Most current off road race engines will need the seats cut every 40 to 60 hours, or whenever valves are replaced due to wear. High RPM dirt bikes wear out valves much quicker than road race engines due to the digestion of abrasives past the valves and seats. The seats wear convex and the valves wear concave. When a new valve is installed on worn seats the mating surface becomes very thin and won’t transfer heat, resulting in even faster wear on the valves. We can do general maintenance on your seats by re-cutting the seats, or major head repair such as replacing seats and guides
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Old vs. New Machining Techniques.
Briefly, this is why old school seat machining and the machines don’t work with the newer high RPM performance multi-valve head designs. It’s virtually impossible to machine near perfect valve seat concentricity without the use of the newer style machine technology and a knowledgeable operator. Perfect concentricity is creating the exact same distance from the center-line of the guide to the seat. The seat also has to be exactly perpendicular to that same center-line. The heads used on the newer multi-valve sport and off-road bikes run very small diameter valve guides that are placed a lot further from the valve seat. This is due to the huge ports needed for flow volume and also for an unobstructed flow around the valve stem. The valve seat surface itself is much larger than those used on earlier engines, especially on the single cylinder, large displacement, off-road race bikes.
Seat Machining Process.
A pilot shaft is fitted into the valve guide in order to achieve a center axis for the seat cutting tool. The actual cutting tool can be a grinding stone, multiple blade carbide, or a multi-angle single blade cutter; with the newest design being a single cutting point that is computer controlled to cut the angles. Just remember that all of these cutters get the center-line axis location through the pilot. The cutters, or grinding stones, can be turned by a hand crank or a motor drive. The pilot can also be stabilized from above the tool holder in a spindle to hold a truer axis for the cutter. Grinding stones and multi-cutting blade single angle tooling usually can’t use a top supporting system. The pilot can either be dead or live. Dead pilots are tapered and fit tight in the guide and the cutting tool holder spins on the pilot shaft. Live pilots are straight and turn in the guide and the cutting tool holder is anchored to the pilot.
Machining Factors on Cencentricity Tolerances.
The newer heads run a valve with a smaller diameter stem. Consequently the pilot used is also smaller creating the possibility for more flex in the pilot shaft when cutting. To reduce, flex pilots have gone from tool steel to carbide, which reduces the flex by about 10 times. The cutter and pilot will try to follow the old seat diameter even if it has become elliptical from wear or less precise previous seat cutting. Grinding wheels and multiple blade angle cutters are used to create a naturally more concentric diameter, because they even out the pressure on the cutting edges either all the way around, as in a stone, or in several points, like in the neway cutters, compared to single edge cutters. Unfortunately these type of cutters are only supported with the pilot in the lower guide and not the top, which creates a huge possibility for lateral pressure on the pilot while turning it by hand or by hand help motor. The other set back by these now ancient methods is that they can only cut one angle at a time. Whenever one angle is cut the width of the adjoining angle is decreased, creating a painstakingly long, drawn out time to create a good valve job that aligns on the face of the valve properly and is exactly the same dimension and depth as the seat next to it. The current cutter is a single blade that has all the angles and radius built in so you can duplicate in the next seat. Sense the cutting edge puts all its pressure against the seat in one point it tries to flex the pilot in the opposite direction and will follow the old seat. To reduce flex and cut on center, the pilot on this type of cutting is also anchored at the top and both axis should be in perfect alignment. A carbide pilot has to be used especially on the smaller diameter guides. The blade is adjusted in or out depending on the location needed to center on the valve face. Once this is set, you really only need to measure the depth of the cut toward the guide so that it is the same as the seat next to it. This keeps the valve shim size the same. Too deep of a cut will diminish spring tension, reduce shim size and also lower the compression because the valve will sit deeper in the head. Every time a mistake is made, you have to cut deeper to clean it up. It is really easy to make mistakes, especially when using non-top supported pilots and having to measure cut widths when using single angle cutters.
The Main Job.
In order for a head to be considered a “race head,” you would have to get all the seats concentric to .0005″ or less. That’s on half of a thousandth. The seats would also have to be smooth without chatter marks, because you can’t lap in titanium valves with a nitrate or some other micro hard surface coating. For racing, and not necessarily endurance, you would probably see a .8mm intake and a 1.0mm exhaust seat width. The valve cools mostly through contact with the seat and so you can’t cut the face of modern valves or you will lose the hard facing and they all should be exactly the same. Sense all the valves are identical the shim size would be the same because the machinist cut each seat to the same depth. The guides are reamed to spec or at least within wear limits before machining the seats. All this would be done most likely on the live pilot machine and most likely a Serdi. Set up is just as critical as the actual machine from this point on. In order to keep super tight tolerances, the machinist would not only have to know the feel and familiarity if the machine he is working on, but also need to use the tightest pilots with zero clearance in the spindle bearings and be able to align the pilot perfectly between the guide and the spindle. On a dead pilot machine, the spindle ball mechanism would have to be free of play. Most high dollar machines have an air float system for the motor/spindle and been even on the bed where the head is clamped. Certain machines have heavier floating parts that can overcome the strength of the smaller pilots used in small multi valve car, karting, and motorcycle race engines. The spindle on most modern seat and guide machines can cut on an angle to adapt to the angle of the guide. Gravity from the weight of the spindle can also cause the pilot to become off-center from the seat. It is better to tip the head until the guide is in line with the pilot and spindle. A lighter spindle/motor/floating head is better alignment on motors. Certain companies are building equipment with this in mind, but most will still do heavy automotive heads. The most common 3 angle seat is a 60/45/30. The angle the valve comes in contact with the most is usually always the 45 degree angle. There are hundreds of different cut bits, including 3 and 5 angle and radius for racing. Most machines use a similar tool holder layout and will accept the original Serdi style tooling.
The End Result.
You can gain 10%-15% more horsepower between an acceptable and a real race prepared valve job. The better seal you have on the valves the higher the compression will be. Especially at low RPM, when the compression has more time to leak out. For those racers that are on and off the throttle, like Motocross and tight course racing, the largest gain from a perfect seat is in the lower RPM range. After that the flow characteristics kick in from the angles of the seat and the porting. The valves will cool better and have less hot spots in the seat because they are touching the seat all the way around. If you pick the right multi angle cutting bit for a better flow, you will pack a lot more mixture into the chamber. Carburetor tuning is more consistent because the valves aren’t constantly moving around on the seats. Valve spring harmonics can also be reduced due to the way the valve hits the whole seat at the same time. Imagine a valve hitting the high part of a seat that wasn’t cut perpendicular to the center-line of the guide. The stem of the valve actually had to flex when the head of the valve is forced to conform with the seat surface under pressure. This eventually weakens the valve and the head can break, causing catastrophic damages. Top race tuners know that given a certain engine size and compression, the 5mm area that includes the seat and the valve is the most critical in achieving the greatest amount of power and yet is still usually overlooked. If you can’t put it on a shelf in every performance shop in the country, you really can’t market it. Most mechanics and race shops don’t have the equipment to do a standard valve job, so they opt to sell bolt-on performance or sell new heads if needed.