Cylinder Head Maintenance and Repair.

We use the new Rottler SG7 race designed seat and guide machine

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 faster than road race engines due to the digestion of abrasives past the valves and seats.  The seats wear convex and the valves concave.  When a new valve is installed on worn seats the mating surface becomes very thin and won’t transfer valve heat resulting in even faster wear on the valve.

We can do general maintenace on your seats (seat cutting) or major head repair (Replacing seats and guides)

Saves money over replacement heads!
Seats cut for max power or endurance, your choice!
We offer quality valves, seats, guides and springs!
Seat blending, porting, surfacing available!
Stainless steel or titanium valve kits designed for your needs!

Give us a call for options and price quotes
435-668-7776

Modern performance head machining and design

Old vs New machining techniques:

Briefly, this is why old school seat machining and the machines themself 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 newer style machine technology and a knowledgeable operator. Perfect concentricity is creating the exact same distance from the centerline of the guide to the seat. The seat also has to be the exactly perpendicular to that same centerline. The heads used on the newer multi valve sport and off road race bikes run very small diameter valve guides that are place a lot farther 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 in a nut shell.

A pilot shaft is fit into the valve guide in order to achieve a center axis for a the seat cutting tool. The actual cutting tool can be a grinding stone, a multiple blade carbide cutter or a multi angle single blade cutter and 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 concentricity 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 this 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 its has become elliptical from wear or less precise previous seat cutting. Grinding wheels and multiple blade angle cutters try 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 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 a hand held 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 decrease 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 dimensions 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 try 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 a the seat next to it. This keep the valve shim size the same. To 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. Its really easy to make mistakes especially when using non-top supported pilots and having to measure cut widths when using single angle cutters.

Now that you have an idea on how these different cutting methods are used we will focus on all the factors that come into play in order to reduce the out of round and non-centered seat. Lets start with the pilots. A tapered dead pilot can’t move in the guide but it can tip or be tipped due to it’s nature and the wear in the guide. A live straight pilot can only tip as far as the angle created from the upper and lower edge of the guide and the clearance of the pilot to the guide. A dead pilot will have more of a tendency to cut concentric seat if it doesn’t tip while cutting and if the spindle in the machine which is live doesn’t have any play. With a dead pilot system the valve stem would have to angle itself or flex when seated if the pilot was off centered even though the seat diameter was perfectly concentric. Normally with a dead tapered pilot system you will only need one or two pilot guide diameter. Say your valve guide is 5mm and not two worn you might just have to stock a 4.98 and a 5. In a life pilot system you want a tight fit but it has to turn so you might need a 4.96 to a 5.1. Each carbide pilot can cost $125 or more. Now you are asking yourself witch is the best system, dead or live. Most of the high volume automotive machine shops tend to like the taper pilot system. They are faster and easier to set up and can stay within factory specs. Race shops, on the other hand usually prefer the live pilots because they obtain a higher degree of accuracy. Especially shops that deal in smaller high performance multi valve heads. You have to remember that any tolerances in the pilot to guide and the spindle will show up equally in the concentricity of the seat. A standard automotive valve job on either machine may have one or two thousands concentricity and still only create 10 to 15% leak down. A leak down test is performed by filling the cylinder at TDC on the compression stroke with a steady supply of pressure, say 100 lbs and what leaks off is the amount escaping through the valve seats, piston rings or gaskets. 0-5% perfect for race engines, 10-15% usually won’t change tuning to much but will be down on performance. 20% or more means the engine needs to be rebuilt depending on the source of the leak. Valve clearance less than 0 will also cause leak down so check that first. Leak down or head problems on a single cylinder race machine that doesn’t have a few extra good cylinders to cover up a problem is even more critical on a perfect valve job.

The perfect head job.

Other than your wife or girlfriend. They seem to be masters and have it down to a science. I’m talking about the upper head. Back to the wonderful world of engines. In order for a head to be considered a race head you would have to have to have all the seats concentric to withing .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 need a bit more on the exhaust and smaller for better flow on the intake. This is also were concentricity comes in. If your valve is only touching part of the seat it won’t transfer heat near as fast. You will be using new valves because you can’t cut the face of modern valves or you will loose the hard facing and they all should be exactly the same. Sense all the valves are identical the shim sizes would be the same because the race machinist to each seat to the same depth. The guides would reamed to spec or at least within wear limits before machining the seats. All of this would be done most likely on live pilot machine and most likely a Serdi. Set up is just as critical as the actual machine at from this point on. In order to keep super tight tolerances the machinist would not only have to know the feel and familiarity of the machine he’s working on but also use the tightest pilots, have 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 a air float system for the motor/spindle and some 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 machine can cut on an angle to adapt to the angle of the guide. Gravity from the weight of the spindle can also off center the pilot to the seat. Its therefor better to tip the head until the guide is in line with the pilot and spindle. A lighter spindle/motor/floating head is better for alignment on small 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 is almost always 45 degrees. There are hundreds of differently 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 to 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 as in 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 the seats. Valve spring harmonics can also be reduced due to the way the 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 centerline of the guide. The stem of valve actually has to flex when the head of the valve is force to conform with the seat surface under pressure. This eventually weakens the valve and the head can break off causing catastrophic damage.

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 it is usually overlooked. If you can’t put it on a shelf in every performance shop in the country you can’t really 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.