Saturday, May 28, 2011

Servicing valve-seat inserts

A valve-seat insert would only be replaced if it had reached a stage where it was pitted or burnt and its seat could not be reconditioned. It would then be necessary to remove the insert from the cylinder head and replace it with a new one.
Inserts are an interference fit in the cylinder head, so are not easy to remove. They can be removed by being cut out with inserting equipment, or they can be shrunk to make them easier to remove. This is done by running a bead of weld around the inside of the insert, which shrinks as it cools. This loosens the insert in the head.

  • There are precautions to be taken during welding so that the cylinder head is not heated and distorted.  
The objectives, when fitting a valve-seat insert in a cylinder head, are:
1. To cut a recess in the valve port that is concentric with the valve guide.
2. To fit an insert that will bring the valve seat back to standard and remain securely in position.

Fig 3.40
Cutting a recess
A valve-seat recess cutter is shown in Figure 3.40. It consists basically of an anchor that is mounted on the cylinder head and a spindle with a handle. A cutter is fitted to the bottom end of the spindle. The tool is adjustable so that the cutter can be positioned accurately over the valve port.
Hand-operated equipment is shown, but machine- operated equipment is used in reconditioning workshops.
To cut a recess for an insert, a cutter to suit the diameter of the insert is selected. The cutter is fitted to the spindle and a stop on the tool is adjusted for the depth of cut. The spindle is turned by the handle to rotate the cutter and, at the same time, the feed nut is turned. The feed nut applies a light load to the cutter and keeps it cutting.
When the feed nut reaches the stop that was previously set, the cutter will no longer cut and the recess will be at the correct depth.
Selecting an insert
The insert is selected for type of material and size of the valve port. Alloy inserts are usually fitted to both the intake-valve and exhaust-valve ports.
Fig 3.41
The size of the insert is found by measuring the original insert, or from the insert-maker’s data chart. Three dimensions are needed: outside diameter, inside diameter, and depth, as shown in Figure 3.41.
When the correct cutter is used, the recess will be slightly smaller in diameter than that of the new insert to be fitted. This will provide an interference fit of approximately 0.15 mm to 0.25 mm.

Fitting an insert
A punch is used as shown in Figure 3.42. The insert is driven firmly on to its seat. No lubrication is used.
Fig 3.42
After the insert is installed, the final operation is to cut and finish the valve seat to the correct angle. This is the same as for a normal valve-seating operation.
With aluminium cylinder heads, the valve-seat inserts are removed from and fitted to a cylinder head when it has been heated. Heating is done as indicated previously for fitting valve guides (see Figure 3.38). The inserts can also be chilled in dry ice before being fitted. With a heated head and chilled insert, very little punching is needed.

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