Fastheads is a leader in motocross cylinder head and valve train repairs, upgrades, and modifications, using current advances in technology and precision machining.
This article explains why it is absolutely necessary to check, adjust, and maintain the correct valve clearance, commonly called valve lash. We will also cover what causes valve clearance to change, how to avoid constant adjustments, and some of the major mistakes we have seen over the years.
We will touch on the differences between motocross and automotive race heads, explain a general method for checking and adjusting valve clearance and cam timing, and finish with a review of common installation mistakes and major failures we have witnessed.
What Is Valve Lash?
Valve lash is the gap measured between the top of the valve and the cam. A measuring tool called a feeler gauge is used to check this gap. It is normally inserted between the top of the lifter bucket and cam, or between the valve adjusting shim and rocker arm, depending on the valve train design.
This measurement must be taken when the cam is in the correct position. Basically, the top of the cam lobe should be opposite the lifter, on the smallest part of the cam’s radius.
Manufacturers make this location fairly easy to find. Most manuals will tell you to check the gap when the piston is at top dead center on the compression stroke, with the marks on the cam sprocket aligned with the mark or location on the head. Always check your owner’s manual or service manual for your particular model.
If you are not at top dead center on the compression stroke, one of the cam lobes, either intake or exhaust, will usually be depressing the valves. There is usually a mark on the flywheel that indicates top dead center, commonly called TDC.
How Valve Shims Work
To adjust the gap, most modern motocross engines use a round disc shim of various thicknesses, usually in increments of .05mm / .002". Some OEM shims can be found in .025mm / .001" increments for even finer adjustment.
The smallest shim is usually around 1.20mm, and the largest is around 3.50mm. The shims are held in place by the valve spring retainer, but they sit directly on top of the valve stem itself. The top of the shim is pushed by the raised pad in the bottom of the lifter bucket or by the pad on the rocker arm.
The shim cannot fall out unless the clearance becomes larger than the distance between the valve stem and the top of the retainer. We will go into more detail on that later under common mistakes and major disasters.
Why Valve Clearance Gets Tighter Over Time
Due to valve face and valve seat wear, the valve clearance usually becomes smaller over time. In off-road race engines, this gap can diminish very quickly, especially if the air filter is not maintained properly or if it leaks.
If the gap becomes too small, zero clearance or less, the valve will not close all the way and compression will be lost.
At kicking speeds, compression has time to escape if the valve or piston rings are not sealing properly. This makes the engine very hard, or sometimes impossible, to start. At higher than kicking RPM, the compression does not have as much time to leak off, and the engine has a better chance of starting.
When heat expands the metal in the valve stem, the stem lengthens, which makes the clearance even smaller. This is why valve lash must be checked and adjusted at room temperature.
A hot engine will usually start easier due to thermal efficiency, even at kicking RPM, and even if the engine is not sealing properly. This is worth mentioning because a lot of riders think they do not have a valve problem if the bike starts hot but not cold. That is not always true.
Do Not Increase Valve Lash to Avoid Adjustments
If a rider decides to increase valve lash beyond the manufacturer’s suggested spec in order to go longer between adjustments, the results can be even more damaging.
Cams are ground so the initial opening of the valve is gradual. This eases the valve open and progressively increases valve speed. If the gap is too large, that gradual opening portion of the cam is missed, and the lobe strikes the lifter with a hammering effect.
This creates aggressive spring harmonics, metal fatigue, increased wear, and possibly total failure.
How to Check Valve Clearance with a Feeler Gauge
Each valve train design requires a specific minimum and maximum gap. Most clearances for modern motocross models are somewhere between .004" and .007" for intake valves, and .007" to .011" for exhaust valves.
Factory specs usually give you the desired clearance with a plus or minus limit. For example, the manual may call for .005" with a plus or minus limit of .001". That means if you measure a snug .004", you can reduce the shim size by .002" to get to the maximum clearance of .006". Most shims come in increments of .05mm / .002".
Example Valve Lash Adjustment
Let’s say the factory spec calls for .008" / .20mm, plus or minus .001" / .02mm.
Find the feeler blade that measures .007" and see if it slips between the gap with very little or no drag. If it will not go in, or if it feels tight, go to a .001" smaller blade and try again until you find one that fits with very little drag.
Let’s say you find the clearance to be .004" / .10mm. That is tighter than spec, so you now need to reduce the size of the shim.
After removing the cam and lifter, you find the shim is 2.00mm. All shims are marked in millimeters, so if you think in inches, you will need to convert. As a general rule, .05mm equals approximately .002".
To get back to the required gap of .008" / .20mm, you will need to reduce the shim size from 2.00mm to 1.90mm, which is .10mm / .004" smaller.
Once the new, smaller shim is installed, reinstall the cam and torque it to factory specs. This is very important. Use an inch-pound torque wrench.
Line up the marks on the sprocket, with or without the cam chain installed, and check the clearance again. If the cam chain is not on and tensioned, press down hard with your thumb to simulate chain tension. Do not do this on the non-cam-chain side of the cam.
Always check the valve clearance again after turning the motor over a couple of times.
Feeler Gauge Tips
- Sometimes you have to work the initial edge of the blade into the flat area before dragging it back and forth.
- Try not to flex the blade if possible.
- You may need to put a bend in the blade to clear the head casting. This should be done about 3/4" to 1" from the tip.
- A human hair is about .002" / .05mm thick, which is the difference of one shim size, so keep all surfaces clean.
- On some rocker arm models, such as KTM or CRF exhaust, there is a temptation to use a lever and push down on the exposed retainer to remove the shim without removing the cam. Do not do this. It can dislodge the keepers and possibly bend the valve stem.
- It is always a good idea to check shim size with a caliper, especially if the numbers are worn off.
Fastheads sells shim kits with a feeler gauge and digital caliper for around $120.
When Should Valves Be Replaced?
Any valve should be replaced once you have adjusted the valve gap more than .004", which is about the equivalent of two .05mm shim sizes.
Many people continue adjusting the gap far beyond this point, which can create several problems, including worn and rounded valve seat faces. Once this happens, the face angles must be re-cut. Excessive adjustment can also lead to weak valve spring seat pressure, increased wear, and possible valve float.
When the head is down to the smallest shims, the retainer surface may ride against the lifter rather than the shim. This can unlock the keepers and usually drops a valve.
We have seen people grind down the smallest shim or even remove it just to get in one more ride. This usually ends with disastrous results.
If a larger shim is needed, it usually means there is carbon buildup under the valve, a bent valve, a seat dropping out, or major wear in the cam bearing or journal.
Stainless Steel vs. Titanium Valves
The original fix to reduce valve adjustments and keep valve replacement costs lower was to replace titanium valves with stainless steel valves.
Titanium valves are about 40% lighter than steel, which is why they are used in high-RPM race motors. The horsepower loss with steel valves can be around 7% to 10%, depending on the required spring pressure and RPM.
Lightweight titanium valves have a thin, hard coating infused on top of the softer base material. Once that micro-thin hard coating wears through, valve adjustments are required more and more often.
Stainless steel valves have a chemical or heat treatment that offers around .002" of hard surface before wearing into the softer base material.
Most titanium valves cost about $100 or more per valve. Stainless valves are usually around $40 per valve. However, heavier springs are required with steel valves to keep them from floating, which can add greatly to the cost.
A lot of stainless intake valve kits also come with titanium retainers. Unfortunately, those retainers need to be replaced fairly often, which also adds to the total cost.
Over time, new technology and advances in hard coatings for titanium valves have helped them wear as well as, or even outwear, stainless valves. One of the newer surface coatings is CrN, or Chromium Nitride, often applied using DLC-type technology and PVD, or physical vapor deposition.
Many aftermarket valve manufacturers, and some OEM manufacturers, are using this type of technology. Titanium valve wear in newer EFI-equipped models is seldom an issue due to better seat and valve technology and, believe it or not, no carburetor vent tubes sucking in dirt.
Motocross Heads vs. Automotive Heads
Modern four-stroke motocross engines have a cylinder head and valve train design that is very different from cars, utility vehicles, and even automotive race heads.
The casting has to be extremely light and compact. The valve train also has to be controlled up to 13,000 RPM in order to produce competitive power. This eliminates the possibility of a self-adjusting system.
Unfortunately, valves and seats wear faster in motocross because of dusty and abrasive air conditions, combined with the lightweight titanium valves needed to produce maximum horsepower at very high RPM.
Most of us who have been in the sport for a decade or more came from the two-stroke era and did not have to worry about valve lash adjustments or cam timing. Many riders have come to hate the newer four-strokes, especially those who failed to maintain the engine properly and ended up with an expensive failure.
Let’s stray from the subject for a moment.
The initial reason four-strokes were developed to compete with two-strokes was due to environmental pressure on two-strokes and air pollutants. After years of research and design, four-stroke engines developed a wider powerband with more torque, which offers better traction and control for motocross-style riding.
It will be interesting to see what kind of power two-strokes make as EFI systems continue to develop. They may make a comeback, but for now, let’s focus on maintaining the modern four-stroke properly.
Disastrous Head and Cam Installation Mistakes
Most cylinder head problems can be avoided by understanding the cause of failures.
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.
If the head fails, it is usually due to one of the scenarios below.
We cannot overstate this: there is zero room for error during installation and continued maintenance of your motocross head. Please read the limited warranty policy at the end of this article.
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 helpful.
The list below covers common mistakes associated with the assembly of motocross-style heads. These mistakes are not always found in owner’s manuals or service manuals, so they are included here as warnings 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 seldom 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 is not always 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 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 the 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 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 tools, such as a degree wheel, dial indicator, and special holders, and you 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 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 must 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, 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 and sit above the valve stem, which can create the same problem.
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 it has worn away, 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.
Fastheads Warranty and Repair Policy
As a rule, there is no warranty expressed or implied on machines designed for racing. Your bike most likely did not come with a warranty when it was brand new.
In order for Fastheads to cover, or help cover, any problems you believe may have been caused by our workmanship, please follow the instructions below.
Install, run, and fully test the head within 30 days of our ship date.
If you experience a failure, even after this time, please give us a call and then send the assembled head, cam, and piston in for inspection.
We do not warranty OEM or aftermarket valve train parts that are not manufactured by Fastheads, whether sold individually or assembled in our heads.
Stainless steel valve conversions from titanium are never warrantied because they are 40% heavier and not part of the original design or machine capabilities.
We do offer a valve and seat wear-only guarantee on some of our extreme head packages, which is explained in those package details.
Fastheads does not offer any help on heads we machined but did not assemble, test, and adjust the valve lash on.
Final Thoughts
Everyone at Fastheads lives to ride and loves the sport. Our camaraderie with close riding buddies and customers is a major part of our life.
We fully understand the high cost that comes with dirt bikes, and we will do everything we can to help.
If there is any doubt that something was not perfect on our part, we will either fix the problem for free or offer cost on parts and free labor.
Please send us an email or call us with your thoughts and opinions after putting some time on our products. Feedback is the key to our success.
