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Motocross Cylinder Head Quiz by Fastheads.com

Fastheads (BK)

Test your head knowledge. The following facts are unique to modern 4-stroke motocross style cylinder heads and valve trains. Write T or F in front of the numbers. Grade yourself on the Fastheads.com website.

1. Lapping in a Titanium valve removes the hard anti-wear surface coating and exposes the softer base metal. As the hard coating wears off, either from normal wear or lapping the valve face wears much faster and will necessitate more frequent valve lash adjustments.

2. Titanium valves are 40% lighter than stainless which allows them to run at high RPM’s without losing contact with the cam, commonly known as floating. It takes less force to open a light valve and lighter spring pressures to control it. Reducing the weight of the valve train with titanium valves is necessary to have competitive Hp. In a 250cc motocross engine this equates to about 2 hp at higher RPM’s. This is the reason all current motocross bikes come stock with Ti valves.

3. Modern valves faces should never be ground. Valve manufacturers use several valve face hardening methods and coatings for stainless and titanium valves. Some increase valve life many times longer than others. Special heat treatments, spray welds, chemical hardening, electro plating and vapor particle distribution with diamond like coating processes are used to reduce valve face wear. Surface hardening penetrates the surface from a few microns to a couple of thousandths. The hardening techniques are necessary in race motors and in dusty abrasive environments.

4. Motocross 4-stroke cylinder heads and valve trains are unique to all other race heads. First there is only one cylinder so it has to seal and function perfectly or the engine will be very hard to start, especially without the aid of other cylinders. Most dirt bikes are kick started which moves the piston at slower speeds and only through one or two cycles at a time. At slower piston speeds the compression has more time to escape through the valves and rings. Factors that lower the compression are, zero or tight valve lash which holds the valve from closing, worn, damaged, slightly bent or carbon caked valves and seats and worn piston rings. Pushing a bike in gear builds more compression which will allow it to start when kick starting won’t.

5. If there is a need to reduce the valve adjusting shim more than .004"/.10mm, about two shim sizes, the valve should be replaced. This indicates the hard surface coating has worn through. Re-shimming more than this will ultimately round out the seat surface which then requires re-machining of the seat. Your owners manual doesn’t mention this.

6. Port and polish myths. The factory uses sophisticated computers and field testing to aid in their port and head design. Its hard to find a significant power gain from porting alone. The stock ports and valve diameters are designed to offer the optimum dimensions for most racing applications where a wide power band is needed, such a motocross. Oversized valves and larger ports may be desired when the engine volume in increased or maximum power is desired at the higher range of the RPM as in drag racing, super moto, flat track etc. Cam durations and lifts are designed to offer a wide variety of power characteristics including more low or top end. Cleaning up the mis-match between the seat and port, known as bowl porting and removing any large casting flaws is usually all that’s needed to improve on the original design. Most of the ports in the EFI heads have been decreased in size to offer higher flow velocities for low end and still maintain more top end than the older larger ports due to the efficiency of the EFI system.

7. Motocross heads are built to be very small and light. The castings are very thin in area’s that utility and automotive heads are not. This make them vulnerable to damage from being over torqued, over heated, welded and improper guide installation. All these can offset the way a valve centers itself on the seat. Only machine shops that understand the complexity of lightweight motocross style race heads should rebuild this type of head. 8. If the support casting in front of the guide is removed from porting, the guide can tip forward when the thicker casting on back side of the guide expands from heat. The valve being guided by the guide will then sit off center on the seat. This enable compression to be lost making the bike hard to start and create tuning problems when small puffs of compressed and burned fuel are sent back into the intake port. It also wears the valve, guide and seat prematurely. Many of the new generation of MX heads are build with this in mind and build up the casting in those area’s.

9. Every time a valve guide is installed the valve seat has to be re-cut to properly align the guide axis with the seat face. This measurement is called concentricity and should be kept under .001". A concentricity gauge that reads in tenths of a thousandth is required for race tolerances.

10. Many factory replacement guides are oversized and requires that the guide bore to be reamed to size before installation. If the guide is not properly sized for the guide bore and is to tight, the guide bore may crack or distort the head casting during installation. The thin casting on MX heads make them vulnerable to this type of damage. The interference fit should always be check even if the guide is marked standard. The normal interference fit is no more than .0015" to .002".

11. It is impossible to cut a concentric round seat face when the cutting tool is piloted by a worn guide.

12. When a 2.05 valve shim is replace by a 2.00mm the difference is about the same as a human hair .002"/.05mm. Most after market shims come in .05mm. Most valves lash specs are plus or minus .001" which offers a .002" variance. As the valve face wears the gap gets tighter, therefore it’s better to set the gap on the maximum tolerance and let it wear to the minium. Valves do not actually stretch, the valve face and seat wear which simulates stretching. If a valve bends or there is carbon build up on the face the gap will increase.

13. A new valve on a worn or non-concentric seat will wear many times quicker. Seat face wear, unless extreme, can not be detected by sight alone. Most people including mechanics will determine the valve contact area on a seat is good until they actually measure the width and check it with a concentricity with a gauge. If the valve has grooves from wearing against the seat, the seat face surface should be machined before installing a new valve.

14. Guides wear in an hour glass shape and oval front to back. The wear is much more prevalent when nudged by a rocker arm compared to a lifter bucket type valve actuating systems.

15. Valve seats have to be cut perpendicular to and the exact radius from the centerline of the valve guide. The measurement of this is called concentricity. Seat concentricity on a motocross head should be under .001". Anything more can create a poor valve seal, hard starting and potential valve and seat wear. A concentricity gauge is used to measure the variance and should read in tenths of a thousandths. The tool itself rotates around a pilot in the valve guide with a dial indicator arm resting on the seat face.

16. Surfacing or decking a head surface changes the cam timing on heads with over head cams. Surfacing should only be done fix a warped head and not to raise compression. Most MX cams do not have adjustable sprockets to correct cam timing. A stretched cam chain has similar timing effects.

17. All valve seats are cut using a cutter that rotates around a pilot that is centered in the guide. Multi-valve race heads have very small diameter valve stems to decrease weight and increase flow. Small diameter pilots flex easier which can allow the cutter to take a path of less resistance making it very hard to cut a concentric seat. Solid carbide pilots and high tech machining equipment with air float centering heads have to be used as opposed to old school hand cutting methods. For instance the valve stem diameter on a CRF50 is 6mm and larger than a YZ250f’s at 4mm, cars are usually around 9mm. Motocross heads should only be machined by machine shops that specialize and understand the complexity of these types of heads. About a quarter of the heads we repair come from dealerships, back yard mechanics and auto machine shops that attempted cutting the seats and failed.

18. Cam journals can be ruined from lack of oil, oil pressure, improper cam cap bolt torque, extreme over heating, improper warm up and dry starts. Motocross heads with cam journals machined in the casting usually retails around $600 or more without the valve train. You may want to think twice if you punch a hole in your case and hope to make it back to the pits.

19. The piston passing cylinder ports in a 2-stroke has the same function as valves and cams in a 4-stroke.

20. Parts like guides and valve seats can be nudged around when the thicker and thinner parts of the casting swell from heat creating a loss of compression due to poor seat to valve concentricity. This can create hard start and tuning problems. Engineers over the last few years have taken note and incorporated this error into their designs which have made the heads a lot more dependable. Improper porting and removing casting material from around the guides and bowl can reverse the solution.

21. High lift cams may require a spring that won’t coil bind but not necessarily a stiffer spring. Stiffer springs are required if the ramp on the lobe of the cam is steeper which opens the valve at higher speeds or if a heavier valve is used.

22. Valve float happens when the valve spring is not strong enough to keep the valve from leaving the lobe of the cam. Some ultra high tech designs incorporate a controlled degree of valve float off the back side of the lob and then meets up at the lower part of the cam before closing at high RPM. The back side of the cam lobe is a bit concave giving the valve a lower lift for higher air velocities at low RPM’s. This is seen in some of the newer KTM’s, F1 etc.

23. Valve bounce happens when the valve hits the seat and bounces off again. This can be caused by improper spring pressure, poor cam control or if the valve to seat concentricity is way off letting one side of the valve hit before the other. This can damage the seat and valve and create a tuning nightmare.

24. Coil valve springs are the limiting factor in RPM’s. Valve springs are subject to high frequency vibrations that resinates back and forth through the wire in the coils. The shock of the lobe striking the lifter, the valve hitting the seat and oscillations set up from other parts of the bike can all have an effect on the springs ability to perform as designed. When opposing frequencies meet somewhere in the winding it can actually deaden the spring pressure. Kind of like a spasm. Just milliseconds of reduced spring pressure at a critical moment can create damage and troubleshooting nightmares. Cam and valve train engineers try to keep the spring deadening pitch above the red line. Many methods are used to raise this point such as longer wire, different size coils, dual springs wound opposite and rubbing against each other, progressive windings with the lower windings touching each other all the time and more tension. Coil springs are limited to around 15,000 RPM. FI motors that turn much higher RPM’s use pneumatic springs. An air pressurized spring system that keeps a progressive and varying tension on the valve and can enable the current motors to run up to 23,000 RPM’s.

25. Most motocross cam sprockets or sprocket holders are pressed onto the cam shaft and are not keyed. In certain instances the sprocket can slip which changes the cam timing. If this happens the index marks on the sprocket for timing can still be lined up at TDC but the cam lobes will open the valves at the wrong time. If only a slight slip a change in performance or compression may be noticed. If it slips enough the valve will probably hit the piston and result in major damage. The cam sprocket may slip on the camshaft if there is major resistance on the shaft such as journal seizure, over torque cam cap, lifter buck seizure, broken valve or if something gets caught in the cam chain. The factory does not scribe marks that indicate the shaft to sprocket alignment. It’s a good idea to scribe your own index mark on the sprocket and shaft when its running good in order to rule it out when troubleshooting a poor running condition.

26. The only way to check if a cam is opening a valve at the correct time in relation to the crankshaft is to degree it in using a degree wheel on the crank and a dial indicator to indicate when the valve opens. This is not an easy task on most MX heads as you will need to make a special fixture to hold the indicator over the valve lifter. This is nearly impossible as the valve lifters are hard to rest the point of the indicator on. You will also need the duration specs on the cam being tested. I usually suggest just buying a new cam.

27. In one dusty ride, a dry or partially oil covered foam air filter can destroy a top end. In certain cases it can jam a valve stem in its guide but usually sticks the piston rings to the piston and creates undo wear. Spray on filter oil has to be rubbed in and squeezed for complete saturation.

28. Problem inherent to specific models. Honda has cured the intake valve wear problems associated with the earlier non EFI models. All the CRF carbureted models had premature intake valve and seat wear and exhaust guide wear. The original fix was stainless valves with heavier springs the current and correct fix is CrN PVD coated Titanium valves and preferably on Beryllium Copper seats. This outlast all the stainless setups and less obtrusive to the rest of the valve train, especially at high RPM’s. The Kawasaki KX 250's has a problem with the intake seats crumbling and cracked valves which is more prevalent in the EFI models and especially on modified high compression motors and or running. The earlier 04-06 RMZ250 and KX250 had premature valve wear and seat problems due to poor seat concentricity, light intake springs and poor valve coatings. The YZ250F 5 valve will eventually break the intake valves due to the small 4mm stems and the fact they hardly ever wear out and people leave them in way past their life cycle. Usually the middle intake valve wears first. The new YZ250F four valve is to new to evaluate. The KTM250F comes stock with Del West Ti valves and good seats with the proper springs and wear isn’t a real issue. Most the ones we repair have slipped timing which caused the valves to break after hitting the piston and rocker lever wear from oiling problems. The newer KTM250 heads don’t run bearings on either side of the cams but has not been and issue yet. The Honda CRF450 had the same exact issues as the CRF250's above. The new EFI heads are now as good or better than the other MX bikes. The KX450 valve train wears really well but if the head is overheated it’s prone to warping and the exhaust guides tip which offsets the valve on the seats creating a concentricity problem. This usually causes the bike to be a bit harder to start and can create future valve and guide problems if not correct by re-cutting the seats. The problem is often first diagnosed as a blown head gasket but the head usually warps from lack of coolant and then the gasket fails. The first three years of the RMZ450 heads had a lot of problems with cam seizure and cam cap breakage and cam chain tensioner failures which in turn caused the valve to hit the piston. In 2007 Suzuki went to 5.5mm from 5mm valve stems and in 2008 revamped the head into something super reliable. We don’t see a lot of KTM 450's and YZ450's until they have a huge amount of hours.

29. 450's last a lot longer than 250's because very few riders use all the power in a 450.

30. The newer motocross 4-strokes, especially the EFI equipped models are a lot more reliable and wear a lot better than those of just a few years ago.

31. If your motor was built to compete with the top factory pro’s, it would need to be torn down, inspected and rebuilt with new race proven parts every 10 hours or less. Many are only built to last the length of a race. If you only have one bike to practice and race with, on a budget and don’t have a engine builder for a mechanic you may be better off leaving your motor stock.

32. Most engine mods require re-jetting or mapping for optimum performance. This should be done on a dyno in order to apply a load on the motor and an exhaust gas analyzer along with other sensors to acquire the proper data. Lean or rich conditions can then be corrected. EFI equipped models can be re-programed instantly after each run.

We hope the above facts provided you with some valuable insights and knowledge about motocross heads and their unique design. If you have any questions, arguments or have some helpful tips contact Brent Kirk through the Fastheads.com website.

Fastheads / Racers E.R. has been building and rebuilding motocross engines for the last 46 years. Fastheads currently offers new assembled motocross heads and head rebuilds along with customized mods for riders worldwide. P.S. If you answered true to all the above you passed.


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