Mitsubishi Pick-ups and Montero 1983-1995 Repair Guide

Valve Seats


See Figures 1 and 2


As in the case of valves, seats may corrected for inadequate seal width and trueness with machining. After the seat is machined the valve must be lapped in. If the valve seat insert shows severe damage or wear it must be replaced.

The seats should be replaced by an experienced machine shop. The procedure requires precise measuring and milling of the head. The new seat is installed through the use of extreme temperatures which require the use of special safety and handling equipment.

Before removal, the seat thickness is removed through drilling or reaming, leaving about 0.5 mm around the outside. The seat can then be collapsed out of the head. After cleaning the area, the opening in the head is precisely enlarged to accept the new, oversize valve seat. The hole in the head must conform exactly to the outer diameter and depth of the seat insert. These dimensions vary for each engine; select your parts carefully.

Click image to see an enlarged view

Fig. Fig. 1: Installing a common valve seal

Click image to see an enlarged view

Fig. Fig. 2: Mitsubishi recommends using a tool to fit the seals to the guides-but a properly sized socket (neither too large nor too small) can be used

On most applications, the seat is installed after the head is heated to 482°F (250°C). The room temperature seat is pressed (not hammered) into the heated head. As the head cools, it grips the seat very tightly.

After the seat is installed, the metal must be allowed to return to normal temperatures slowly. Resist any temptation to hurry the heating or cooling; sudden temperature change can cause the metal to crack. Simply let the head sit, usually on the press or workbench for a few hours.

After the head has returned to normal temperature, the new seat insert must be machined to the correct angles. The valve is then inserted and the valve and seat lapped with compound to assure perfect contact.