A process known as synchro-seating can be used to produce valve seats with a fine finish and at exactly the same angle as the valve faces.
With this method, the valves are refaced in the usual way, and the seats in the cylinder head are rough-finished to the correct angle with a stone.
After rough-finishing the seats, the carrier with its stone and pilot is mounted in the valve refacer in the same way as a valve (Figure 3.31). The grinding wheel is then used with a very light feed to dress the stone, just as if grinding a valve. The stone will have a fine finish at exactly the same angle as the valves.
The seats are then given a finish grind with the finely dressed stone. With this method, the seat angle will match that of the valves. The valves can be checked for correct sealing with bearing blue, but lapping is normally not required.
Figure 3.32 shows valve-seat cutters that are used to restore valve seats. These have a number of tungsten steel cutting blades. They can be used with a T-handle, or they can be power-driven. Figure 3.32(a) is a seat cutter with blades at 30° and 45°. Figure 3.32(b) is a narrowing cutter with its blades at angles for top- narrowing and bottom-narrowing of seats.
|Fig 3.32 (a & b)|
A cutter of the correct size for the valve seat is selected and the blades adjusted to suit the seat diameter. The cutter is fitted to a carrier with a handle. A pilot of the same diameter as the valve stem is installed in the valve guide and the carrier with its cutter is installed on the pilot. To make a cut, the cutter is turned clockwise while
maintaining a light downward pressure. A finish cut of one or two turns is made without pressure.
The procedure for cutting a valve seat is shown in Figure. 3.33, is as follows:
3. Take a top cut with a narrowing cutter until the seat width is slightly narrower than required — this operation lowers the seat.
4. Finish cut with the seat cutter, cutting lightly until the seat is the correct width.
A valve-seat cutter of the type also referred to as a reamer is shown in Figure 3.34. The cutter has a number of cutting teeth machined at an angle, and a tapered hole in the centre, which enables it to be fitted to a pilot.
Valve guides can be checked for wear in a general way by using a new valve as a gauge. The valve stem should be a free-sliding fit in the guide without excessive free play. The stem-to-guide clearance is about 0.05 mm to 0.1 mm, with the exhaust valve usually having a slightly greater clearance than the intake valve.
To measure guide wear, the valve is held slightly clear of its seat by a spacing sleeve. A dial gauge is mounted on the cylinder head, with its plunger against the head of the valve (Figure 3.35). The valve can then be moved from side to side of the guide, to indicate the clearance on the dial gauge.
but does not accurately measure the clearance.
Most valve guides are replaceable, but valve guides can also be cast integral with the cylinder head. Methods of service differ with each type.
With replaceable guides, the old guides are either pressed or driven out of the cylinder head (Figure 3.36). This is often done from the spring end of the guide because the valve-port end can become brittle and coated with carbon, making the guide hard to remove.
New guides should have an interference fit in the cylinder head of about 0.1 mm. They are driven or pressed into the head with a stepped punch. When installed, they should project from the top of the head the same distance as the original guides.
When fitting new guides, a guide can be installed in the correct position at each end of the cylinder head. A straightedge is then used between these two guides as a height gauge for the other guides. After fitting, a guide may have to be lightly reamed to provide a fit for the valve stem (Figure 3.37).
Integral valve guides are part of a cast iron cylinder head and cannot be removed. However, worn guides of this type can be reamed oversize and new valves with oversize stems fitted.
aluminium cylinder heads must be heated.
Valve guides are removed from an aluminium cylinder head after it has been heated. Heating can be done in an oven, or in water as shown in Figure 3.38. The head is heated to about 90°C. The guides can be removed once the cylinder head has reached a uniform temperature.
New valve guides must have the correct interference fit in their bores in the cylinder head. Before the new guides are fitted, the cylinder head must be reheated. A stepped punch or press tool is used to replace the guides.
Valve guides can he restored by knurling. With this process, valves with oversize stems are not required. In the knurling process, a special tool with a small knurling wheel is used to form a spiral groove down the inside of the valve guide. The tool displaces metal on each side of the wheel as it works its way down the guide and this reduces the guide’s diameter.
After knurling, a reamer is used to ream the guide to fit the valve stern. When restored, the valve guide will have a spiral groove from top to bottom and the valve stem will have minimum clearance in its guide.
A valve and reworked guide are shown in Figure 3.39.