. Place a large oil drain pan under the engine to catch liquids as you disassemble the engine.
Step 2: Disassembling the engine
Follow the service manual instructions for disassembling the engine in the following order:
1. Remove the cylinder head and cylinder.
2. Inspect the cylinder bore and cylinder head for damage and measure with dial calipers to ensure they are still in spec. Honing may be necessary.
3. Remove the piston and connecting rod from the crankcase.
4. Inspect the piston connecting rod for damage. Check the piston rings and replace if they are worn or damaged.
Step 3: Cleaning and inspecting parts
1. Carefully inspect and clean the engine parts. Remove all carbon deposits from the cylinder head and combustion chamber using a carbon remover.
Step 4: Rebuilding the Bottom End
1. Thoroughly clean and inspect the crankcase, crank shaft and bearings for wear or damage. Replace as needed.
2. Install new crankcase seals.
3. Carefully seat the bearings in the crankcase.
4. Install crankshaft and torque to factory specs.
5. Install new gasket between crankcase halves and torque in a crisscross pattern to specs listed in the manual.
Step 5: Rebuilding the Top End
1. Install a new piston ring set onto the piston. Use the piston ring compressor tool to compress the rings for easy installation.
2. Install the piston pin and circlips.
3. Lubricate the piston with light engine oil.
4. Carefully install the piston and connecting rod assembly into the cylinder.
5. Install the top piston ring onto the piston.
6. Install the oil ring expander onto the piston and install the oil ring.
7. Install the cylinder head gasket.
8. Place the cylinder head onto the engine and install the cylinder head bolts.
9. Torque the cylinder head bolts in the prescribed sequence specified in the service manual.
Step 6: Reinstall Engine and Start Up
1. Reinstall the engine into the frame of the quad.
2. Reconnect the engine to all accessories and fuel line.
3. Change oil and filter.
4. Perform oil change procedures listed in the service manual to prime the oil pump.
5. Start the engine and let it idle for a few minutes to circulate the fresh oil.
6. Continue to let the engine warm up and monitor for leaks.
