Eric Gorr Aritical (Valve train in 4 Strokes)
Posted 30 December 2004 - 11:23 AM
VALVE TRAINS Part 1
By Eric Gorr
Modern four-stroke dirt bike engines have a strong lineage in F-1 and Indy car racing. Yamaha’s partnership with Toyota and Honda’s collaboration with Mugen has given us some incredible dirt bike engines. The average 250cc four-stroke revs to 13,400 rpm. At that rpm the piston is moving up and down in the cylinder at 223 times per second, that’s 111 valve events per second. Multi cylinder F-1 and Indy car engines are toast after a 2-hour race, so why do dirt riders expect their four-stroke singles to last for years without maintenance? The answer for most riders is the fear of dealing with complicated valve train systems. Modern valve trains wear at about the same rate as the piston and rings. Some models have characteristic problems that require specialized repairs. Here is a guide to how valve trains work, how to know when something is awry, and some choices on how to fix up your four-stroke dirt bike.
WHAT’S IN A VALVE TRAIN?
A valve train consists of all the parts that work together to control the phases of a four-stroke engine, intake, compression, power, and exhaust. The valve train consists of a cam chain, cam and crank sprockets, cam chain tensioner, cam chain guides, camshaft, tappets or rocker arms, valves, valve springs, valve seats, and valve guides.
The crankshaft drives a sprocket with a cam chain that runs on plastic guides and backed by a tensioner mechanism. The cam sprockets spinning at half the speed of the crankshaft are fastened to the cam with bolts or by interference press-fit. The camshaft lobes ride against rocker arms or tappets that look like buckets and contain a shim-pad to adjust the valve lash. At the heart of the cylinder head is the valve assembly that includes a valve sealing against a seat that is cast into the head and machined to a series of precise angles to maximize flow. The valve is loaded by pressure from a spring, and sandwiched by a base and a top keeper, and held together by a set of retainers. The valve slides through a tightly fitted valve guide with an oil seal to reduce the leakage into the ports.
WHAT GOES WRONG
Valve train parts wear out. The cam chain and sprockets wear just like the parts that drive the rear wheel of your bike. The plastic chain guides wear down and the automatic chain tensioners eventually extend so far that they fail. The tappets fit between the cam and valve so if the clearance is too tight or loose they take a beating and can eventually fracture. The valve stems run through the valve guide and both of those parts wear allowing the valve to wallow against the valve seat. Eventually the valve face wears, if the valve is left in too long it damages the valve seat. If the engine still starts and runs it will break the head off the valve. You’re lucky if the engine won’t start because the alternative is massive meltdown!
REASONS FOR BREAKING
Valve train parts break for five main reasons. Metal fatigue, lack of maintenance, dirt contamination, loss of lubrication, and riding errors.
The most common reason for valve train problems is metal fatigue. Engineers rate valve train parts in number of cycles. The better the grade of material the higher the number of cycles. Valve springs are the highest wear part in a four-stroke engine. When the spring sacks out the valve is allowed to bounce off the valve seat and accelerate wear of the valve and seat. Stock valve springs last about the same as a piston, 20 to 50 engine hours.
Periodic maintenance like adjusting the clearance between the tappets or rocker arms and the cams is critical to finding fatigued parts and replacing them before an engine failure occurs. Normally the wear pattern makes the clearance tighter as the valve wears against the seat. The rule of thumb is if it takes three or more shim sizes to loosen up the valve clearance then the valve face is worn too far and must be replaced. Shimming the valve with a lot of clearance might buy some riding time but will also cause other valve train parts to be over stressed and break.
Another common problem is damage from dirt seeping past the air filter. The abrasive dirt wears off the protective oxide coating on valves made of titanium. The soft titanium wears fast and typically a cupped shape forms on the valve face, making it impossible to seal. Rust forms on the valve seat, on engines that are stored for long periods of time in warm, humid climates like the southeastern US. When the titanium valve strikes the rusty valve seat the coating wears off the valve. Cupped and leaking valves will usually make the engine hard to start. If the worn valves are left in the engine too long, they can break at the head and stem, causing expensive engine damage.
Running low on oil can cause a variety of problems in the engine, but the most vulnerable area is the cam bearing surface. Yamaha, Kawasaki, and Suzuki machine the head and cam cap as the finished bearing surface. If the engine runs low on oil, the cam can seize in the bearing and in most cases it can damage the cylinder head requiring complete replacement.
The most unusual and catastrophic valve train failures are caused from over-revving the engine to the point where the valve springs can’t react quickly enough and allow the valves to float and collide with the piston. The valves break causing the severe engine seizure and stick you with a repair bill of about $2,000. Riding circumstances that cause valve train failures involve downshifting for turns or as an engine brake on steep down hills. In motocross, landing hard from a big jump in too low of a gear can cause the engine to over rev and float the valves. Although all modern four-stroke dirt bikes have rev limiters built into the ignition system, those devices can’t prevent the engine from over-revving from gearbox loads like downshifting on jumps.
LOOKING FOR TROUBLE!
Worn valve train parts give you a fair bit of warning in advance. Funny noises, vibration, oil leaks, hard starting, popping, and just general loss of power are all red flags signaling potential engine problems. If you remove the valve cover every four rides and perform a quick check of the valve clearance, you’ll have a better chance of detecting worn valve train parts before they cause an expensive engine failure.
When the valves get cupped and lose seal, the first indication will be hard starting. The next sign is popping on idle and at high rpm. A grinding and slapping noise at idle might be worn cam chain guides or the tensioner. An unusual sign of trouble is the carb blowing off of its mounting spigot. That is caused by combustion gasses burning past the valve seat and through the carb, causing fresh gasses to ignite and blow the carb off of the rubber-mounting spigot.
When rocker arms and tappets start to crack the engine will make a loud clattering noise moments before it stops abruptly. Well before a failure occurs, the tappets or rockers will show wear patterns on the rolling surfaces. Tiny nicks or fracture lines on tappets or groove marks on rocker rollers are a sure sign of immanent failure.
Cams can seize from lack of oil and stall an engine. When the engine cools down and is possible to start, a seized cam bearing will make a shrieking sound or noticeable clicking sound at idle.
Most bikes have automatic cam chain tensioners. As the chain and guides wear, a spring puts pressure on a plunger that rides against the rear chain guide. The greater the wear on the chain and guides the less the spring tension applied by the tensioner. Sometimes the tensioners fail abruptly allowing the chain to jump one tooth on the cam and crankshaft sprockets. The engine may even continue to start but the power will be sluggish and make a clicking sound at idle.
Posted 30 December 2004 - 11:24 AM
By Eric Gorr
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.
Posted 30 December 2004 - 11:25 AM
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.
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
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.
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.
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.
Posted 16 January 2005 - 08:48 PM