Let’s Restore the Condition of the Engine with Increased Mileage by Cutting the Valve Seat

As the mileage of both motorcycles and cars increases, it is undeniable that fatigue builds up in each part, and performance decreases. when the condition of the intake and exhaust valves, which are the most important part of the cylinder head of a 4-stroke engine, decreases, various effects such as decreased punching power and fuel consumption will occur. In such cases, ask an internal combustion engine shop to cut the valve seats.

The way the valve seat meets the valve face is crucial.

The combustion chamber of the Yamaha Tricker had a large amount of carbon deposits on both the intake and exhaust valves after 40,000km. Since the motorcycle is mainly used for commuting, there are not many opportunities to rev the engine to high rpm, so the carbon deposits were not burned off. The low viscosity oil used in the air-cooled single-cylinder engine, which is difficult to raise the oil temperature in the winter, may have caused a large amount of blow-by gas to be generated. The valve seats barely have a metallic sheen to them, but there are traces of carbon seized all around.

The valve face has a strip of carbon residue where it meets the valve seat, and even after removing the carbon buildup on the valve umbrella and stem, the carbon black spots remain. On engines with stronger valve springs and higher lift camshafts, the face where the valve seats meet can become gouged over the course of many miles.

The intake and exhaust valves of a 4 stroke engine are required to open and close at the right time during all stages of intake, compression, combustion, and exhaust. A lot of attention is paid to the tuning elements around the cylinder head such as camshafts, valve springs, large-diameter valves, and port polishing, but in the end, the condition of the valves determines whether or not these high-performance parts can be utilized.

Specifically, it is important to close the valves firmly. No matter how much lift or action-angle a camshaft has, it is useless if the valves do not close and compression pressure escapes. In order for the valves to close properly, it is necessary to manage and adjust the tappet clearance, but even more important than that is the condition of the valve seats and valves.

The valve seat is cast into the cylinder head to receive the valves that open and close and has the role of keeping the combustion chamber sealed and releasing the heat from the valves to the head side. On top of that, the valve seat must be durable and hard to wear out even if it is struck by the opening and closing of the valves. On the other hand, the valves also need to be durable enough to resist wear even after repeated collisions with the valve seats.

The pistons are subjected to explosive combustion, the connecting rods and crankshafts rotate at high speed, and all of the engine parts work under extreme conditions, but there is no lubrication between the valve seat and the valve face to soften the impact and prevent wear. The stem of the valve body has oil lubrication between it and the valve guide, but the valve seat and valve face are direct metal to metal contact.

What's more, while oil-lubricated parts are lubricated with oil that has been filtered out, the valves and valve faces are also exposed to the hard carbon deposits that have built up around the intake ports and combustion chambers.

An important guideline in determining the condition of a valve seat is the width of its contact with the valve. Both the valve and the valve seat are designed to withstand repeated impact, but this does not mean that they will not wear. The greater the valve lift and the greater the tension of the valve spring, the greater the force of the collision between the valve and the valve seat, so it is only natural that as the mileage increases and the normal operating speed increases, the contact surface becomes wider.

Carbon entrapment and increased hit width cause performance degradation

It is important to check not only the valve seat but also the valve itself. Shake the valve umbrella to check the rattle between the valve stem and guide. Since both the stem and the guide wear out, it is ideal to use a new valve with an unworn stem in order to check the rattle of the guide. If the valve guide is worn out, the contact position between the valve and the seat will not be consistent, which will accelerate the seat wear. Also, the valve stem seal at the top of the valve guide is subjected to extra force, which can cause the oil to drop noticeably.

Let's list some of the effects of wider valve seat contact. First and foremost, tappet clearance is reduced. When the valve seat is struck by the valve face, the valve generally moves deeper into the combustion chamber. This causes the valve stem to stick out toward the cam and rocker arm, which reduces tappet clearance.

Secondly, the increase in hit width reduces the surface pressure when the valve closes, which reduces the airtightness of the combustion chamber. In other words, it is easier for compression to leak out. As the surface pressure decreases, the force to knock off the carbon sludge on the contact surface of the valve also decreases. You may think that carbon should be soft compared to the force of the valve opening and closing. However, carbon sludge formed from the unburned components of gasoline and oil is the valve's worst enemy.

Carbon caught in the valve seats does not necessarily result in a significant drop in compression pressure. However, in a leakdown test, where the intake and exhaust valves are closed and compressed air is pumped through the plug holes, the leakage figure will be higher. Whether to prioritize the dynamic measurement of compression pressure or the static measurement of leakdown depends on the person, but there is no doubt that the loss of airtightness due to increased hit width and carbon entrapment is a performance loss.

I will focus on the valve seats here, but the valves themselves are also very important to the condition of the cylinder head. The valve stems and the valve guides have a proper clearance to open and close at high speed, but as the mileage increases, the clearance increases due to wear of the guides. This causes the valve to swing open and close, which results in uneven contact with the valve seat, leading to poor airtightness and increased contact width. Therefore, when checking the condition of the valve seat, it is necessary to check the valve guide at the same time.

Sheet cut to restore airtightness. Polishing of the stem head may be necessary.

General valve seat cutting is done by changing seat cutters of three tapered levels, but the machine Inoue Boring has introduced is characterized by the use of a blade that can cut three sides at the same time. This is where the seat cut position is measured with the valve stem as a reference and the position is set to the 3-sided cutter.

The head is bolted to the workbench and the automatic centering function measures the angle of the valve stem to set the optimum seat cut position. If you turn the handle carefully, you can cut the seat by any amount. It is important to use the minimum amount of cut considering the increased protrusion of the valve.

While clamping the stem and turning the valve, the valve face is polished with a grinding wheel turned by another motor. This process also corrects carbon deposits and bumps with a minimum amount of polishing. The Tricker is a single-cylinder, two-valve engine, so the seat cutting and face machining can be done twice, but for a four-cylinder, sixteen valve engine, the same process must be done sixteen times, and the cost to fix the problem is high. However, this work can only be done by a professional of internal combustion engine machining.

There are standard dimensions for valve seat contact width, and the standard for general production models seems to be around 0.5 to 1.0mm. The wider the valve seat, the lower the surface pressure will be, and at the same time, the more carbon deposits will be trapped due to wear. In order to correct the valve seats that have become wide, valve seat cutting is required.

The valve seat is shaped like a circle of pipe made of metal material such as sintered alloy or phosphor bronze, and is press-fitted into the cylinder head. The design of the contact surface with the valve is very important for the flow of intake and exhaust air, so it is shaped in multiple steps to ensure smooth flow. For example, using the valve stem as a reference point, the cross-section of the valve seat from the intake side is 60 degrees, 45 degrees, and 32 degrees from the port side to the combustion chamber. The 45-degree surface is the part that contacts the valve.

The width of the hitting surface is 1.If it increases to more than 0mm, how do you correct it? The first step is to use a valve seat cutter to remove any carbon deposits from the 45-degree surface that is in contact with the valve, and then grind the 32 degree and 60-degree surfaces. 32-degree surface cutters will reduce the outside diameter of the contact point with the valve, and 60-degree surface cutters will reduce the inside diameter of the contact point. If you use a 60-degree face cutter, you can reduce the width of the contact point by grinding the inside of the contact point. If you have the right valve seat cutters, you can do this work yourself, but in general, you should ask a factory that processes internal combustion engines to do this work for you.

Inoue Boling in Saitama Prefecture, the company that commissioned us to cut the valve seats for the Yamaha Tricker, utilizes a state-of-the-art machine with a blade that can cut the seats in three stages in a single operation. This blade can be changed according to the angle of the machined surface and, of course, adjusted to the diameter of the valve seat. Furthermore, this valve seat cutter is equipped with an automatic centering function that allows the seat to be cut at the correct position relative to the valve stem, so that the valve and valve seat are perfectly aligned without any deviation in centering. After the valve seat is cut, the face of the valve must also be prepared. The valve stem is clamped and the valve itself is rotated while a grinding wheel spinning on a different axis is applied to the face to polish the surface.

These operations will improve the valve seat to valve face contact to an ideal condition, but it may be necessary to address the overall length change of the valve stem. In most cases, cutting the valve seat will lower the valve position and increase the cam side protrusion. This will cause the tappet clearance to decrease. If you are using a head that adjusts the tappet clearance with adjusting screws like the Honda horizontal engine or the Tricker, you can easily adjust the tappet clearance, but if you are using a head that adjusts the tappet clearance with tappet shims, you need to measure the clearance and replace the shims if necessary. In the case of seat cut, it is common to replace with thinner shims.

However, if you are using the thinnest of the replacement shims, there is no way to increase the clearance. This is why the internal combustion engine shop also offers full-length valve stem matching. Specifically, they measure the amount of valve protrusion before the seat cut and grind the difference between the amount of protrusion increased by the seat cut. If you do this, theoretically the tappet clearance will not change even if you use a shim-adjustable head. Of course, there will be a slight change by breaking in the motorcycle right after the seat cut, so you need to check again after driving for a while.

However, if the valve seat contact width is wider than the standard value, or so-called "sticky contact", it is not enough to secure airtightness because the surface pressure will decrease. If you have a carbon deposit on the valve seat and the contact width is wider than the standard value, you can ask your internal combustion engine shop to cut the seat and polish the valve face.

Interview cooperation: Inoue Boring

The beautifully re-cut seat ring is a professional job with the correct width and uniform finish all around. The carbon deposits have been removed, and the airtightness of the new engine has definitely been restored. Inoue Boring charges 3,200 yen per seat cut for 125 to 250cc engines.

After polishing the face, the valve seat was mated with the valve seat, so there are some traces on the contact surface with the seat. We didn't specify a narrower hit width, so it's a standard finish for a street model, but the carbon band around the entire circumference is gone and the finish is beautiful. The valve face polishing at Inoue Boring is 1300 yen per valve face.

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