What is the Procedure for Valve Seat Replacement and installation on Diesel engines?

Diesel engine valve seat replacement is the main process where a technician needs to be precise and perfect. Here are some details about the replacement process. 

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Valve seat- 

The valves and valve seats are ground to the correct size and shape so that the value may sit appropriately on the seat. For effective valve seating and sealing, the valve face must be concentric with the valve stem; and the valve guide must be concentric with the valve face. Also, the valve face angle must watch the valve seat angle. 

Procedure for Replacement- 

If it is determined that the valve seat must be replaced because of cracks or excessive width, the following procedure should be used: 

1. Using a removing tool, remove the seat carefully to prevent damage to the cylinder head.

Note- 

Use extreme caution when removing the valve seat. Damage to the cylinder head in the valve seat area may render the head unfit for further use.

2. Clean the valve port and seat area with a carbon brush and compressed air.

Note. It is recommended that after the removal of a valve seat, the seat counterbore should be enlarged to allow the installation of an oversize seat. Although this practice is used most often, with additional experience the mechanic will be able to determine the type of cylinder heads that will allow the successful replacement of valve seats without enlargement of the counterbore. A slightly oversize valve seat (0.013 to 0.25 mm) is generally available and may be used to insure a good seat fit in the counterbore if the counterbore is not recut.

Valve guides must be in good condition or replaced before any attempt to cut the valve seat counterbore or replace valve seats. 

3. If it is decided to enlarge the valve seat counterbore or to cut a counterbore in a head not originally equipped with valve seats, a cutting tool or repeater should be used. Many types of counterbore-cutting tools are available. 

Procedure for Installing- 

1. After the valve seat counterbore has been enlarged or considered usable, select a mandrel pilot for the insert that will fit snugly in the valve guide.

2. Obtain a new valve seat insert that will fit the counterbore using the engine parts manual as a reference or using the chart supplied with the valve seat insert cutting tool set.

3. Visually inspect the counterbore, making sure that it is free from metal particles and rough edges. Select a driver that has an outside diameter slightly smaller than the seat.

4. Place the ring insert over the driver pilot onto the cylinder head counterbore.

5. Place the driver onto the pilot and with a hammer drive the valve seat into the counterbore using sharp, hard blows.

6. After driving the seat in place, it is recommended that the seat be staked or knurled in place. If a knurling or staking tool is not available, a 7 mm round end punch may be used to stake insert around its outer circumference.

Note- 

If the seat is of cast iron, no knurling or staking is necessary as the seat has the same coefficient of expansion as the cylinder head. Seats that are made of steel alloy require staking since their expansion rate does not match that of cast iron. As a result, they may fall out during engine warm-up.

Alternate methods of valve seat installations are :

1. Shrinking valve seat by cooling and then driving them in. 

2. Warm the cylinder head in hot water and then install the seat.

Caution- 

Safety glasses should be worn during this operation as valve seat inserts are very brittle and may shatter, causing eye damage.