Most cylinder head problems can be avoided by understanding what causes failures in the first place. A head that is properly assembled and machined, with valve train components engineered to handle the RPM of a particular engine, will seldom fail on its own.
Fastheads accurately machines, assembles, and tests every head. When a head fails, it is usually due to one of the scenarios below. We cannot express this enough: there is zero room for error during installation and continued maintenance of your motocross head.
You do not have to be a skilled mechanic, as long as you take the time to follow the instructions outlined in your owner’s manual. There are also many online videos and picture instructions that can be very beneficial.
The list below covers common mistakes associated with the assembly of motocross-style cylinder heads. These mistakes are not usually found in owner’s manuals or service manuals, so they are mentioned here as a warning and food for thought.
You will need an inch-pound torque wrench, a foot-pound torque wrench, a feeler gauge, and a metric socket and wrench set.
New valves will not break or bend unless they hit the piston, hit each other, or lose contact with the cam. Valve float occurs when the valve lifts off the top portion of the cam lobe due to improper spring rates for the weight of the valve and the opening speed. In many cases, the valve can slam shut on the valve seat without touching the backside of the lobe.
1. Make Sure the Piston Is Installed in the Correct Direction
If you removed the piston, make sure the new one is installed in the correct direction. Valve pockets are machined into the piston for the intake and exhaust valves. Installing the piston backwards will bend the valves.
What looks obvious may not always be correct. Some pistons have an arrow or dot that indicates the exhaust side.
When installing modified cams, oversized valves, or aftermarket pistons, it is a good idea to check piston-to-valve clearance.
The best way to do this is to put a thin piece of soft clay across the top of the valve pockets, torque the head to spec, set the timing and valve clearance, and turn the crank one complete revolution.
The closest possible contact with the valve is usually a little before the valve opens or closes, not necessarily at top dead center.
Remove the head and check the thickness of the clay. Most engine builders suggest .060" / 1.5mm, building in a safety factor for heat expansion, crank bearing wear, and carbon buildup.
Lube the head bolt threads or nuts, and both sides of the washers, with engine oil before torqueing. Dry or dirty threads and washers can add up to 10 pounds of resistance and create an improper torque reading.
Always use new gaskets.
2. Torque Cam Journal Caps Correctly
Cam journal caps must be tightened to the correct torque. Over-tightening can cause the aluminum to bulge inward and bind the cams.
Lube the cam journals before installing the cam. It is also a good idea to kick the motor over with the kill button applied ten or more times to fill the oil galleries. This is one of the reasons motors should be warmed up at low RPM.
Note: Early KX250F and Suzuki manuals stated 109 inch-pounds, but that was later changed to 86 inch-pounds.
3. Set Cam Timing Correctly
Cam timing to crank position must be set exactly as prescribed by the manufacturer. If timing is off by one tooth or more, you may bend a valve.
If your cams previously experienced rotational resistance while the motor was running or being kicked over, the sprocket may have slipped on the camshaft. This can happen because many cam sprockets are pressed on with no indexing.
This can be caused by a broken lifter bucket, cam journal seizure, bent valves from slipped timing, or other failures.
If you think this happened, it would be wise to degree the cams or replace them. Manufacturers usually do not scribe index marks between the camshaft and cam sprocket, so it is a good idea to scribe your own marks on known good cams for future reference.
If your cam sprocket slipped, you may still be able to align the marks at TDC, but the cam lobes will open the valves at the wrong time. If this happens, you may end up with bent valves the first time the motor is turned over.
Usually, when the sprocket slips, it moves a noticeable amount. If you can find another like model and view the lobe location at TDC, that may help.
If slippage is questionable, we usually suggest replacing the cam. Otherwise, unless you have the proper tools, such as a degree wheel, dial indicator, and special holders, and know how to physically check the degree at which the valve opens, it is usually cheaper to buy a new cam.
4. Understand Cam Journal Damage
KX, RM, YZ, and most KTM motocross heads have cam journals bored directly into the head. They do not have replaceable plain bearings or ball bearings.
If the cam seizes in the journal due to lack of oil, lack of oil pressure, or over-tightened cam caps, the head may have to be replaced. Most of these heads cost over $600 without the valve train.
We can fix some slightly seized journals, but it is never as good as the original.
Cam caps are only sold with the head and cannot be purchased separately because they are mounted in the head when line bored.
It is easy to break a cam cap if the bearing-locating half washer is not centered when the cap is tightened. Using the wrong valve cover bolts or seal washers with the wrong spacing can also break the cap on models that bolt directly to the cam caps.
5. Inspect CRF Cam, Rocker, and Lifter Components
CRF heads have ball bearings on both ends of the cam, so there is less chance of ruining the head from a cam seizure.
If these heads run low on oil, the lifter bucket may seize in its bore, just like any lifter bucket-style head.
If you bought a new head, or had yours repaired because of this type of failure, you will need to check the rocker arm bearing, cam, and lifters.
Parts that have gotten hot enough to bake the oil have become extremely hot, and their molecular structure and heat treatment may have changed. One indication is if the part has turned black or discolored compared to stock.
Using these parts again is a sure way to create another failure.
Lifter bucket bores can be oversized and fitted with larger-diameter buckets on some models. If the head is worn and needs to be rebuilt along with lifter bore work, it is usually cheaper to buy a new head.
Lifters are also called tappets or buckets.
6. Make Sure Adjustment Shims Are Seated Correctly
Adjustment shims may only touch the top of the valve stem and the shim pad in the ceiling of the lifter bucket, or the pad underneath the rocker arm.
If the spring retainer gets depressed instead of the valve stem, the keepers can become dislodged and the valve can drop. We see this a lot, and we often get blamed for the failure.
The most common problem happens when the lifter bucket is lifted above the width of the shim. The shim sticks to the lifter and then slides to one side. When the lifter is pushed back down, the shim rests outside the retainer bore and depresses the retainer.
Always take the shim out of the bucket and install it in the retainer before installing the bucket.
The same thing can happen with the rocker on CRF models. The shim can fall out or get cocked sideways on the retainer.
Every time the camshaft is reinstalled, and after the cam caps are torqued, re-check the valve lash. If the gap is extremely loose or extremely tight, you will know the shim has become dislodged.
The keepers, also called locks or cotters, are wedged into a groove near the top of the valve stem. If the retainer gets pushed down rather than the valve stem, you should visually inspect the keepers to make sure they are still in the groove. Then set them by tapping lightly on the retainer with a socket that is larger than the shim bore.
Aftermarket lifter buckets often have larger-diameter shim pads in the ceiling of the bucket. If the surface of the shim sits below the top of the retainer, the lifter may contact the top of the retainer rather than the shim, which can dislodge the keepers.
Some aftermarket retainers are taller above the valve stem, and the same problem can happen.
Even if the keeper does not completely dislodge, it may not wedge correctly. Eventually, it can wear out the stem groove and the valve will drop.
This can also happen when a seat has been cut deep, raising the stem in the head and requiring a much smaller shim.
If you think this may be a problem, check the valve lash without a shim in the retainer. You will need at least .010" / .25mm more clearance than the required maximum valve clearance.
For example, if your maximum valve clearance is .006", you should have at least .016" clearance when the shim is not installed.
7. Follow Cam Manufacturer Instructions for High-Lift Cams
If your head was assembled and tested by Fastheads, and we adjusted the valve shims with a stock or aftermarket cam, you can skip this section.
If you are installing a cam with higher lift or a steeper ramp than stock, you need to strictly follow the cam manufacturer’s spring and installed height instructions.
If the seats have to be cut more than .010", spring shims should be installed under the spring locators to correct the installed spring height.
All of the above scenarios must be followed carefully.
8. Replace Worn or Stretched Cam Chains
If the cam chain is stretched, worn, or kinked, replace it.
Cam chain tensioners have a greater tendency to fail with loose or kinked cam chains. When the tensioner can no longer do its job, the timing may slip. This can cause the valve to contact the piston or create a noticeable change in power.
If you hear excessive noise in the top end, check the cam chain tension, cam timing, and valve lash.
9. Keep Small Parts Out of the Cam Chain Area
Be careful not to let dowels, shims, bearing retainers, head bolt washers, or dirt fall into the cam chain area.
These parts usually stick to the magneto and can cause the cam chain to skip or damage the stator and trigger coils.
If you cannot find a part while reinstalling the head, remove the stator cover and look for it there.
10. Be Careful When Installing Your Own Valves
If you are installing your own valves and assembling the head, keep the following in mind.
Many heads have different intake and exhaust valve stem diameters and use different-sized keepers, retainers, seals, and springs. Do not mix them up.
Apply a thin amount of anti-seize or engine assembly oil on the valve stem whenever new guides or valves are installed.
Tap on the retainers a few times to wedge and seat the keepers.
Read everything above for a better understanding of potential problems.
Never use lapping compound on a valve that has had a coating applied to the face. This includes all titanium valves and some stainless valves.
The hard coating is only microns thick. Once the coating has worn off, you are into the softer alloy.
All modern valves have been surface hardened using various techniques. Grinding valve faces on modern valves is not feasible, and the valve will wear out in a few hours.
Some machine shops still offer valve grinding, but it should only be done on old, low-RPM automotive engines designed before modern unleaded fuel and modern valve coatings.
Why Proper Motorcycle Cylinder Head Machining Matters
We repair a lot of heads that were recently rebuilt by shops using old-school seat grinding and valve guide work, combined with a poor understanding of the close tolerances required in high-RPM race motors.
These heads are not like cars or utility vehicles. They do not even compare with motorcycle heads from just a few years back.
Concentricity has to be within tenths of thousandths to avoid premature wear, poor sealing, and possible valve breakage.
Concentricity refers to the angle difference between the guide axis and the seat face, as well as the roundness of the valve face diameter compared to the seat face diameter.
For example, every time a guide is installed, the concentricity has to be corrected by re-cutting the seats. There is no getting around it.
We see hundreds of heads come in ruined due to improper guide installation.
Even porting methods are changing. Removing too much casting around the guide, or cutting the guide flush with the port, can affect concentricity when the head gets hot.
These heads have very thin castings to reduce weight and increase port size. If the porter cuts out the guide support casting for better flow, the side of the guide with the greatest amount of casting will expand differently and can tip the guide toward the leaner side.
Be aware of this if you send your head out for machining or porting.
Most of the catastrophic and expensive problems we see in four-stroke race motors come from improper assembly by someone who did not follow the instructions described in the manual.
