STEEL PISTONS

Auckland Star, Volume LXXII, Issue 165, 15 July 1941, Page 10

STEEL PISTONS

MOTOR NOTES

ENGINES MODIFIED

METALS SUBSTITUTED

According to motor engineers, the substitution of alternative materials in construction will not lead to any change in the standards of safety, durability, performance and comfort established in 1941 automobiles. Whether or not prices will be affected is a different proposition, as in virtuallv all cases parts made of alternative materials cost more than those made of the original materials.

The most important development, as has been pointed out previously, is the substitution of cast-iron or cast-steel pistons. These pistons are approximately 50 per cent heavier, have less than half the thermal conductivity of aluminium alloy and less than half the thermal expansion. Also, the increase in reciprocating weight means that piston pins, connecting rods and connecting-bearing rods must be checked for strength and rigidity against the higher stresses to which they will be subjected.

The cooling system, too, must be checked against the lower thermal conductivity, and the piston-pin fit against the lower thermal expansion. In many cases the compression ratio will have to be lowered and the spark plug timing altered to suit the new piston set-up.

Heavier, stiffer connecting rods have been found to be necessary on some engines, the added weight being employed to increase the section through the connecting rod shank and the thickness of metal around the big end.

On one line of cars approval has been obtained for switching to a connecting rod about five per cent heavier to bearings with longer fatigue life on both connecting rod and main bearings. These bearings are of the type that employ a porous nickel-bronze matrix to bond the thin "babbit". lining to the steel backing. This company will incorporate the heavier connecting rods and new bearings into the engine designs as soon as present parts are exhausted. Then, if cast-iron pistons must be substituted, the only changes necessary in the piston assembly will be the piston change. plus certain detailed adjustments of the piston-pin fit.

The other main change necessitated by scarcity of materials is the minimising of the amount of nickel used in passenger cars in general, and several companies report that they will be able to eliminate up to 80. per cent of the nickel in their cars. Zinc alloy die castings that serve merely to decorate the car should logically be the first to go, and this has been the policy pursued by all makers. Fortunately, it is in these parts that the greatest saving of weight can be made, and the hope is that these savings will be great enough to permit retention of zinc alloy die castings for important functional parts.