Why is the modern 4-stroke MX dirt bike engine not built to do a high number of hours?
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 14,000 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. Genuine valves and springs have a higher wear than the piston and rings.
In order for the model 4-stroke dirt bike engine to be competitive with 2-stroke power output, they need the capability to rev higher and make more power. Since there are a lot more moving costly parts in a 4-stroke than a 2-stroke, genuine manufactures are forced to make parts from low grade cheaper materials or poor manufacturing process, such as cast pistons, two piece titanium valves and low cost valve spring material to keep the overall cost down.
The manufactures factory race teams Honda, Yamaha, Suzuki, KTM, Kawasaki do not use these parts in their engines because of reliably issues, such items as genuine cast pistons, two piece valves and stock valve springs these first parts thrown in thrash. If Jo public’s bikes engine was fitted with what the factory teams use from new the overall cost of the bike would be much higher and new bikes are built to a budget. But you can buy exactly the same parts as the factory teams use form aftermarket companies and normal at a lower price than you pay for inferior genuine parts.
Most people and shops often overlook the valves and springs because they do not realize how very important these two parts are. Valves are a plug and seal compression in the cylinder, what is the point just installing a new piston & rings when you may also maybe losing more power through poor valve sealing. That’s right poor valve sealing can rob the engine of more poor than a worn rings.
Also leaving the genuine titanium valves in when they need to be placed may not have a very nice outcome if one breaks in half, it will most likely destroy the head & barrel or the complete engine if you are up in the rpm when this happens. A lot of manufactures workshop manuals state the titanium valves should be check every 10-15 hours and replaced between 30-40 hours. So what they are saying is they are not made to last, so why would you replace them with more genuine valves again? That are designed not to last very long at all! This is when you should go for a aftermarket valve, because like most aftermarket parts they are made to outlast genuine.
Thank goodness for aftermarket companies that manufacture higher quality parts that are normally at a better price and quality than genuine. That’s right you would not see any real professional 4-stroke race teams or engine builders using genuine, valves, valve springs, pistons these parts are the first that are thrown in the trash, when building a real 4-stroke race engine. (there are many other genuine parts that a thrown the rubbish bin)
Below we will explain about valve train the good the bad - information on different type of valve and valve springs materials, manufacturing processes, and how to fix valve problems alot of 4-stroke engine suffer from.
Most modern 4 stroke motocross bike come fitted with titanium intake and exhaust valves...which might make you think they are made to the same spec or even higher quality as aftermarket manufacturers' valves.
You couldn't be more wrong.
In fact, most genuine titanium valves are very poorly designed. The valve material is for short life applications, and way the valves are manufactured is not the best, all this is done to save a few dollars and there is warranty on new bikes so they can get away with this, and so if the parts have a short life it is better for the manufactures as it means they are hoping you will replace the worn part using their genuine parts again. This is called “Plan obsolescence” (google it) and manufactures make big dollars out of this, yes they make more in selling parts than bikes.
And yes, up to 2015, valves are still being manufactured in this two-piece design. In fact, most genuine manufacturers are still using the same valves as in the early models - they have NOT fixed the problem on new bikes!
What are the problems with genuine titanium valves?
Most genuine titanium valves are made as a two piece parts and valve wield together! Why is this?
Cheap and fast manufacturing to save a few bucks. The reason most genuine valves are made as a two piece part is to keep the cost down. By buying a 4mm-5mm bar as the per made valve stem and then welding the valve head to this bar youn have got a fast cheap made valve. This cheap as manufacturing process has less wastage, thus less cost. The correct way to machine strong reliable valve is from a one piece billet bar which is very expensive. Because genuine parts are made to a budget and motocross bikes have no real warranty, long-lasting engine components are not high on the manufacturers list.
The problem with the genuine two piece valve design is the welded together area now becomes the weak point in the valve train and after hours of use the low quality genuine valve spring losses some of its pressure and cannot control the valve train and the valve gets valve bonce, this places immense stain on the valve and it breaks are the weakest point the weld. With two-piece valves, the junction between the stem and head is where the valve often cracks and separates, not because the valve is defective (though a poor weld could be a factor) but because this area is highly stressed due to the joining of two alloys with a weld.
Another problem with Genuine Titanium valves is the soft material they are made from.
Another issue is using titanium as a valve material is titanium is soft and will wears out very fast in stock form, so how does the manufacture get around this? A very thin hard layer is put on outside of the valve; this is around the thickness of a human hair. This offers some protection and slows the wearing process down.
The problem with all coatings they wear off. As the engine does more hours the valve springs wear and cannot control the valve correctly, so valve starts to get some valve to seat bonce which causes the coating to start to come off. Another major causes is if engine sucks in a bit of dust this acts like rubbing sand paper on the valve coating and erodes it away at a very fast rate.
Once the very thin outer protection layer is gone, the valve is then into the soft material and valve sealing face wears at a fast rate, this is called valve cupping or valve recession (view images below). This valve cupping/recession causes the valve to move back into the cylinder heads valve seat, this is moving the valve up towards the top of the engine, in turn closing the valve clearances up. The result is the valves sealing face is held off the valve seat, so it cannot hold the compression in the combustion chamber, making the engine hard to start, low in power and pop or backfire through the carburetor.
At this point do not even try to shim the valve to get the clearance back as this will be very short term fix and you will be wasting your money getting a shop to do this as this fix will only last around a few hours. Only a very poor mechanic or shop will do this because they have no glue of what they are doing and why the valve clearances have gone.
Diagram on the left below is of a engine with valve recession, you will see the valve shim has no clearance between the valve stem end and adjustment shim (part in red), the valve has move back into the valve seat because the hard layer has worn of the titanium valves face - this is marked RECESSED the arrow slows the valve has moved back towards the top of the engine, in turn closing the gap between the end of the valve and the valve shim. This is called no valve clearance all engines must have valve clearance to operate correctly. This engine needs new valves and valve springs and valve seats recut.
Diagram on the right below is of a engine with good valve clearance, there is the correct gap between the end of the valve stem and adjustment shim (part in red), the valve head is above the valve seat - this is marked NORMAL. This is how the a correct valve train should look.
The engine is losing a lot of power with poor valve clearances- Valve to seat sealing is just like a good sealing plug in a bath tub full of water, if you pull the plug off the plug seat it lets out water. That is just like how the valve seals the engines compression inside the combustion chamber. When the valve moves back into the valve seat (recession) it closes the valve clearances up, this holds to valve off the valves sealing face leaking out compression & horsepower.
Not all titanium valve are bad - High quality titanium valves do have their place – factory race teams use them, so I should?
High quality one piece bullet titanium aftermarket that cost around $200+ each are good for full blow races, this is factory race team use. (pro engine builders or real pro race teams would never use genuine two piece valves). These bullet titanium valves are really made for the professional racer at the top of the sport, that needs a full blown race engine that requires every ounce of acceleration, as the rider’s lively hood is riding on every bit of power they can get out of the bike to try and win. But these engines have a very short life and are torn down every few racers and the valves/springs and most other internals are replaced. (So do not believe that because some big name rider or team uses a part in an engine it will be good for you).
Factory teams also installed new valve seats into every new cylinder head this cost around around $250 each and there are 4 of these valve seats. This makes the cylinder head truly compatible with titanium valves. The genuine stock valve seats are the wrong material for titanium valves in a high performance application. If pro teams left the genuine valve seat inserts in and run them even with the best titanium valves, the valves may become cupped and get valve recession in a short time and engine will lose power.
So once again another the poor designed cheap made genuine part is thrown in the trash bin. And once again the poor public once gets another cheap designed genuine part in their bike, all because of the high cost of doing the job correctly. Have a look at the Pro Circuit's website and many other top engine builders are offering replacing the valve seat inserts to make the engine strong & reliable with titanium valves.
Why are Titanium valves installed in most bikes engines from new?
1. New bike tests are all off peak horsepower this makes sales. It is all about sales not how long it will last.
2. Most off road dirt bike engines have no real warrantee, so if it does not last it’s your problem (buy genuine parts they are the best the dealer will say..wot!) If a new car manufacture installed titanium valves in a road going car engine they would have that many claims they would go bust!
3. Off road riding is a very low hour sport normally in short durations, if your car engine did the same hours as the average rider in a year, it would still virtually be new. Lets say you drive from Auckland to Wellington 7-8 times this would be around 64 hours. At this time or often less a bikes engine with genuine parts will most likely need a piston, gasket, valve & springs that is if being rebuild correctly.
We hear this a lot only buy genuine parts! Why? because they have a shorter life than a lot of aftermarket parts, or you just like paying for over priced parts that make genuine manufactures richer! Thats right they make a lot money from parts than selling bikes. Google this term and you will see how it works - Planned Obsolescence.
Titanium valves where never originally never designed to use in engines for the Jo public everyday trial or average motocross rider. These riders require longevity and reliability from their engines and low maintenance cost. Stainless steel is the most durable valve material known to man, as it is hard all the way to the core, has a very low wear rate, and can withstand the highest combustion temperatures of any valve material.
High quality stainless valves are manufactured from a one piece billet, so there is no weak point so the valve will not break. And when installed correct and with the correct valve springs can outlast titanium valves up to 4 times.
Stainless steel requires no hard outer lay placed over the outer surface is the same hardness right to the core. This means no outer protection layer to wear off so the stainless valves will require a lot less maintenance.
Very important don’t be fooled, not all valves are made the same– There are some nasty valve brands out there that are not stainless steel and they are only a low grade steel and some are made as a 2 piece steel valve. These valves are not rated that great and can cause problems at high RPM and suffer from valve burn, not a high heat rating. Ask the person you are getting the valves from to prove they are stainless steel (what grade) and are a one piece valve. If they can not prove it (not by just saying it), don't buy them as it will cost you more in the long run (we know the names of these cheap made brands).
Motoxparts only stock one piece billet stainless steel valve kits. Motoxparts would never bother stocking Ti valves as we do not want to sell parts that give problems. We give our customers valve from money and supply parts that last - We stock: Duramax, Faction MX and Prox valve kits.
The Honda CRF engine is the only dirt bike to have stainless steel valves fitted to the exhaust from new and these very really give a problem. In fact we sell a very high number of stainless steel inlet valve kits for the CRF to replace the worn out titanium valves. But we do not sell many exhaust valves as customers say the the steel exhaust valves are in good condition and the valve clearances still perfect.
Every car, truck, road bike and other road going engines are fitted with steel valves from new so there will be no engine problems, this is one reason road going vehicles can have a five year warranty. If they were fitted with fast wearing titanium valves it would send the manufactures broke as the warranty claims would be so high from titanium valve recession problems.
More coming soon