The use of lead free petrol in many older (pre 1987) engines requires the fitting of harder valve seat inserts. The lead previously found in petrol provided a protective layer on the exhaust valve and valve seat reducing the amount of wear. When lead free petrol is used the seat will rapidly wear unless they are replaced with a harder material.
The rapid wear from running lead free fuel can clearly be seen in the Ford Pinto cylinder head pictured. The exhaust seat was originally machined with a similar seat to the inlet but is now deeply recessed leading to loss of flow on this big valve head.
First the old seats are machined out to leave a counterbore which will give the correct interference fit (the hole is smaller than the seat which is to be fitted). Then the cylinder head is heated to 200oC in a large oven and the seat inserts are frozen in Liquid Nitrogen. This results in aprox 400oC temperature difference between the valve seat and the head. The hole in the head expands due to the heat while the diameter of the insert reduces allowing it to be placed into the hole.
When the head has cooled the force of the interference fit between the seat and head holds the insert in place.
After cooling the new insert has the final seat profile machined into it to the correct depth with the Serdi Millenium seat cutter.
When there is doubt about the quality of the valve material we fit new exhaust valves as we can check the technical spec to ensure it is up the job.
Badly Recessed Valve Seat
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