Eric Gorr
Engine Builder
- Jun 29, 1999
- 384
- 12
In the second part of our series on four-stroke valve trains we spotlight the characteristic problems with late model bikes. Now that these bikes have been on the market for at least three model years, dirt riders have tortured these bikes in all sorts of conditions. The information expressed in this article is based on the author’s experience at operating an engine rebuilding business. By examining engine failures, usage patterns, and interviewing customers on maintenance practices, the author has formed opinions on the potential problems in late model bikes and offers a variety of maintenance solutions.
Let’s start by bringing you up to speed with a crash course in valve train materials and modern designs.
VALVE TRAIN MATERIALS AND DESIGNS
In order for late model 4-stroke dirt bikes to be competitive with 2-stroke power output, they need the capability to rev higher. By reducing the valve train mass with titanium valves, softer springs can also be used. Valve spring pressure produces parasitic losses on the engine (drag) so designers try to keep spring rates in check with valve materials. There are two types of titanium valves, solid and two-piece. All OEM titanium valves are made of two pieces with a cast head inertia welded to a stem rod. Inertia welding involves spinning the stem and head in opposite directions at high rpm and merging the two pieces together with pressure. When a two-piece valve breaks, it is usually on the stem next to the weld. Pro Circuit and WMR are marketing solid titanium valves. These valves are machined from a solid bar of titanium so there is a lot of wasted material in exchange for the extra strength. Solid titanium valves cost about $165 each that is about twice as much as OEM two-piece titanium valves.
Some OEM exhaust valves are made of steel because it is inexpensive and durable. Stainless steel is a popular choice of aftermarket valve manufacturers like Kibblewhite and Ferrea. Aftermarket stainless steel valves are the most durable valves and are priced about the same as OEM steel and half the price of OEM titanium. However the biggest cost of installing stainless steel valves is the springs and top retainers.
Valve seat material is also an important factor in the overall design of a valve train. Steel valve seats are the most common material used in motorcycle cylinder heads. Steel seats are easy for the manufacturers to install and the material is inexpensive. However there are two materials that would make better choices for valve seats when using titanium valves. If money were no object and performance is the primary concern, bronze or copper-beryllium are softer materials and offer better heat transfer. A softer material serves to reduce valve bounce and wear on the valve, however the valve seat would need frequent service. Bronze seats are less expensive than copper-beryllium from two perspectives, material cost and installation. Copper-beryllium is the most popular valve seat material used in NASCAR and INDY racing engines because of its superior heat transfer characteristics. Installation of copper-beryllium valve seats is the biggest challenge since the exotic beryllium material’s dust is toxic and requires special machining centers to contain the dust particles. Currently there is no aftermarket company offering copper-beryllium valve seat installations for dirt bikes, but WMR offers bronze seat replacement for KXF/RMZ models.
Springs and Things
The purpose of a valve spring is to keep the valve in contact with the cam lobe, dampen the valve from bouncing of the valve seat when closed, and prevent the valve from floating when the engine is over revved. That’s a lot of work for $8, the average cost of an OEM valve spring for a ti-valved 250cc dirt bike. As mentioned in the valve materials paragraph, springs must be carefully designed taking into consideration factors like the valve mass, cam profile, and engine operating conditions. There are four different types of valve springs used in modern high performance 4-stroke engines. They include coil springs that are single and double straight wound and single conical designs. F-1 engines use gas springs, also referred to as pneumatic valve trains. Gas springs are used on engines that rev beyond the limits of coil spring technology. Most modern dirt bikes use single straight wound coil springs with the exception of some KTM models that use conical springs. OEM valve springs are the weak point of modern dirt bikes, with the relatively cheap springs suffering from metal fatigue before the piston wears out. The difference in price between cheap OEM and expensive aftermarket is profound. A stock spring is priced between $6-8 whereas an aftermarket spring can be as expensive as $50, and you get what you pay for!
Camshafts and Built-in Harmonics
Cam designers make a compromise between performance and valve train reliability. If the motorcycle manufacturers could actually count on dirt bikers to service their engines we could have bikes with considerably more power than current designs. However some models skirt the line more towards performance than reliability. Aftermarket cam manufacturers position their products towards performance or longevity. Generally speaking Hot Cams makes cams that are easier on the valve train components and designed to tolerate stock springs. Web-Cam makes cams that are performance orientated and require the use of aftermarket springs, shortened valve guides, and special performance machining to the cylinder head.
Tools of the Trade
With so many factors to consider, you may wonder how motorcycle manufacturers and aftermarket companies design valve train parts. The answer is in software. There are several professional development suites available to aid designers in selecting the right materials for springs, seats, and valves as well as spring rates, cam profiles, and the position of the valves in the cylinder head. If you’re scientific minded and are curious to learn more, check out this web site on a popular set of software used by NASCAR and Indy engine builders. www.profesorblairandassociates.com Now that you have a basic understanding of the materials and designs used in OEM and aftermarket valve train parts, let’s look at the common problems that occur on modern dirt bikes.
COMMON PROBLEMS WITH LATE MODEL THUMPERS
Patterns of problems with certain year/model dirt bikes are starting to develop. These problems are more rider usage, maintenance, and materials related rather than being design flaws. Keep in mind that the manufacturers are cost limited on certain valve train components and intend for the owner to replace parts as they become fatigued.
Honda CRF models
2004 CRF250
Although no specific service limit is advised in the Honda service manual, you should replace all the valves and springs at the same time as the piston. Honda recommends changing the piston every 15 hours, and in practice I estimate the valve train and piston to have a useable service life of about 30 hours. In fact the valve train usually wears out before the piston. The dangers of leaving the valves in place until the engine won’t start anymore, is that the valve seats will become damaged and require an expensive service of valve seat reconditioning. In a case like that, considering that new head is so inexpensive, it may be cheaper to buy a new head rather than fix the old one.
Another problem that is rare for the CRF250 is a failure of the left side cam bearing. If the engine becomes particularly noisy at idle, or if there is a thick black residue of rubber present under the valve cover, then the cam bearing is worn and placing a greater side load on the cam chain guide. The rear tensioner chain guide is coated with rubber, which accounts for the thick black residue.
2002-04 CRF450
The two most common problems with this model include intake valve wear on all model years and cam chain tensioners that go slack on the 2002 and 03 models. At first the intake valves wear was attributed to dirt bypassing the air filter, causing wear to the protective oxide coating on the titanium intake valves. Some of the other theories touted on the Internet included valve seats that are too hard, too soft of a valve spring, and too steep of a closing ramp on the camshaft. The most reliable solution for the intake valve problem is to install a Kibblewhite spring kit and Black Diamond stainless steel intake valves. The parts cost about $300. Ferrea also makes stainless steel valves and their product features lighter weight and are designed to work with RD brand single coil spring kits. Ron Hamp Cycle offers a choice of lightened Ferrea stainless valves or solid titanium valves with DLC (diamond-like coatings) in standard and oversized along with custom machine work.
There are two options for the cam chain tensioner. The least expensive choice is to buy a tensioner from a 2004 CRF450, which sells for about $52. The 2004 and later Honda part is greatly improved but it should be replaced every 100hrs. Factory Racing of Italy chose a different approach to the problem; they make a manual cam chain tensioner. However manual cam chain tensioners require frequent adjustment and a careful touch. Another area of concern on the Honda is a worn exhaust rocker arm roller. Look for a deep groove to appear in the center of the roller. That indicates that the bearing is worn. You can’t replace the roller itself because it isn’t available from Honda separately. You need to replace the entire rocker assembly, which sells for about $120. If you are a fervent Internet news group reader there are two things that are recommended that should never be attempted. They include grinding down the valve shim pads to give valve to tappet clearance, and installing Honda ATV steel intake valves with the stock springs. When a valve is worn so far that Honda doesn’t offer a small enough shim, its junk and needs to be replaced. With regards to changing valve materials, when you switch from a lightweight titanium valve to a heavier steel valve, you must also install a stiffer spring. One last thing that contradicts the Honda manual and Internet myths, never attempt to use valve grinding compound to pre-finish a titanium valve prior to installation. The gritty compound will damage the oxide coating designed to protect the valve.
Kawasaki, Suzuki KXF/RMZ250
In the first year of production these models suffered growing pains. Some of the problems associated with the valve train parts include an aggressive cam profile that challenges the valve springs and tappets. Riders who constantly bang the engine up against the rev limiter aggravate the problem. This model uses a two-piece titanium valve with a relatively soft spring. If the valve starts to get worn and cup-shaped it will not contact the valve seat evenly which could lead to the valve head breaking away from the valve stem. When checking the valve clearance, if you have to install a shim pad that is two sizes smaller than the shim pad that you’re replacing, then you should assume that the valve is worn out and needs replacement. As in the case with all modern 250cc 4-strokes, replace the piston kit, valves, and springs all at the same time if using OEM parts. Typical service limits range from 15hrs for experts to 40hrs for novice riders. There are two other parts of the valve train that should be examined every time that the valve cover is removed, the tappets and the camshaft lobes. If the tappets have a defined circle in the top center, the tappet should be replaced because it is in danger of breaking. The small circle is about the same size as the shim pad, because the worn valve spring is allowing the valve stem to slam the shim pad up against the underside of the tappet.
Pro Circuit sells a high performance package for these models that includes solid titanium valves, a stiffer spring kit, and the optional labor to perform a multi-angle valve job, porting, and installation of the valves. WMR offers a similar approach using the solid titanium valves, conical springs, and the option of installing bronze valve seats that are more compatible with the titanium valves.
If you’re more interested in long-term reliability and lower operating costs then Kibblewhite has the solution. They offer stainless steel valves, a dual coil spring kit with titanium retainers, and hardened tappets. Hot Cams offers a cam profile with a less aggressive opening and closing ramp that puts less strain on the valves, guides, and tappets. However there is a slight loss of low end but more power from 8,000-11,000 rpm.
Yamaha YZF models
YZ250F
The YZ250F is the most reliable 250 4-stroke, but that tends to make owners lethargic about maintenance. Some of the issues with this bike include valve seats that can rust in humid climates, valves springs with a short lifespan, and a cam chain of relatively poor quality that tends to wear the sprocket that is part of the crankshaft. If the cam chain isn’t replaced with every top end rebuild, it can cause wear dot to appear on the sprocket teeth. When the sprocket wears the crankshaft must be replaced since the sprocket isn’t removable. There is no better alternative for the cam chain; stock OEM is the only choice available.
Riding situations that tend to stress the limits of the valve train and can cause valve floating and engine failure is downshifting in the air over big jumps and landing with the throttle on. The kinetic energy of the motorcycle contacting the ground serves to raise the engine rpm past the limits of the valve springs enabling the valves to float and contact the piston. Rev limiters don’t offer any protection when you downshift and land from a big jump.
When checking the valve clearance, if the clearance tightens up so much that you need to install a shim pad 2-3 sizes smaller, the valve is in danger of breaking the head and the valves and springs should be replaced.
Kibblewhite offers a White Diamond stainless steel valve kit and dual coil springs as an endurance alternative to the stock titanium parts.
YZ400/426F
The steel valves used on the 1998-2000 models have an excellent reliability record. However there are some typical problems with the later model titanium valves. Cheap OEM valve springs have a short and violent life. When extended past the service limit they tend to crack, sending the pieces throughout the cylinder head, typically getting ground up by the camshaft and tappets. One unusual problem observed on YZFs is rusty valve seats. This problem seems to be more common on bikes used in humid climates like the Southern and Mid-Atlantic States. When the engine is shut off, kick the engine over until the piston comes to TDC. If any of the valves are open, moisture in the air will condensate on the valve seat causing corrosion. That’s not a problem for steel valves but a rough valve-seating surface will damage the oxide coating of a titanium valve.
YZ450F
The new generation YZF had some minor problems in the first year of production 2003. The rubber cap used to plug the old manual decompressor passage tends to pop out, allowing a loss of oil from the front of the engine. If the rider doesn’t immediately notice the leaking oil, the cams will seize to the head. Several companies make billet caps with fastening bolts. The cam chain tensioner can loose spring pressure and retract, which may cause the cam chain to jump off of the cam sprockets allowing the valves to contact the piston. The only aftermarket alternative is Factory Racing’s manual tensioner.
The stock titanium valves have a good service record in comparison to the CRF450. If you’re looking for better top end power and longevity, consider a Kibblewhite oversize stainless steel valve and spring kit.
MAINTENANCE PLANS
This section gives you estimates of the cost per hour of maintenance for OEM parts, aftermarket endurance parts, and high performance aftermarket valve train and top end parts. Of course this figure is for parts only. If you don’t have the tools or skills to work on a 4-stroke engine, then you’ll have to consider dealing with a local franchised motorcycle dealer or non-franchised dirt bike specialist. Mail-order service is an excellent alternative because the talent base is likely to be deeper than your local area.
The maintenance intervals quoted in hours are very lenient compared to factory recommendations but they tend to err towards the conservative. These recommendations are based on real world experience. And of course some dirt riders will try to push the limits of maintenance intervals in order to reduce the operating cost per hour, but if the engine suffers a catastrophic breakdown it can easily double the cost of the hourly maintenance quoted in this article.
New OEM Parts
In a previous issue of Dirt Rider, David Dye demonstrated top end rebuilding for the YZ250F and provided some guidelines on service limits. He suggested that the average intermediate rider who races and practices on the same bike should do a complete top end job twice a year. A complete top end rebuild includes the piston and gasket kit, cam chain, and all of the valves and springs. The average price of Yamaha OEM parts is about $600 for approximately 30 hours of service or about $20hr. That’s twice as long of a service interval than Yamaha recommends but if the bike is well cared for with frequent oil changes and filter cleaning, the average 250cc 4-stroke will perform well for a 30hr period. 450cc 4-stroke dirt bikes will last about 60hrs. So maintenance costs for the average 450cc dirt bike are about $10hr.
Aftermarket Performance Solutions
If ultimate performance is your motivation then pry-open the piggy bank! Aftermarket performance solutions always require specialized machining. Performance valve train parts can include items like solid titanium valves ($640 set) a spring kit with titanium retainers and chrome silicon dual or conical springs ($225) and bronze valve seats ($350 installed). While you have it apart you might as well have it ported and a multi-angle valve job performed ($550+). Most aftermarket performance companies recommend a specific cam to compliment the package. Cams cost about $170 each, most dirt bikes have two cams. You could easily spend $1,600 to $2,000 on a performance cylinder head and valve train with a part life of 60hrs. or $15hr. So generally speaking, performance parts have a lower hourly operating cost than OEM parts but the upfront machining costs are much greater.
Aftermarket Reliability Solutions
If you’re less concerned about performance and want to lower your maintenance costs and extend service intervals, here is a reliability solution. Kibblewhite makes the most durable valve train parts. Their stainless steel valves are hard coated with either nitro-meg or hard chrome based on the valve guide material. The spring kits feature dual coil springs with lightweight titanium retainers. The valve and spring kits don’t require special machining just a good quality spring compressor and some common sense. Stainless steel valves can be lapped for a better seal but if the engine was run too long with worn valves, the valve seats may need to be reconditioned with a valve job. Valve jobs average $40 per seat or $160-200 per cylinder head. The cost of a set of Kibblewhite valves and springs is $360 and the average life span is 300hrs. for an average cost of $1.20hr.
Let’s start by bringing you up to speed with a crash course in valve train materials and modern designs.
VALVE TRAIN MATERIALS AND DESIGNS
In order for late model 4-stroke dirt bikes to be competitive with 2-stroke power output, they need the capability to rev higher. By reducing the valve train mass with titanium valves, softer springs can also be used. Valve spring pressure produces parasitic losses on the engine (drag) so designers try to keep spring rates in check with valve materials. There are two types of titanium valves, solid and two-piece. All OEM titanium valves are made of two pieces with a cast head inertia welded to a stem rod. Inertia welding involves spinning the stem and head in opposite directions at high rpm and merging the two pieces together with pressure. When a two-piece valve breaks, it is usually on the stem next to the weld. Pro Circuit and WMR are marketing solid titanium valves. These valves are machined from a solid bar of titanium so there is a lot of wasted material in exchange for the extra strength. Solid titanium valves cost about $165 each that is about twice as much as OEM two-piece titanium valves.
Some OEM exhaust valves are made of steel because it is inexpensive and durable. Stainless steel is a popular choice of aftermarket valve manufacturers like Kibblewhite and Ferrea. Aftermarket stainless steel valves are the most durable valves and are priced about the same as OEM steel and half the price of OEM titanium. However the biggest cost of installing stainless steel valves is the springs and top retainers.
Valve seat material is also an important factor in the overall design of a valve train. Steel valve seats are the most common material used in motorcycle cylinder heads. Steel seats are easy for the manufacturers to install and the material is inexpensive. However there are two materials that would make better choices for valve seats when using titanium valves. If money were no object and performance is the primary concern, bronze or copper-beryllium are softer materials and offer better heat transfer. A softer material serves to reduce valve bounce and wear on the valve, however the valve seat would need frequent service. Bronze seats are less expensive than copper-beryllium from two perspectives, material cost and installation. Copper-beryllium is the most popular valve seat material used in NASCAR and INDY racing engines because of its superior heat transfer characteristics. Installation of copper-beryllium valve seats is the biggest challenge since the exotic beryllium material’s dust is toxic and requires special machining centers to contain the dust particles. Currently there is no aftermarket company offering copper-beryllium valve seat installations for dirt bikes, but WMR offers bronze seat replacement for KXF/RMZ models.
Springs and Things
The purpose of a valve spring is to keep the valve in contact with the cam lobe, dampen the valve from bouncing of the valve seat when closed, and prevent the valve from floating when the engine is over revved. That’s a lot of work for $8, the average cost of an OEM valve spring for a ti-valved 250cc dirt bike. As mentioned in the valve materials paragraph, springs must be carefully designed taking into consideration factors like the valve mass, cam profile, and engine operating conditions. There are four different types of valve springs used in modern high performance 4-stroke engines. They include coil springs that are single and double straight wound and single conical designs. F-1 engines use gas springs, also referred to as pneumatic valve trains. Gas springs are used on engines that rev beyond the limits of coil spring technology. Most modern dirt bikes use single straight wound coil springs with the exception of some KTM models that use conical springs. OEM valve springs are the weak point of modern dirt bikes, with the relatively cheap springs suffering from metal fatigue before the piston wears out. The difference in price between cheap OEM and expensive aftermarket is profound. A stock spring is priced between $6-8 whereas an aftermarket spring can be as expensive as $50, and you get what you pay for!
Camshafts and Built-in Harmonics
Cam designers make a compromise between performance and valve train reliability. If the motorcycle manufacturers could actually count on dirt bikers to service their engines we could have bikes with considerably more power than current designs. However some models skirt the line more towards performance than reliability. Aftermarket cam manufacturers position their products towards performance or longevity. Generally speaking Hot Cams makes cams that are easier on the valve train components and designed to tolerate stock springs. Web-Cam makes cams that are performance orientated and require the use of aftermarket springs, shortened valve guides, and special performance machining to the cylinder head.
Tools of the Trade
With so many factors to consider, you may wonder how motorcycle manufacturers and aftermarket companies design valve train parts. The answer is in software. There are several professional development suites available to aid designers in selecting the right materials for springs, seats, and valves as well as spring rates, cam profiles, and the position of the valves in the cylinder head. If you’re scientific minded and are curious to learn more, check out this web site on a popular set of software used by NASCAR and Indy engine builders. www.profesorblairandassociates.com Now that you have a basic understanding of the materials and designs used in OEM and aftermarket valve train parts, let’s look at the common problems that occur on modern dirt bikes.
COMMON PROBLEMS WITH LATE MODEL THUMPERS
Patterns of problems with certain year/model dirt bikes are starting to develop. These problems are more rider usage, maintenance, and materials related rather than being design flaws. Keep in mind that the manufacturers are cost limited on certain valve train components and intend for the owner to replace parts as they become fatigued.
Honda CRF models
2004 CRF250
Although no specific service limit is advised in the Honda service manual, you should replace all the valves and springs at the same time as the piston. Honda recommends changing the piston every 15 hours, and in practice I estimate the valve train and piston to have a useable service life of about 30 hours. In fact the valve train usually wears out before the piston. The dangers of leaving the valves in place until the engine won’t start anymore, is that the valve seats will become damaged and require an expensive service of valve seat reconditioning. In a case like that, considering that new head is so inexpensive, it may be cheaper to buy a new head rather than fix the old one.
Another problem that is rare for the CRF250 is a failure of the left side cam bearing. If the engine becomes particularly noisy at idle, or if there is a thick black residue of rubber present under the valve cover, then the cam bearing is worn and placing a greater side load on the cam chain guide. The rear tensioner chain guide is coated with rubber, which accounts for the thick black residue.
2002-04 CRF450
The two most common problems with this model include intake valve wear on all model years and cam chain tensioners that go slack on the 2002 and 03 models. At first the intake valves wear was attributed to dirt bypassing the air filter, causing wear to the protective oxide coating on the titanium intake valves. Some of the other theories touted on the Internet included valve seats that are too hard, too soft of a valve spring, and too steep of a closing ramp on the camshaft. The most reliable solution for the intake valve problem is to install a Kibblewhite spring kit and Black Diamond stainless steel intake valves. The parts cost about $300. Ferrea also makes stainless steel valves and their product features lighter weight and are designed to work with RD brand single coil spring kits. Ron Hamp Cycle offers a choice of lightened Ferrea stainless valves or solid titanium valves with DLC (diamond-like coatings) in standard and oversized along with custom machine work.
There are two options for the cam chain tensioner. The least expensive choice is to buy a tensioner from a 2004 CRF450, which sells for about $52. The 2004 and later Honda part is greatly improved but it should be replaced every 100hrs. Factory Racing of Italy chose a different approach to the problem; they make a manual cam chain tensioner. However manual cam chain tensioners require frequent adjustment and a careful touch. Another area of concern on the Honda is a worn exhaust rocker arm roller. Look for a deep groove to appear in the center of the roller. That indicates that the bearing is worn. You can’t replace the roller itself because it isn’t available from Honda separately. You need to replace the entire rocker assembly, which sells for about $120. If you are a fervent Internet news group reader there are two things that are recommended that should never be attempted. They include grinding down the valve shim pads to give valve to tappet clearance, and installing Honda ATV steel intake valves with the stock springs. When a valve is worn so far that Honda doesn’t offer a small enough shim, its junk and needs to be replaced. With regards to changing valve materials, when you switch from a lightweight titanium valve to a heavier steel valve, you must also install a stiffer spring. One last thing that contradicts the Honda manual and Internet myths, never attempt to use valve grinding compound to pre-finish a titanium valve prior to installation. The gritty compound will damage the oxide coating designed to protect the valve.
Kawasaki, Suzuki KXF/RMZ250
In the first year of production these models suffered growing pains. Some of the problems associated with the valve train parts include an aggressive cam profile that challenges the valve springs and tappets. Riders who constantly bang the engine up against the rev limiter aggravate the problem. This model uses a two-piece titanium valve with a relatively soft spring. If the valve starts to get worn and cup-shaped it will not contact the valve seat evenly which could lead to the valve head breaking away from the valve stem. When checking the valve clearance, if you have to install a shim pad that is two sizes smaller than the shim pad that you’re replacing, then you should assume that the valve is worn out and needs replacement. As in the case with all modern 250cc 4-strokes, replace the piston kit, valves, and springs all at the same time if using OEM parts. Typical service limits range from 15hrs for experts to 40hrs for novice riders. There are two other parts of the valve train that should be examined every time that the valve cover is removed, the tappets and the camshaft lobes. If the tappets have a defined circle in the top center, the tappet should be replaced because it is in danger of breaking. The small circle is about the same size as the shim pad, because the worn valve spring is allowing the valve stem to slam the shim pad up against the underside of the tappet.
Pro Circuit sells a high performance package for these models that includes solid titanium valves, a stiffer spring kit, and the optional labor to perform a multi-angle valve job, porting, and installation of the valves. WMR offers a similar approach using the solid titanium valves, conical springs, and the option of installing bronze valve seats that are more compatible with the titanium valves.
If you’re more interested in long-term reliability and lower operating costs then Kibblewhite has the solution. They offer stainless steel valves, a dual coil spring kit with titanium retainers, and hardened tappets. Hot Cams offers a cam profile with a less aggressive opening and closing ramp that puts less strain on the valves, guides, and tappets. However there is a slight loss of low end but more power from 8,000-11,000 rpm.
Yamaha YZF models
YZ250F
The YZ250F is the most reliable 250 4-stroke, but that tends to make owners lethargic about maintenance. Some of the issues with this bike include valve seats that can rust in humid climates, valves springs with a short lifespan, and a cam chain of relatively poor quality that tends to wear the sprocket that is part of the crankshaft. If the cam chain isn’t replaced with every top end rebuild, it can cause wear dot to appear on the sprocket teeth. When the sprocket wears the crankshaft must be replaced since the sprocket isn’t removable. There is no better alternative for the cam chain; stock OEM is the only choice available.
Riding situations that tend to stress the limits of the valve train and can cause valve floating and engine failure is downshifting in the air over big jumps and landing with the throttle on. The kinetic energy of the motorcycle contacting the ground serves to raise the engine rpm past the limits of the valve springs enabling the valves to float and contact the piston. Rev limiters don’t offer any protection when you downshift and land from a big jump.
When checking the valve clearance, if the clearance tightens up so much that you need to install a shim pad 2-3 sizes smaller, the valve is in danger of breaking the head and the valves and springs should be replaced.
Kibblewhite offers a White Diamond stainless steel valve kit and dual coil springs as an endurance alternative to the stock titanium parts.
YZ400/426F
The steel valves used on the 1998-2000 models have an excellent reliability record. However there are some typical problems with the later model titanium valves. Cheap OEM valve springs have a short and violent life. When extended past the service limit they tend to crack, sending the pieces throughout the cylinder head, typically getting ground up by the camshaft and tappets. One unusual problem observed on YZFs is rusty valve seats. This problem seems to be more common on bikes used in humid climates like the Southern and Mid-Atlantic States. When the engine is shut off, kick the engine over until the piston comes to TDC. If any of the valves are open, moisture in the air will condensate on the valve seat causing corrosion. That’s not a problem for steel valves but a rough valve-seating surface will damage the oxide coating of a titanium valve.
YZ450F
The new generation YZF had some minor problems in the first year of production 2003. The rubber cap used to plug the old manual decompressor passage tends to pop out, allowing a loss of oil from the front of the engine. If the rider doesn’t immediately notice the leaking oil, the cams will seize to the head. Several companies make billet caps with fastening bolts. The cam chain tensioner can loose spring pressure and retract, which may cause the cam chain to jump off of the cam sprockets allowing the valves to contact the piston. The only aftermarket alternative is Factory Racing’s manual tensioner.
The stock titanium valves have a good service record in comparison to the CRF450. If you’re looking for better top end power and longevity, consider a Kibblewhite oversize stainless steel valve and spring kit.
MAINTENANCE PLANS
This section gives you estimates of the cost per hour of maintenance for OEM parts, aftermarket endurance parts, and high performance aftermarket valve train and top end parts. Of course this figure is for parts only. If you don’t have the tools or skills to work on a 4-stroke engine, then you’ll have to consider dealing with a local franchised motorcycle dealer or non-franchised dirt bike specialist. Mail-order service is an excellent alternative because the talent base is likely to be deeper than your local area.
The maintenance intervals quoted in hours are very lenient compared to factory recommendations but they tend to err towards the conservative. These recommendations are based on real world experience. And of course some dirt riders will try to push the limits of maintenance intervals in order to reduce the operating cost per hour, but if the engine suffers a catastrophic breakdown it can easily double the cost of the hourly maintenance quoted in this article.
New OEM Parts
In a previous issue of Dirt Rider, David Dye demonstrated top end rebuilding for the YZ250F and provided some guidelines on service limits. He suggested that the average intermediate rider who races and practices on the same bike should do a complete top end job twice a year. A complete top end rebuild includes the piston and gasket kit, cam chain, and all of the valves and springs. The average price of Yamaha OEM parts is about $600 for approximately 30 hours of service or about $20hr. That’s twice as long of a service interval than Yamaha recommends but if the bike is well cared for with frequent oil changes and filter cleaning, the average 250cc 4-stroke will perform well for a 30hr period. 450cc 4-stroke dirt bikes will last about 60hrs. So maintenance costs for the average 450cc dirt bike are about $10hr.
Aftermarket Performance Solutions
If ultimate performance is your motivation then pry-open the piggy bank! Aftermarket performance solutions always require specialized machining. Performance valve train parts can include items like solid titanium valves ($640 set) a spring kit with titanium retainers and chrome silicon dual or conical springs ($225) and bronze valve seats ($350 installed). While you have it apart you might as well have it ported and a multi-angle valve job performed ($550+). Most aftermarket performance companies recommend a specific cam to compliment the package. Cams cost about $170 each, most dirt bikes have two cams. You could easily spend $1,600 to $2,000 on a performance cylinder head and valve train with a part life of 60hrs. or $15hr. So generally speaking, performance parts have a lower hourly operating cost than OEM parts but the upfront machining costs are much greater.
Aftermarket Reliability Solutions
If you’re less concerned about performance and want to lower your maintenance costs and extend service intervals, here is a reliability solution. Kibblewhite makes the most durable valve train parts. Their stainless steel valves are hard coated with either nitro-meg or hard chrome based on the valve guide material. The spring kits feature dual coil springs with lightweight titanium retainers. The valve and spring kits don’t require special machining just a good quality spring compressor and some common sense. Stainless steel valves can be lapped for a better seal but if the engine was run too long with worn valves, the valve seats may need to be reconditioned with a valve job. Valve jobs average $40 per seat or $160-200 per cylinder head. The cost of a set of Kibblewhite valves and springs is $360 and the average life span is 300hrs. for an average cost of $1.20hr.
