Tech Feature: Valve Seat Selection and Machining – UnderhoodService

Tech Feature: Valve Seat Selection and Machining

The type of seat material that's "best" for a particular cylinder head will depend on the application, fuel used, how much power, type of valves, and longevity and durability the engine is expected to deliver.

0; This means high chromium, cobalt or nickel alloy seats are needed for the intake valve seats to resist corrosion and wear.

Today’s engines are also producing a lot more horsepower and torque from the same number of cubic inches. That means more heat load on the engine and valves, which requires a premium valve seat material with high thermal conductivity. At the same time, the seat alloy must also provide high temperature wear resistance for hundreds of thousands of miles of operation.

Many of the big, high output diesel engines today are running steel pistons rather than aluminum pistons. This means a catastrophic valve seat failure can cause a lot more damage inside the engine.  So the last thing you want is a seat alloy that’s prone to cracking.

Though powder metal seats are used in most late model passenger car and light truck engines with aluminum cylinders, powder metal seats are not used in most heavy-duty diesel engines. Why? Because the type of powder metal seats that perform well in automotive engines generally don’t do well in a heavy-duty diesel environment. That’s why heavy-duty diesel OEMs typically use stellite, chromium, cobalt or nickel alloy seats, or the new proprietary cast iron high heat alloys that have been developed for these engines.

Another change that’s taken place in the heavy-duty market is that many OEMs have stopped selling replacement valve seats because they want to sell complete cylinder heads.  Others only offer replacement seats in standard sizes (which is good for aftermarket valve seat suppliers provided they have oversized replacement seats in the right alloys for the application). Many of the OEM seats are also pre-finished, which means you don’t have to cut them after they’ve been installed.  But it also means the seat counterbore had better be concentric with the valve guide, otherwise you’re going to have problems.

If a valve seat is not concentric with the center of the valve guide, it may prevent the valve from sealing tightly causing a compression leak.  It can also cause the valve stem to flex slightly every time the valve opens and closes, which can lead to metal fatigue in the valve stem and valve failure down the road.  A nonconcentric seat can also cause one side of the valve to run hotter than the other, increasing the risk of valve burning and seat erosion.

Interference fit is one of the main concerns when installing valve seats.  You want the seat to fit tightly so it doesn’t fall out, even if the engine overheats.  But you don’t want it so tight that there’s a danger of cracking either.

On passenger car and light truck engines with aluminum heads, the seats are usually factory installed with about .002˝ to .003˝ of interference fit.  Some say powder metal seats require a little more interference fit than cast iron alloys, while cobalt alloy seats require a little less because of their higher coefficient of thermal expansion.  

Keep in mind these numbers are for brand new heads with brand new seats.  After tens of thousands of miles, seat counterbores can become distorted and eroded, requiring an increase in interference to keep the seat tight.  

The most common recommendation from valve seat suppliers for cast seats being installed in aluminum heads is .003˝ to .005˝ of interference fit.  If you are installing powder metal seats, use .005˝ to .007˝ of interference.  If you are using beryllium-copper seats, go with .004˝ to .0045˝ of interference fit.

Many valve seats have a radius or chamfer on the outside bottom edge to make installation easier.  Seats with square cut corners are more difficult to install and may damage the counterbore if they snag any metal or become cocked while they are being driven in.

Chilling the valve seats in a freezer and preheating the head are often recommended to make installation easier, especially if you are using a lot of interference fit.  Using a lubricant also helps.  When heating the head, don’t get carried away.  You only need about 160° to 180° F.  If you get the head too hot, say 200° to 250° F, things can start to move around and change the alignment between the valve guide and seat.

Unless you are installing a pre-finished seat, the seat will have to be cut after it has been installed in the head.  This, of course, requires installing or reconditioning the valve guides before you do the seat work.  The center line of the valve guide will determine the location and concentricity of the valve seat.

Accurate seat refinishing requires a valve-and-seat machine that is in good condition and can hold tight tolerances.  You can’t have a couple thousandths of an inch of slop and do a good valve job.  The pilot to guide clearance should be down around .0002˝ or less for accurate machining. One way to achieve that is to use a high pressure lubricant on the pilot.

The seat cutter must be sharp and spun at a high enough speed to produce a high quality finish on the seat. If you’re getting chatter while cutting a seat, the problem may be too much play between the pilot and valve guide, the speed of the cutter, or the machine is out of level.  Using a coolant when cutting hard seats will also reduce chatter.

The profile of the seats will depend on what you are trying to achieve. A single cut 45° seat may be all that’s needed for a low output stock engine. But on a high performance engine, a multi-angle valve job is an absolute must to optimize the breathing potential of the cylinder head.

The commonly used 30-45-60 degree three angle performance cut certainly flows better than a seat with a single 45 degree cut.  But more angles breathe even better. Adding additional cuts under the seat, and using steeper angles generally helps the airflow numbers even more. Some well-known performance engine builders say they see the best flow numbers using a 58° undercut below the primary seat, and a 70° cut below that. Using a steeper top angle of 33° to 37° on the intake seats also helps reduce turbulence as the air enters the combustion chamber. Others use special cutters with an infinite radius or a CNC-controlled single point cutter to contour the seat above and below the primary 45° seat to optimize flow.

Unfortunately, there’s no tried-and-true formula that works best for every engine. It takes a lot of time on a dry flow bench and wet flow bench to figure out which angles actually produce the best results. And sometimes what looks good on a dry flow bench doesn’t always produce more power because of fuel separation problems at the valve and seat interface. Because of this, it often takes some trial-and-error experimentation to find the optimum combination of angles or seat profile that delivers the most power and throttle response.

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