REPLACING VALVES IN ENGINE

IN THIS SECTION, YOU WILL FIND THE FOLLOWING SUBSECTIONS:

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

LAPPING THE VALVES

Once the valve seat has been ground to its proper size and shape, the valve and seat are prepared for lapping. Lapping(also known as reseating) is the process of mating the valve and its seat to ensure a tight secure fit. Valves should be lapped to their seats any time they are removed from an engine, even if they are in good shape and do not require reconditioning. Brand new valves should also be lapped before they are installing in an engine.

APPLYING THE LAPPING COMPOUND

Valves are lapped with carborundum grinding paste, or lapping compound, a substance that, in sight and texture, resembles toothpaste, however its composition contains fine abrasive grains. When the lapping compound is rubbed onto metal, it is these grains that smooth the metal surface. The compound is available in different forms, each with a unique abrasiveness. A coarse grain lapping compound will frequently be used in the initial stages of the lapping procedure, while final lapping is done with a finer grain compound.

Organizing the valves: before you begin lapping, it is a good idea to organize the valves which are to be installed in an engine. You may wish to temporarily insert the valves into their guides to keep them organized. They would then be removed one at a time, and each lapped to its own seat. Another means of keeping valves organized is to drill holes into a flat section of like a yardstick. The holes should be slightly larger than the valve stem diameters. The wood piece can then be used as a valve holder, with the engine location of each valve carefully noted on the surface of the wood.

Spreading on the compound and inserting the valves: to begin lapping the valves, you should apply a thin coat of the lapping compound to the valve face or seat, either one. After one of these contact areas has been covered, the valve should be inserted into its guide, and pressure applied until the face and seat make contact.

From there, each valve will be rotated rapidly in its seat. When a valve is rotated, the abrasives in the lapping compound will produce extremely light superficial wear on the two surfaces, allowing them to mate to one another. Each valve should be mated only to its own seat; care should be taken to install each valve in the seat to which it was lapped.

USING THE LAPPING STICK

A lapping stick is a tool used to rotate valves. It consists of a wooden or plastic shaft with a suction cup on one end. This suction cup attaches to the valve head. Some technicians will moisten the cup before it is attached to improve the seal between it and the valve head. After the lapping stick has been attached, the valve can be rotated back and forth by spinning the shaft of the tool between your palms. While you are rolling the shaft back and forth, apply a moderate steady pressure downward; this will help the lapping compound mate the valve face and seat surfaces.

When using a coarse grain lapping compound, do not apply too much downward pressure, or lap for an extended period; these practices can easily result in too much metal being removed from the surfaces. Also, try pausing regularly to check the progress of the seating procedure.

CHECKING THE VALVE SEATING

To check valve seating, you must remove the valve from the engine, and clean it of lapping compound with solvent and a clean cloth. After the valve has been cleaned, apply a thin coat of blue dye to its face. Insert the valve back into its guide, and, applying slight downward pressure with your thumb, rotate the valve a fraction of a turn. Remove the valve and observe the seat; if the dye is evenly distributed around the seat circumference, the valve is now properly lapped. If the dye is distributed unevenly around the seat, more lapping compound should be applied, and the valve relapped.

After both valves of a cylinder are appropriately lapped, the valves and seats should be cleaned with solvent, followed by soap and water. This will remove leftover lapping compound. Should lapping compound penetrate the engine, it can cause harm especially to bearings.

CHECKING THE VALVE CLEARANCE

Valve clearance is the small amount of space between the end of a valve stem and the push rod or valve lifter. Clearance must be correct for a valve to function properly. If clearance is too small, it indicates that the valve stem extends too far through its guide; in such a case, the valve will not close correctly. If the clearance is too large, it indicates that the valve stem does not reach far enough past its guide to effectively engage the push rod or valve lifter.

WHY THE VALVE CLEARANCE CHANGES

When you grind an engine valve seat, you remove metal from the seat area. For this reason, a valve reinstalled in the engine after grinding will sit lower than did originally. If a large amount of metal is removed, the valve will sit significantly lower than before. This is why grinding a valve seat can have serious effects on valve clearance.

Because valve clearances might have been changed by grinding and lapping, they should be measured and the results compared with service manual specifications. If valve clearance has been reduced too much, it will need to be adjusted. In some old L head engines, the valve lifters can be adjusted to correct clearance issues. In most engines, however, valve clearance must be corrected by grinding the end of the valve stem. Valve stem size can be reduced with a valve grinding machine, or the stem can simply be shortened by hand with an ordinary metal file.

HOW TO CHECK AND ADJUST THE CLEARANCE

Usually valve clearance is checked during valve installation. Some engine manufacturers recommend that the clearance be checked and adjusted prior to installation of the valve spring. Other manufacturers recommend that the valve spring be installed to close the valve completely before clearance is checked. You can temporarily install valves in an engine to check their clearances, or, if preferred, you can wait to perform the clearance check until the engine is about to be reassembled. Either way, valve clearance must be checked to ensure that it is within service manual specifications. If you do decide to put checking the clearance off until you are ready to reassemble the engine, be sure that the valve components are clean of dirt and lapping compound before measurements are taken.

Checking the clearance: after the parts have been sufficiently cleaned, you can install the valves in the engine, and take clearance measurements. Note that the camshaft and valve lifters, or push rods and rocker arms, will also need to be installed to perform these checks.

Valve clearance is normally checked with the piston at top dead center(TDC). At top dead center during the engine compression stroke, both valves for the cylinder will be tightly closed. Check the clearance at this point using a feeler gauge. The feeler gauge blade should be inserted between the valve stem and valve lifter of an L head engine, or between the push rod and rocker arm of an overhead valve motor. Keep inserting feeler gauge blades until the proper thickness is found. A blade of the correct size will resist with moderate drag when you attempt to pull it from between the valve assembly components. If a blade moves with no drag, you can assume that it is too thin. On the other hand, if you cannot insert a given blade between the assembly components, it is too thick.

If an engine manufacturer recommends that valve springs be installed before clearance is checked, be sure that the correct springs and valves are used together. On some engines, the intake and exhaust valve springs are of different sizes and strengths. If a manufacturer recommends that valve springs should not be used in checking clearance, press firmly on the top of the valve to ensure that it is closed as you take clearance measurements.

Adjusting the clearance: if you find that clearance is too small, you will need to remove the valve, and file or cut its stem to make it shorter. To determine how much metal to remove from the stem, subtract the clearance measurement from the service manual from the clearance you just measured. The difference is the length of metal which needs to be removed from the stem. Use a micrometer or caliper to measure the stem length. After you have trimmed the stem, insert the valve into the engine, and recheck its clearance. Keep repeating this process until the proper clearance is achieved.

As you grind the valve stem, remove only a little bit of material at a time. This will prevent the valve from overheating, and also keep you from inadvertently removing too much stem metal. If you are using a standard file to remove metal from the stem, be certain that the end of the stem remains flat, and that it maintains a strict right angle to the length of the stem.

Valve clearance may also prove to be too large when measured. If it is, the valve seat and valve may need to be refinished, and lapped again to reposition the valve lower in its seat. Lowering valve position in its seat will reduce clearance. If a valve cannot be refaced due to a too narrow margin, it may have to be replaced. Most of the time grinding the seat itself will effectively lower valve position and correct clearance.

On older engines that have been serviced repeatedly, valve seats may have been ground enough times so that they are too deep to be used again. You will need to determine if a given seat remains usable, or if it has been stripped of too much of its metal in past rebuilds. A good way to do this is to compare a new valve to the existing one(the valve that was removed from the engine). Measure the distance on each valve from the stem to the lock groove which supports the valve spring retainer. If this distance measurement on the old valve is less than half the distance measurement on the new valve, the seat has been ground too deeply for reuse; the seat must be replaced if you expect to reinstall the old valve. After you have adjusted valve clearance, the engine valve assembly should be checked, and reconditioned where necessary. This will leave the valves ready to be reinstalled with the rest of the valve assembly components.