Dirt Bike Valve Job and Reassembly Part 2 - Dr. Dirt - Dirt Rider Magazine

Before starting assembly, the head must be thoroughly cleaned with solvent, soft-bristled brushes and Scotch-Brite pads until there is no oil or carbon left. Clean the head well with contact cleaner until there is no telltale solvent smell. Let the cleaner evaporate thoroughly while you get organized. You\'ll need to have new guides sitting in the freezer inside a sealed plastic bag as well as the proper driver to install the new guides. Wood uses a trick stand, but you\'ll most likely need to have two-by-fours arranged that will support the head without damaging the gasket surfaces. Once everything is ready, heat the head to 300 degrees in an oven or on a hot plate. Make sure you have the proper gloves to handle hot metal. You\'ll need to work fairly quickly once you get going.

Use a bit of extreme pressure lubricant on the leading edge of the guide. It will help the guide go in without taking any metal out of the guide bore.

Use the driver to install the guides. It is not merely a punch. It has a tip that slips into the guide itself, so it drives the guide without mushrooming the opening. This operation is much more risky when using a normal flat punch. Wood machined this outer sleeve, then marked the punch so he can see exactly how deep to drive in the guides.

You will need to measure as you go to make sure you attain the correct depth. The manual has the specs for the guide height.

This shot provides a good look at how clean Tokyo Mods got the head casting before beginning the guide work. The next step is to ream the new valve guides to size. Since this Honda has different diameter stems for the intake and exhaust valves, two different reaming tools are required.

After the guides are reamed, it is time to use a machinist\'s dye on the valve seats. The color lets you see where and how much material is being removed. We\'ve heard of using a permanent marker for this task.

This is the 45-degree cutting head from a Neway valve seat cutting set. It makes the initial and widest cut, and this angle will be where the new valve seals and transfers heat.

The Neway set uses a precision hand crank to make the cuts. The tool has a shaft that slides into the valve guide to ensure the cutting head stays in place and cuts properly.

After the initial cut, the remaining machinist dye shows the seat was no longer shaped precisely and the hole was no longer perfectly round thanks to the worn guides letting the valve move around.

The completed 45-degree cut is clean, wide and perfectly round. A new valve will seat very well to this surface.

In this shot the outer 35-degree cut has been made. Note that the 45-degree cut has been narrowed somewhat.

With the final, deepest 55-degree cut made, this is your proverbial three-angle performance valve job. When a head is ported, mismatches like the one pointed out by the arrow are blended in. Tokyo Mods also offers porting.

Here is the completed head with all four seats cut and ready for fresh valves. We cleaned it, then colored in each angle of the cut seat to make it easier to see.

These are the rest of the new parts that will go into the rebuilt head. Most Tokyo Mods customers are looking for maximum performance, so most choose to replace the valves with stock Honda valves. Since this engine sees a lot of off-road use, we chose to use stainless steel valves from Faction MX for longer valve life off-road. The stainless valves require their own matched valve springs.

To lubricate parts as he goes along, Wood uses a small squirt bottle of oil, and it gets a workout! Lube the spring seat area and the top of the guides with engine oil. Oil is your friend. Use plenty of it.

Set the valve spring seats on the guides, and oil them.

Again, since this model has different size valve stems, the seals are different and must go on the correct pair of valve guides.

In this case, the brown ones go on the intakes. You can tell them apart with help from the manual or by using the part numbers. The stem seals also get oiled.

Honda recommends a molybdenum (moly) paste on the valve stems, but Wood uses moly paste mixed with engine oil. The moly is to prevent damage at start-up and during break-in.

With the new seals, the valves pop into the head and they stay put nicely. Wood popped in all four valves before installing the springs.

Oil both ends of the new valve springs before installing them. All valve springs are assembled with the tighter ends of the coil down. The other end is lighter, so it is better to have it up. The ends are usually painted for identification. Honda springs are painted on top; the springs for the stainless valves we used are painted on the bottom.