Davis Diesel Development introduced their head for converting Cox TD and Medallion .049’s in late 1976 and is still available. Cox International also sells an almost identical head of this type. They has a loosely fitted contra-piston and compression adjusting screw with a coil spring for retaining the desired compression setting. To seal against leaks, the system uses a disc punched from thin fluorocarbon plastic sheet between the head and cylinder. Compression is adjusted in the usual way, as the disc conforms to the slight changes in combustion chamber volume. This arrangement always backs off when desired – no sticking contra-piston here. However, the plastic disc will fail if the engine runs too hot and must then be replaced. An alternative diesel head uses a rubber O-ring fitted to a groove in the aluminium contra-piston to make a gas tight seal.
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