Honda TRX 400 Top End Rebuild Part 2: Cylinder Head Inspection

After we disassembled the top end of our Honda TRX 400EX quad, the next step was to inspect the cylinder and piston before reassembling the engine. 

Watch the video above and follow the steps below to learn how to inspect the cylinder head on a TRX 400EX Honda ATV.  Use our Honda TRX 400 parts diagrams for additional reference.

Tools and Parts – Honda TRX 400 Top End Evaluation

Buy Motion Pro valve spring compressor tool

Buy Honda ATV valve stem seals

Honda TRX 400EX Cylinder Head Inspection

Step 1. Remove the valves from the cylinder head using a valve spring compressor.

Honda TRX 400 top end rebuild valve inspection

PRO TIP: Number each valve and the port it’s in so you can reinstall each valve in the correct port when reassembling the top end.

Step 2. Inspect the valves for wear, and check the valve seats in the cylinder head for anything that might prevent the valves from seating properly.

Honda TRX400EX top end rebuild valves inspection

Honda TRX400EX top end rebuild valve seats inspection

Step 3. Check the valve stem seals in the cylinder head for signs of leaks.

Honda TRX 400 ATV cylinder head inspection

Honda TRX 400 ATV valve stem seals inspection

PRO TIP: Replace the valve stem seals. They’re inexpensive and should be replaced while the engine is dismantled, regardless of their condition.

Step 4. Inspect the cylinder for scratches or wear, and feel for any imperfections on the inside wall of the cylinder.

Honda TRX 400 ATV cylinder inspection

Step 5. Inspect the piston rings for wear, and feel around their outside edges for uneven surfaces.

Honda TRX 400 EX piston rings inspection

If you find the piston rings or the cylinder is worn, you don’t need to throw them away. You can move up a +1 piston size and piston ring size, take the cylinder to a reputable machine shop and get it bored out to match.

Honda TRX400 cylinder bore top end rebuild

After the piston and cylinder inspection, it’s time to reassemble the top end. Watch the video below to see the third part our Honda TRX 400EX top end rebuild.



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