Thursday, May 26, 2011

Valve-seat reconditioning

Valve-seat reconditioning
Worn or burnt valve seats in the cylinder head can be restored by either grinding or cutting. This removes pit marks and bums and produces a seat that is concentric with the valve guide. The seats can also be narrowed to give them the correct width.

Fig 3.27
Valve-seat grinding 
 With this method, small grinding stones are used to restore the valve seats. The stones are available in various diameters and are dressed to suit the valve-seat angle. Narrowing stones are dressed to other angles. The stones are mounted on the threaded end of the
stone carrier (Figure 3.27). To grind the valve seat, the carrier is mounted with a pilot in the valve guide and driven by an electric drill.
The carrier has a clutch as part of its drive. The clutch is designed to have some slip and this can be adjusted by the knurled sleeve at the top of the carrier. Clutch slip causes the stone to chatter on the seat that is being ground and this assists the grinding action. The clutch can be adjusted for roughing or finishing.
Pilots There are two types of pilots on which the carrier is mounted: expanding pilots and solid pilots. They are used in the valve guide to support the stone carrier as it rotates. This keeps the stone grinding the seat true with the valve guide.
An expanding pilot is held stationary in the valve guide and the carrier spins on it. A solid pilot is fitted to the stone carrier and rotates in the valve guide.
The valve guides must be clean and not excessively worn, and the pilot must be correctly fitted. Any errors or looseness will cause the seat to be ground eccentric with the guide so that the valve will not be able to seat properly.
Stone dresser A jig is used to dress the grinding stones to the correct angle. The carrier, with the stone attached, is spun on a pilot on the jig by the drill. A small diamond in the end of a holder is moved across the stone at the desired angle. This dresses the stone. A light pass will produce a fine finish on the stone and a heavier pass will produce a coarse finish.
Grinding valve seats
The procedure for grinding a valve seat is as follows:
1. Clean the valve guides thoroughly so that the pilot will centre correctly in the guide. This ensures that the valve seat is ground concentric with its guide.
2. Select a stone to suit the diameter and the angle of the valve seat. Fit it to the carrier and dress if necessary.
3. Select a pilot to suit the valve guide. Fit this to the carrier or into the valve guide. (This will depend on the type of equipment.)
4. Place the carrier on the pilot and spin it with the electric drill, applying a light pressure to grind the valve seat. The clutch in the carrier can be adjusted to produce chatter, which assists with grinding.
5. Make a light grind to check that the stone is grinding correctly, then continue to clean up the seat.

·     Care must he taken to ensure that the valve seat is ground concentric with the valve

Fig 3.28
Fig 3.29
Lapping valves After the valves have been refaced and seats ground, then the valve and seat can be lapped to each other as follows:
I. Place a small amount of fine valve-grinding paste evenly around the face of the valve, keeping it away from the valve stern.
2. Place the valve in its guide and, by means of a suction cup and handle, rotate the valve back and forth a few degrees on its seat, using a light pressure (Figure 3.28).
3. Raise the valve frequently during lapping and turn it to a new position after a few laps.
4. Lap only until a continuous but narrow lapping mark is obtained on both the seat and face.

5. To check for correct seating, the valve face can be given a light coating of bearing blue, installed in its guide and turned against its seat (Figure 3.29).
·     After lapping, all traces of grinding paste must he
      cleaned from both the seat and the valve.
Narrowing valve seats
The width of a valve seat is important. A narrow seat is desirable to give a good seal, while a wider seat is needed to transfer heat from the valve head to the cylinder head. A compromise is therefore necessary, with the intake valve seat usually being narrower than the exhaust valve seat.
Specifications vary for different engines but examples of valve-seat widths are:
Intake valve 1.5 to 2 mm
Exhaust valve 1.8 to 2.5 mm.
Fig 3.30
If a seat is too wide, it can be narrowed by grinding the top and/or bottom of the seat to reduce its width (Figure 3.30).
The position of the valve on the seat can also be altered. Narrowing by removing metal from the top of the seat will lower the seat in relation to the valve face. Narrowing by removing metal from the bottom of the seat will raise the seat in relation to the valve face.

·     During reconditioning, it is often necessary to grind
      angles at the top and bottom of the seat to obtain the
      correct seat.
See Synchro Seating valves

1 comment:

  1. nice blog.Dev Precision Engineers is the brand that is synonymous with the export and manufacture of Engine spares and parts Engine Valves and Guides.