THE VALVES

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

THE VALVE ASSEMBLY
STEPS TO REBUILDING A VALVE ASSEMBLY
VISUALLY INSPECTING THE VALVES
CLEANING THE VALVES
VALVE DAMAGE
REFACING THE VALVES
THE VALVE GRINDING MACHINE
RECHECKING THE MARGIN AND CLEANING THE REFACED VALVE
LAPPING THE VALVES
APPLYING THE LAPPING COMPOUND
USING THE LAPPING STICK
CHECKING THE VALVE SEATING
CHECKING THE VALVE CLEARANCE
WHY THE VALVE CLEARANCE CHANGES
HOW TO CHECK AND ADJUST THE CLEARANCE

THE VALVE ASSEMBLY

The valves of a four stroke perform two functions: the intake valve allows the fuel air mixture to enter the combustion chamber where it can be burned. The exhaust valve allows combustion gases to exit. The valves must seal tightly when closed so that the fuel air mixture will not escape. The valves must be in good condition to ensure maximum engine power.

The valve dimensions that you will need to check during a rebuild are: valve head diameter, valve face angle, valve seat angle, valve margin width, and valve stem diameter.

Heat: during engine operation, valves can reach temperatures of over 1,000 degrees F. This type of intense heat wears away exposed surfaces, valve heads in particular. It is not uncommon for valves to warp or melt, especially if an engine operates at disproportionately high temperatures.

Friction: friction between a valve stem and its guide will inevitably produce wear. Each valve must open and close once for every power stroke of a four stroke engine. This entails that the valves open and close roughly 1,500 times a minute. Opening and closing this rapidly will produce friction between the stem and guide, eventually wearing one or both.

Impact: the rapid opening and closing of the valves will batter the valve seats. Eventually the seats will become distorted, permitting gases to leak past a valve head even when it is closed.

For these reasons, an engine valve assembly must be thoroughly inspected and reconditioned as part of any overhaul.

STEPS TO REBUILDING A VALVE ASSEMBLY

Before you can rebuild a valve assembly, each valve, and valve component, must be cleaned and inspected to determine its fitness for reuse. After a visual inspection, a valve assembly can rebuilt according to the following steps:

1)Inspect and recondition both the intake and exhaust valves; inspect each for physical damage, and take measurements to clarify if it remains within service manual specifications
2)Refinish each valve face so that its shape is smooth and uniform
3)Inspect the valve guides for physical damage, and repair or replace any that are outside of service manual specifications
4)Inspect and recondition the valve seats; on some engines, damaged seats can be replaced
5)Lap the valves; this means that each valve is mated to its seat to guarantee an airtight seal. This process mating the valves to their seats is referred to as valve lapping

VISUALLY INSPECTING THE VALVES

You should first check the valves themselves. A valve must be in good condition before the guides or seats can be addressed. A worn valve can give inaccurate component measurements. Inspecting the valves first will ensure accurate results later.

CLEANING THE VALVES

Before they can be inspected, the valves must be cleaned. Due to the high temperatures in which they operate, any material that contacts the valve(oil, gas, carbon, etc.) will bake onto its surface. Ordinary cleaning solvents may not be sufficient to remove this residue completely. The best way to clean a valve is with a wire brush. A wire wheel mounted to a bench grinder is an even better alternative because it does the job much more quickly and easily. After you have cleaned the valves with a wire brush, wash them in cleaning solvent to remove lingering dirt particles. When the valves are clean, they can then be visually inspected.

VALVE DAMAGE

Valves can be damaged as a result of several conditions. The most common cause of valve damage, however, is heat. If a valve overheats, its edges can melt or its head crack. Pieces of the valve may even break off. There are four types of potential valve damage to be on the lookout for:

1)Burning
2)Preignition
3)Cracking
4)Necking

The valve margin: you should measure the margin to detect signs of distortion. The margin is the area between the valve head and the beginning of a valve face. Margin is usually measured with a small ruler, or a caliper can be used instead.

The margin of a new valve is usually around 1/32 inches. A valve whose margin width is as low as 1/64 inches will generally be good enough for reuse. If a valve margin is less than 1/64 inches, it must be replaced. This small a margin will not withstand combustion chamber heat.

When checking valve margin, always consult service manual specifications for the engine in question. The manufacturer may recommend that you replace any valve whose margin is less than 1/32 inches. Remember that any valve with a knife edge, or nonexistent margin, should be replaced.

The valve stem: by comparing actual valve stem diameter to service manual specifications, you can check for valve necking, a narrowing of the valve stem. An outside micrometer can be used to measure stem diameter(MICROMETER). It is good practice to measure a valve stem at three positions, near its top, someplace in its middle, and at its bottom. The diameters of all three locations should be checked against service manual specifications.

Typical service manual specifications for a valve stem will include the minimum valve stem O.D.(the minimum stem diameter allowable for a reusable valve).

REFACING THE VALVES

If you inspect the valves and find them in satisfactory condition, they can be reused. All valves will experience some degree of wear and distortion from routine engine operation. Before you reuse a valve, its face should be reconditioned to give it a smooth, uniform appearance. The process of reconditioning a valve face is commonly called valve grinding or valve refacing. The only time you would not reface a valve before engine reassembly is when new replacement valves will be installed instead. Most new valves come with their faces machined to a proper angle.

THE VALVE GRINDING MACHINE

The valve face must be beveled or slanted. The bevel or face angle allows the valve face to perfectly fit its similarly angled seat. A special piece of equipment called a valve grinding machine is needed to perform this refacing effort.

The valve grinding machine uses a rotating grinding wheel to refinish the valve face. The grinding wheel must be adjusted so that the valve face is machined at the proper angle. The valve is clamped into a device called a chuck(similar to the chuck that holds an electric drill bit); the chuck spins the valve as it is being ground. When the valve has been clamped in, the grinding wheel is moved into contact with the spinning valve face. This enables the valve face to be ground uniformly around its circumference.

Dressing the wheel: before you begin refacing a valve on a grinding machine, you must check the condition of the grinding wheel that will be used. The wheel itself must be smooth and straight. If its surface is glazed, rough, or irregularly worn, the wheel itself must be resurfaced before it can be used on a valve. Resurfacing of a grinding wheel is known as dressing the wheel. Procedures for dressing a grinding wheel vary depending on its manufacturer. If a wheel must be redressed, refer to instructions by the manufacturer of the grinding machine.

Adjusting the machine: to set the proper angle for grinding, you must check the service manual dimensions for the valve in question. Usually a valve face will be ground to match a seat angle of 45 degrees; however some older engines may require a steeper valve and seat angle, say 60 degrees or so.

After the grinding machine has been adjusted to produce the proper face angle, the valve should be inserted into the machine chuck and securely tightened. Turning on the machine will begin spinning the valve. Check to see that, as it spins, the valve head does not wobble. If you detect a wobbling head, first check the tightness of the chuck. If the chuck was tight and yet the valve continues to wobble, the valve itself must be bent and therefore should not be reused. A valve can be bent by debris inside an engine, or it can be warped by excess amounts of engine heat. In either case, the valve should be replaced.

The valve grinding machine will require grinding oil to lubricate its wheel as grinding occurs. A pump is used to spread oil onto the rotating grinding wheel. When you switch on the grinding machine, check to be sure that its oil pump is operating effectively. If you use a grinding machine absent proper lubrication, the valve face will be ground improperly.

Grinding the valve face: when the machine is adjusted and the valve clamped into place, you can begin the process of grinding the valve face. Remember to wear safety glasses whenever you are using a grinding wheel. Move the valve face into light contact with the grinding wheel. Grind lightly, moving the spinning valve back and forth across the wheel until the grinding sounds smooth and uniform across the entire valve face surface. When you first begin to grind, the valve surface will not be smooth and thus the grinding sounds will be sporadic and herky jerky. When the sound is uniform, you know that the wheel surface has begun to contact the valve face around its entire circumference. Move the valve away from the wheel, and check the face to see if its surface appears to be as uniform as the grinding wheel implied. If the surface does appear uniform, the grinding procedure is complete. If the surface does not appear uniform, proceed with the grinding operation, however just a little at a time, pausing frequently to recheck valve face condition. Stop grinding when the face surface appears uniform.

RECHECKING THE MARGIN AND CLEANING THE REFACED VALVE

Fter the valve has been refaced, measure the margin to determine if is still within usable parameters. The margin must be remeasured because its width has likely been reduced by the grinding process. The specification for useable valve margin width will be listed in the engine service manual. In most cases, a margin must be at least 1/64 inches wide, or at least half of the margin width specified for a brand new valve. If the margin of the refaced valve proves to be too narrow, the valve must be replaced.

The valves employed by most four strokes are similar. The only obvious difference between them will be their size; larger engines will require larger valves. The facing and grinding procedures, regardless of engine size or valve size, are identical. There will also be no variation in the grinding procedure for valves that are located in an engine block(as would be the case with an L head motor), or an engine cylinder head(as would be the case with an OHV or OHC motor).

After the valves have been resurfaced and inspected, they should be cleaned in solvent before they are reinstalled in the engine. Solvent will remove any traces of lubricating oil, metal particles, or grinding wheel residue from a valve surface.

LAPPING THE VALVES

After both the valve face and valve seat have been ground to proper size and shape, they are ready for lapping to occur(VALVE SEATS). Lapping(also called reseating) is the process of mating the valve and valve seat to ensure that they fit tightly. Valves should be lapped to seats whenever they are removed from an engine, even if the valve and seat are in good shape and will not need reconditioning. Even brand new valves should be lapped before they are installing in an engine.

APPLYING THE LAPPING COMPOUND

Valves are lapped using carborundum grinding paste, also known as lapping compound, a substance that looks like toothpaste but contains abrasive grains. When the compound is rubbed onto metal, its grains will smooth the metal surface. Lapping compound is available in forms that harbor varying degrees of abrasiveness for different purposes. A coarse grain compound is frequently used as the lapping procedure begins, and then a finer grain compound substituted when finishing touches must be added.

Organizing the valves: before you begin lapping, you should organize the valves that will be installed in an engine. You may even want to temporarily insert the valves into their guides to help keep them organized. They can then be removed one at a time, and each lapped to its own seat. Another way you can keep the valves organized is to drill holes into a flat stick of wood(a yardstick for instance), each hole with a diameter slightly larger than the valve stem diameter; the stick can then be used as a holder for the valves, the location of each valve noted on the wood surface near it.

Spreading on the compound and inserting the valves: to begin lapping the valves, apply a thin coating of lapping compound to either the valve face or valve seat. After you have covered this contact area completely with compound, insert the valve into its guide and press it firmly against its seat.

Each valve will then be rotated rapidly within its seat. As the valve is rotated, abrasives in the lapping compound will administer slight wear to the two surfaces, thereby mating them to one another. Each valve should be mated only to its own seat; when the valves are installed, take care to install them in their correct seats.

USING THE LAPPING STICK

The lapping stick is a tool used to rotate the valves. It consists of a wooden or plastic shaft that has a suction cup on its end. The suction cup is attached to the valve head. Sometimes it helps to moisten the suction cup prior to attaching it to improve the seal between it and the valve head. When the stick has been attached, the valve can be rotated back and forth quickly by spinning the tool shaft between the palms of your hands. While the shaft is being rotated back and forth, moderate downward pressure should be applied so that the valve face and seat are in solid contact. This will help the lapping compound mate the two surfaces.

When using a coarse grain lapping compound, be cautious not to apply too much downward pressure, or to lap for too long a period. Doing either can cause too much metal to be removed from the seat. Also, pause frequently during the lapping process to recheck valve seating.

CHECKING THE VALVE SEATING

To check valve seating during the lapping process, remove the valve from the engine, and cleanse it of lapping compound with solvent and a clean cloth. As soon as the valve is clean, apply a thin coat of blue dye to its face; then insert the valve back into its valve guide. Apply slight downward pressure with your thumb, and rotate the valve slightly in its seat. Remove the valve and observe its seat; if the blue dye is evenly distributed around its entire circumference, the valve has been properly lapped. If the blue dye is distributed on the seat unevenly, more lapping compound should be applied and the valve relapped.

After all engine valves have been appropriately lapped, the valves and their seats should be cleaned, first with solvent, then with soap and water. This will remove any lapping compound residue. If lapping compound is allowed to get into an engine, it can harm bearings and other internal surfaces.

CHECKING THE VALVE CLEARANCE

In an L head motor, valve clearance is the diminutive amount of space that exists between the end of the valve stem and the valve lifter. In an OHV or OHC engine, valve clearance is the amount of space between the valve stem and rocker arm end. In either case, the valve clearance must be correct for a valve to function properly. A clearance that is too small will not allow a valve to completely close when it should. A too large clearance will aggravate the tightness between the valve face and valve seat to a point where the engine cannot be turned over smoothly, and where deterioration of both surfaces is excessive. It will also limit the amount a valve can open.

WHY THE VALVE CLEARANCE CHANGES

When you grind the valve seat in an engine, you remove metal from the seat area. For this reason, when a valve is reinstalled in an engine it will sit lower in the seat than it did originally. If you remove a large amount of metal, the valve will sit significantly lower in its seat, drastically affecting valve clearance.

Because valve clearances may have been changed by the grinding and lapping processes, they should be measured, and those measurements compared with service manual specifications. If you learn that valve clearance is now outside of specifications, it must be adjusted.

In some L head engines, the valve lifters themselves can be adjusted to obtain the correct clearance. In most engines, valve clearance must be corrected by grinding end of valve stem. Valve stem can be cut down using fixture in valve grinding machine, or stem can be shortened with ordinary metal file.

HOW TO CHECK AND ADJUST THE CLEARANCE

L Head

Some L head engine manufacturers will recommend that valve clearance be checked and adjusted without the valve spring installed. Others will recommend that the valve spring be installed so that the valve is completely closed when measurements are taken and adjustments made. Valves can always be installed temporarily to check clearances. Clearance can also be checked when the valves are lapped, or later, when engine reassembly is about to occur. Either way, when measurements are being taken and adjustments made, be certain that valve components are free of dirt or lapping compound.

Checking the clearance: after the parts are cleaned, install the valves in the engine. The camshaft and valve lifters must also be installed for valve clearance to be checked.

Valve clearance is normally checked with the piston at top dead center(TDC) of the compression stroke, when both intake and exhaust valves are completely closed. Check the valve clearance with a feeler gauge. The feeler gauge blades should be inserted between the valve stem and valve lifter until a blade is found which produces a mild to moderate drag when it is drawn from between the components; you should not need to tug with force to extract the blade, however friction or pull should be detectable. If a blade slides between the valve stem and lifter with zero drag, it is too thin; any blade that cannot be inserted between the components at all is too thick.

If an engine service manual recommends that valve springs be installed before clearance is checked, be certain to install the correct springs on the correct valves. On some engines, the intake valve and exhaust valve springs are of different sizes and strengths. If a manufacturer recommends that valve springs not be installed, you should press firmly on the valve head to ensure that clearance is checked with the valve closed.

Adjusting the clearance: in some L head engines, the valve lifters themselves can be adjusted to obtain the correct valve clearance. In most, however, a valve clearance that is too small must be corrected by grinding the end of the valve stem. The valve stem can be cut down with a valve grinding machine, or the stem can be shortened with an ordinary metal file.

To determine how much metal to remove from the end of a valve stem, deduct your clearance measurement from the service manual specification for valve clearance. The difference between the two denotes the length of valve stem that must be removed. Use a micrometer or calipers to measure this length of metal that must be removed from the stem(MICROMETER, BEYOND THE MICROMETER). After the valve stem has been trimmed, insert the valve into the engine, and recheck clearance. Keep repeating this process until the proper valve clearance is obtained.

When the valve stem is being ground, try to remove a little bit of metal at a time. This will prevent the valve from becoming either too hot, or too short because too much metal was eliminated. If you will be using a file to remove stem metal, be sure that the valve stem end remains flat, and perpendicular to the stem length.

Valve clearance in an L head motor may also be too large. If so, the valve face and seat may need to be refinished and lapped again, thereby lowering the height at which the valve sits. This reduction in height will reduce valve clearance. If a valve cannot be refaced because its margin is too narrow, it may need to be replaced. Most of the time, however, a valve can be lowered simply by regrinding the seat.

In older engines that have seen repeated service, the valve seats may have been ground several times, and be too deep for further service to occur. One method for determining if too much metal was removed from a seat for further service is to compare a brand new valve to an existing one. The distance from the valve stem tip to the lock groove for the valve spring retainer should be measured on each. If the distance measured on the existing valve is less than half the distance measured on the new valve, you know that the valve seat has been ground too deeply for the old valve to be reused again.

Overhead Valve or OHV and Overhead Cam or OHC

For valve clearance to be checked and adjusted on one of these motors, the entire valve train normally must be installed; this includes the camshaft, push rods and valve lifters in the case of an OHV engine, rocker arms, valve springs and valves, and the cylinder head.

The piston must be at top dead center(TDC) on its power stroke to ensure that both valves are closed when measurements are taken and adjustments made. If the recoil start is attached to the engine, you can use it to rotate the crankshaft and piston. Otherwise turn the crankshaft by hand.

Try inserting the blade of a screwdriver into the spark plug hole in the cylinder head. This will tell you when the piston has reached TDC. You will be able to detect if the piston is on its compression or exhaust stroke by observing the rocker arms. On the exhaust stroke, one of the rocker arms will be depressing the stem of the exhaust valve, forcing it open. On the engine compression stroke, both rocker arms will be idle, neither applying any pressure to its valve stem.

Look at the rocker arm. Its one end will be in contact with the pushrod, the other will be pressed against the valve stem immediately above the valve spring and its retainer. Measurements are taken with feeler gauge blades between the rocker arm end and the valve stem.