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Every thing you need to know about thumper's Part 2

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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.


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 materialÂ’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. ThatÂ’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 youÂ’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.


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quote; "so why do dirt riders expect their four-stroke singles to last for years without maintenance?”

I’m not sure years with no maintenance, is expected by anyone. However the current amount and its related cost have significantly increased in the last decade.

Personally I am impressed with the current 4 stroke designs life span based on their horsepower outputs, knowing full well that these are volume production units and having a good understanding of horsepower hours.

a motor designed for 50hp hours belivering 1hp per hour will last 50 hours, but if delivering 50 hp per hour, it will last only 1 hour.

So why does the problem quoted above exist?

I believe it is a combination of three key elements.

1. In our society / sport there exists the need to own the baddest thing on the block, the latest and greatest, even if the owner can't use it to its potential.

2. manufactures want to be able to offer competitive machines, but also must be concerned with production volumes. therefore model offerings are limited. typically breaking our sport into the super high output models, or a trail bike for momma.

3. historically 4 strokes have enjoyed a reputation of high reliability and minimum service needs when compared with 2 strokes. I believe that this subliminal mentality leads many buyers into a situation that is currently false.

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