Thrown Valve Seat

Today at work, we had a customer bring in the head of his engine, with a thrown valve seat.  Usually once an engine throws a valve seat, it means its time to replace the head, but there is a trick you can do to try and save the head, and it is success more often than not.

First thing you must do is obviously remove the head and valve covers.  Before doing anything further to repair the seat, you should check the head and engine block to ensure that they are not warped and need replaced anyways [To check levelness of the head, use a flat surface (a plate of glass), and a feeler gauge.  Set the head on the glass and feel around the edges with a .003 in. feeler gauge.  If it does not slip between the head and glass, your head is level, if it does, depending on how badly warped the head is, you can use a fine grit metallic sandpaper laid on your flat surface to shave the head until level.]

Once you are certain that the head is level, clean the head and the valves with a wire wheel or buffer disks (I usually prefer starting with the wire wheel to get the majority of carbon build up off, and then put a nice smooth finish on it with a buffer disk).  You will also need to clean the valves and the valve seats.

Once the thrown seat is clean, use a grinding wheel or coarse sand paper to lightly rough the outer edge and the bottom of the seat.  Now, you will mix a high temperature adhesive (We use JB Quick Weld) and apply it to the outside and bottom of the seat, and put it in place.  Using a socket (Deep well impact sockets usually work best) lightly tap the seat into place until it is flush with the surface of the head.  As an extra precaution, using a center punch, go around the seat, about 1/8 in. out, and tap the head in 1/8 in. intervals all the way around the seat.  This will force the metal of the head around the seat to mushroom slightly over top the seat, holding it in place.

Next, you want to take a fine valve grinding compound and smear it on the valve.  Insert the valve into the seat, and rotate it back and forth, "lapping" the valve and valve seat.  By checking the wear marks made from the compound on the valve and seat, you can tell whether the seat needs to be recut or the valve needs replaced.  If everything is as it should be, you're finally ready to reassemble the engine.