The Five-Angle Valve Job

Admin · February 2, 2022

A titanium intake poppet valve used in a KTM 1290 Super Duke R. (KTM/)

Back in the days of Supersport racing, the “Contingency Gypsies” (riders who lived on the road, paying their bills with factory contingency money) spoke about five-angle valve jobs as though they were a key to achieving a higher state of being.

But what is a valve job?

In a four-stroke piston engine the functions of intake (drawing in a fresh mixture of fuel vapor and air) and exhaust (releasing the burned gases whose work is done) require valves that can open and close at chosen times.

There are several ways to accomplish this. Valves that depend upon friction, such as the slide valves of old-time steam engines, or rotating-ball or disc valves would not last long—combustion heat makes lubricating a sliding surface very difficult. That leaves the so-called “poppet valve,” which looks like a tiny manhole cover on a stick. The “manhole cover” is the valve head, a disc whose edge is ground at a (usually) 45-degree angle to make it self-center against its seat, similarly cut in the cylinder head. Extending upward from the center of this valve head is its stem—a slender cylindrical rod. The rod fits closely into a valve guide. One or more spiral-wound helical springs press against a disc fastened to the far end of the stem—the spring retainer. This spring pressure holds the valve tightly against its seat when closed.

Because the head of each valve faces combustion, the pressure from that combustion forces the valve against its seat, making it seal especially tightly.

Extending upward from each valve seat is a curving port, the curve necessary to avoid hitting the valve spring(s) above. The exhaust port joins a header pipe which is part of the exhaust system. The intake port leads to a device that adds fuel to the incoming air stream—either a carburetor or a fuel-injection throttle body. In a two-valve-per-cylinder engine (older Harleys and US auto engines), there are two valve seats for each cylinder, and two ports—one for intake, the other for exhaust. In a four-valve-per-cylinder engine there are two intake valves side by side, and opposite them two exhaust valves, also side by side. In most cases a single port serves both valves, splitting into two smaller ports inside the head.

Up above each valve is a mechanism that can open and close the valves at the desired points in the engine cycle (either an overhead camshaft or a rocker arm operated by a pushrod). Such mechanism opens the valve by pressing down on the end of its valve stem.

Why a Valve Job?

A valve job becomes necessary when prolonged engine operation results in wear, corrosion or other damage to the 45-degree seating surfaces, causing leakage. The work involves recutting or regrinding those surfaces so that they once again seal tightly against each other. Typically very little metal needs to be removed in order to restore good sealing. Because a valve job moves the valve a few thousandths of an inch toward its seat, it is sometimes necessary to remove a corresponding amount of material from the end of each valve stem.

How does one know if an engine needs a valve job? A compression test will usually tell us. With a pressure gauge screwed into a spark-plug hole and the throttle open, the engine is turned several revolutions. At each revolution the gauge indication rises (a check valve ensures the gauge holds its reading), and when the value ceases to rise, the resulting pressure is noted. If this is less than the specified minimum in the service book, a valve job may be required.

Production engines receive three-angle valve jobs with 15 degrees of change per cut. Here beryllium-copper valve-seat rings are used to seal titanium valves. (Ducati/)

Air “dislikes” flowing around sharp edges, and because of this production engines are given what is called a three-angle valve job. This means that upstream from the 45-degree sealing surface in the valve seat, a 30-degree cut is made, and downstream, on the combustion-chamber side of the 45-degree cut, there is a 60-degree cut. This reduces the angle through which air flowing over the seat must turn to just 15 degrees (45 minus 30 = 15, and 60 minus 45 =15), thus creating less flow disturbance. Less disturbance ultimately increases flow, and both intake and exhaust benefit.

The Five-Angle Valve Job

Taking this one step further, to provide additional smoothing, it was common for performance-seeking Supersport racers to add two more cuts, further reducing the sharpness of the edges. These split the difference between 30 and 45, and between 45 and 60: Voilà—the five-angle valve job. This was both popular and legal because class rules forbade all porting other than reshaping the valve-seat rings.

Purists took this idea further. Back in the late 1950s the innovative rider/engineer Albert Gunter pioneered the blend valve seat. This produced a smooth, continuous curve approaching the upstream and downstream sides of the 45-degree sealing surface. Modern valve and port machines can now produce such blend seats automatically.

An intake valve seated on a blend valve job produces little more than a line contact for sealing the valve to the seat. Because the intake valve is cooled by the inflow of fresh charge (rather than being heated on both faces by hot combustion gas as is the exhaust valve), this narrow seat is durable enough for racing use.

But the exhaust valve is another story. Most of the cooling that the exhaust valves require comes from their intermittent contact with the cooler valve seat, and consequently its seating surface must be made wider than that of the intake valve(s).

Motorcycle cylinder heads are made of aluminum, which is generally too soft to bear the valves seating directly against it. Because of this, each valve seats against a ring or insert made of a more durable material such as bronze or chilled iron. These seats are retained in the head by making them a bit larger in diameter than a corresponding counterbore in the head. Then the head is heated quite hot, the insert is chilled, and the ring is seated in its counterbore,resulting in a very tight fit when their temperatures normalize.

Racers, Lawyers, and Rulebooks

Racers read rule books very carefully. “Look here! It says valve-seat rings may be removed and replaced. But it doesn’t say with what! That means that if we install taller valve seats, pushing the valves toward the pistons, that will increase the compression ratio and boost torque!”

And so it was done. But only until the authors of the rule book changed the wording to add that replacement valve-seat rings must have the same height dimension as the stock parts.

No wonder lawyers are paid so much. Good ones read the law very carefully.

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