An essay by Fastheads, the leader in motocross head and valve train repairs, upgrades and modifications using current advances in technology and machining.
This discussion will cover why it is absolutely necessary to check, adjust and maintain the correct valve clearance, commonly called valve lash We will offer an explanation of what causes this clearance to change and how to avoid the need for constant adjustments. We will cover a few facts on the differences
between MX and automotive race heads, generalize about the easiest method on how to adjust the clearance and cam timing. In the end we will recap and review the common mistakes and major disasters we have witnessed.
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 the gap. It is normally inserted between the top of the lifter bucket and cam, or valve adjusting shim and rocker arm. This measurement must be taken when the cam is in a certain position or basically when the top of the lobe is opposite the lifter on the smallest part of the cams radius. All manufacturers made this location easy to find by having you check the gap when the piston is at top dead center on the compression stroke and then aligning the marks on the cam sprocket with a mark or location on the head. Check your manual or the internet 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 be depressing the valves. There is usually a mark on the flywheel that indicates top dead center (TDC.) To adjust the gap, most modern MX engines use a round disk of various thickness in increments of .05mm/.002″. Some OEM shims can be found in .025mm/.001″ for even finer adjustments. The smallest shim is 1.2mm and the largest is around 3.5mm. The shims are held in place by the valve spring retainer but sits and pushes directly on top of the valve stem itself. The top of the shim is pushed on by the raised pad in the bottom of the lifter bucket or the pad on the rocker arm. The shim can’t fall out unless the clearance becomes larger than the distance between the stem and top of retainer. I will go into detail on this later under major disasters
and common mistakes. Due to valve face and seat wear, this gap usually becomes smaller over time. In off road race engine, this gap can diminish quite fast, especially if the air filter isn’t maintained properly or leaks. If the gap becomes too small, zero clearance or less, the valve will not close all the way and compression is lost. At kicking speeds, compression has time to escape if the valve or piston rings are not sealing properly making the engine very hard or impossible to start. At higher than kicking RPM the compression doesn’t have time to leak off and the engine has a better chance of starting. When heat expands the metal in the valve stem it lengthens which makes the clearance even smaller. This is why it’s necessary to check and adjust the valve lash at room
temperature. A hot engine will always start easier due to thermal efficiency even at kicking RPM and even if the engine is not sealing properly. I mention this because a lot of riders think they don’t have a valve problem if their bike kick starts when hot but not when cold. If a rider decides to increase the valve lash over what is suggested in order go longer between adjustments the results can be even more damaging. Cams are ground so that the initial opening of the valve is gradual in order to ease the valve open and increase valve speed progressively faster. If the gap is too large, that part of the cam is missed and the lobe strikes the lifter with a hammering effect which produces crazy spring harmonics, metal fatigue and increased wear if not total failure.
How to feel the clearance with a feeler gauge, and then make valve lash corrections? Each valve train design requires a specified minimum and maximum gap. Most clearances for MX models are somewhere between .004″/.007 for intake, and .007 – .011″ for exhaust. Factory specs usually give you the desired clearance, say .005″/ with a plus or minus limit of .001″. Which means if you’re a snug .004″ you can reduce the shim size .002″ to get to the maximum clearance of .006″. Most shims come in increments of .05mm/.002″. So this is how it should feel. Let’s say the factory spec calls for .008/.020mm + or – .001″/.02mm. Find the feeler blade that measures .007″ and see if it will slip between the gap with a very little or no drag on the feeler. If it won’t go in or is 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’s tighter than spec, so we will now have to reduce the size of the shim. After removing the cam and lifter you find the shim to be a 2.00mm. All shims are marked in “mm” so if you think in inches you will have to convert (.05mm = .002″.) Now to get back to our required gap of .008″/.020mm we will have to reduce the shim size from 2.00 to 1.90mm which is smaller by .004″/.10mm. Once the new, smaller shim is installed, install the cam and torque it to factory specs, this very important, using an inch pound torque wrench line up the marks on the sprocket with or without the cam chain and check again. If the cam chain is not on and tensioned, press down hard with you thumb to simulate the tension. Don’t do this on the non cam chain side of the cam. Always check again after turning the motor over a couple of times. Tips: Sometimes you have to overcome the initial edge of the blade by working it in to the flat area and then drag 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 ¾”-1” from the tip. A human hair is about .002/.05mm thick or the difference in one shim size so keep all surfaces clean. On some rocker arm models like KTM or CRF exhaust there is a temptation to use a lever and push down on the expose retainer in order to remove the shim without removing the cam. This will dislodge the keepers and possibly bend the valve stem, do not do this! It’s always suggested to check the 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″, about the equivalent of two .05mm shim sizes. Many people continue adjusting the gap way beyond this point which can create several problems, such as worn and rounded valve seat faces, which require the face angles to be re-cut, weak valve spring seat pressures, which increase 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 which can unlock the keepers and usually drops a valve. I’ve seen people actually grind down the smallest shim or even remove it just to get in another ride, usually with disastrous results. If the larger shim is needed it usually means that there is a buildup of carbon under the valve, a bent valve, a seat dropping out, or major wear in the cam bearing or journal.
Stainless vs Titanium valves. The original fix to reduce valve adjustments and keep the cost of valve replacement low was to replace the Titanium valves with Stainless. Titanium valves are 40% lighter than steel which is why they are used in all high RPM race motors. The horsepower loss is around 7% to 10% with steel valves depending on the needed spring pressure and RPM. Lightweight titanium has a thin hard infused coating on the top of the softer base material. Once the micro thin hard coat wears through, valve adjustments are required progressively 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 cost about $40 per valve. Heavier springs are required with steel valves in order to keep them from floating which can add greatly to the cost. A lot of stainless intake valve kits come with titanium retainers which, unfortunately, need to be replace quite often and also adds greatly to the cost. Over time and need, new technology advances in hard coating to the titanium surface has made them wear or even outwear stainless valves. The newest surface coating is a CrN (Chromium Nitride) DLC (diamond like coating) PVD (particle vapor deposited), thanks NASA. Many of the after-market, and some OEM, valve makers are using this method. Titanium valve wear in the 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.
MX vs Automotive heads. Modern motocross four stroke engines have a cylinder head and valve train design that is unique compared to cars, utility vehicles, and even automotive race heads. The casting has to be super light and compact and the valve train has to be controlled up to 13,000 RPM in order to produce competitive power. This eliminates the possibility of a system that self-adjusts. Unfortunately, valves and seats wear faster in our sport due to dusty and abrasive air conditions, and lightweight titanium valves needed to produce max horsepower at very high RPM. Most of us that have been in the sport for a decade or more came from the 2-stroke era and didn’t have to bother with valves lash adjustments or cam timing. Many riders have come to hate the new 4-strokes, especially those that failed to maintain the engine properly and blew it up costing a small fortune. Let stray from the subject for a moment. The initial reason 4-strokes where developed to compete with 2-strokes was the EPA ban on 2-strokes due to air pollutants. Now after a few years of research and design the 4-stroke engines developed a wider power band with more torque which offers a lot better traction and control for motocross type riding. It will be interesting to see what kind of power 2-strokes will make when EFI systems are adapted in the future. They might make a comeback, but for now let’s learn how to maintain the 4-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 can’t overly express the fact that there is zero room for error during installation and in the continued maintenance of your motocross head. Please read limited warranty policy at the end of this letter.
You don’t 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 many on line videos and picture instructions that can be very beneficial. The list below are common mistakes associated with the assembly of motocross type heads. Mistakes are never found in the owners or service manuals and are only mentioned here as a warning and food for thought. You will need a torque wrench both inch and pound, feeler gauge, metric socket, and wrench set.
New valves seldom break or bend unless they hit the piston, each other, or loose 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 speeds. In many cases the valve can slam shut on the valve seat without touching the back side of the lobe.
1. If you removed the piston, make sure the new one is installed it in the right direction. Valve pockets are machined into the piston for intake and exhaust valves. Installing the piston backwards will bend the valves! What may look 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 pistons it is a good idea to check the piston to valve clearance. The best way to do this is put a thin piece of soft clay across the top of the valve pockets, torque the head to specs, 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 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 being torqued. Dry or dirty threads and washer can add up to 10lbs of resistance and improper torque reading. Always use new gaskets.
2. 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 a wise 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 revs. Note: The early KX250F and Suzuki manuals stated 109 inch pounds and it was later changed to 86 inch pounds.
3. Cam timing to crank position must be set as prescribed by the manufacturer. If timing is off a tooth or more, you may bend a valve. If your cams previously witnessed rotational resistance while the motor was running or being kicked over, the sprocket may have slipped on the cam shaft as they are pressed on with no indexing. This can happen due to a broken lifter bucket, cam journal seizure, bent valves from slipped timing, etc. If you think this happened, it would be wise to degree or replace the cams. The manufacturers usually don’t scribe index marks between the cam shaft and cam sprocket. It is a good idea to scribe your own marks on known good cams for future reference. If your cam sprocket did slip you will 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 is a noticeable amount, so if you can find another like model to view the lobe location at TDC it may help. If slippage is questionable we usually suggest replacing the cam. Otherwise, unless you have the tools (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. KX, RM, YZ and most KTM’s motocross heads have cam journals that are bored directly into the head. They do not have replaceable plain or ball bearings. If the cam seizes in the journal due to a lack of oil, oil pressure or cam caps over tightened the head may have to be replaced. Most of these heads run over $600 without the valve train. We can fix some slightly seized journals buts it’s never as good as the original. Cam caps are only sold with the head and cannot be purchased separate as they are mounted in the head when line bored. It’s 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. The CRF heads have ball bearing 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, as on any lifter bucket style head. If you bought a new head or had yours repaired due for this reason you will need to check the rocker arm bearing, cam, and lifters. Parts that have gotten hot enough to bake the oil have become red hot and their molecular structure and heat treatment has been changed. An indication of this is if the part has turned black or discolored compared to stock. Using these parts is a sure way to failure. Lifter bucket bores can be oversized and fitted with larger diameter buckets on some models. If the head is worn and needs rebuilt along with the lifter bore work, it is usually cheaper to buy a new head. Lifters are also called tappets or buckets.
6. 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 on the underneath of the rocker arm. If the spring retainer gets depressed rather than the valve stem, the keepers can get dislodged and the valve could drop. We see this a lot and often get blamed for the failure. The most common problem is when the lifter bucket is lifted above the width of the shim and it 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 can happen with the rocker on the CRF’s. The shim can fall out or get cocked sideways on the retainer. Every time the cam shaft is reinstalled and after the cam caps are torqued, re-check the valve lash. If the gap is super loose or way tight you will know that 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 look to make sure they are in the groove and 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 come in contact with the top of the retainer rather than the shim and will dislodge the keepers. Some aftermarket retainers are taller and sit above the valve stem, and the same can happen. Even if the keeper doesn’t completely dislodge they will not wedge and eventually wear out the stem grove and the valve will drop. This can also happen when a seat has been cut deep, raising the stem in the head which requires 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. I.e. if your valve clearance is .006″ maximum you should have at least .016″ when the shim is not installed.
7. 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 stepper ramp than stock, you will need to strictly follow the cam manufacturers spring and installed height instructions. If the seats have to be cut more than .010″ spring shims should be installed under the spring locators in order to correct the installed spring height. All of the above scenarios have to be adhered to.
8. 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 causing the valve to contact the piston or a change in power. If you hear excessive noise in the top end, check the chain tension and cam timing and then valve lash.
9. 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 can’t find a part while reinstalling the head remove the stator cover and hunt for it there.
10. If you are installing your own valves and assembling the head, please keep the following in mind. Many heads have different size intake and exhaust valve stems diameters and use different sized keepers, retainers, seals and springs. Don’t get them mixed 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 all the above for a better understanding 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. The hard coating is only microns thick. Once this has worn, you’re 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 offer valve grinding but it should only be done on old low RPM cars before unleaded gas.
We repair a lot of heads that have been recently rebuilt by shops that tried using old school seat grinding and valve guide work along with a poor understanding of the close tolerances required in high RPM race motors! These heads are not like cars or utility vehicles nor do they compare with motorcycle heads of just a few years back. Concentricity has to be in the tenths of thousandths to avoid premature wear, proper sealing and possible valve breakage. Concentricity is the angle difference between the guide axis and seat face and the roundness of the valve face diameter to the seat face diameter. For instance, 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 to much of the casting around the guide and cutting the guide flush with the port can affect concentricity when hot. These heads have very thin castings to reduce weight and increase port size. If the porter has cut the guide support casting out for better flow the side of the guide with the greatest amount of casting will expand and tip the guide to 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 the 4-stroke race motors are from improper assembly by someone that did not follow the instructions as described in the manual.
Fastheads Warranty Repair Policy: As a rule, there is no warranty expressed or implied on machines designed for racing. Your bike most likely didn’t 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 adhere to the following instructions. 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, either sold individually or assembled in our heads. Stainless steel valve conversions from titanium are never warrantied as they are 40% heavier and not of 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 packages. Fastheads does not offer any help on heads we machined but did not assemble, test and adjust the valve lash on.
Everyone at Fastheads lives to ride and loves the sport. Our camaraderie with our close riding buddies and customers is a major part of our life. We fully understand the high cost inherent with dirt bikes and 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.