Lapping Valves

When a valve seat contains irregularities that are too large to be removed by grinding-in, you can remove them by lapping. A cast-iron tool (LAP) of exactly the same size and shape as the disk is used to true the seat surface. Two lapping tools are shown in figure 5-55.

Figure 5-55.-Lapping tools.

Here are the most important points to remember while using the lapping tool.

1. Do not bear heavily on the handle of the lap.

2. Do not bear sideways on the handle of the lap.

3. Change the relationship between the lap and the seat so that the lap will gradually and slowly rotate around the entire seat circle.

4. Keep a check on the working surface of the lap. If a groove develops, have the lap refaced.

5. Always use a clean compound for lapping.

_6. Replace the compound often.

^7. Spread the compound evenly and lightly.

8. Do not lap more than is necessary to produce a smooth, even seat.

_9. Always use a fine grinding compound to finish the lapping job.

_10. When you complete the lapping job, spot-in and grind-in the disk to the seat.

Use only approved abrasive compounds to recondition seats and disks. Compounds for lapping and grinding disks and seats are supplied in various grades. Use a coarse grade compound when extensive corrosion or deep cuts and scratches are found on the disks and seats. Use a compound of medium grade to follow up the coarse grade. It may also be used to start the reconditioning process on valves that are not too severely damaged. Use a fine grade compound when the reconditioning process nears completion. Use a microscopic-fine grade for finish lapping and for all grinding-in.

Refacing Valves

Badly scored valve seats must be refaced in a lathe, with a power grinder, or with a valve reseating machine. Use the lathe, rather than the reseating machine to reface all disks and all hard surfaced seats. Work that must be done on a lathe or with a power grinder should be turned over to machine shop personnel. This discussion applies only to refacing seats with the reseating machine.

To reface a seat with a reseating machine (fig. 5-56), attach the correct 45-degree facing cutter to the reseating machine. With a fine file, remove all high spots on the surface of the flange upon which the chuck jaws must fit. Note that a valve reseating machine can be used ONLY with a valve in which the inside of the bonnet flange is bored true with the valve seat. If this condition does not exist, the valve must be reseated in a lathe, and the inside flange bored true.

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Figure 5-56.-Valve reseating machine.

Before placing the chuck in the valve opening, open the jaws of the chuck wide enough to rest on the flange of the opening. Tighten the jaws lightly so that the chuck securely grips the sides of the valve opening. Tap the chuck down with a wooden mallet until the jaws rest firmly and squarely on the flange. Then tighten the jaws further.

Adjust and lock the machine spindle in the cutting position and start the cutting by turning slowly on the crank. Feed the cutter slowly so that very light shavings are taken. After some experience, you will be able to know by the feel whether or not the tool is cutting evenly all around. Remove the chuck to see if enough metal has been removed.

Be sure the seat is perfect. Then remove the 45-degree cutter and face off the top part of the seat with a flat cutter. Dress the seat down to the proper dimensions, as follows:

After the refacing, grind-in the seat and disk. Spotin as necessary to check the work. A rough method of spotting-in is to place pencil marks at intervals of about one-half inch on the bearing surface of the seat or disk. Then place the disk on the seat and rotate the disk about a quarter of a turn. If the pencil marks in the seating area rub off, the seating is considered satisfactory.