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BI-METAL PISTON
AN interesting patented design of piston is being introduced by Bobbies, Ltd., of Dereham, Norfolk. The object of the inventor has been to secure lightness of weight and yet to avoid the tendency, common with certain types of piston, to lap out the walls of cylinders, thus causing comparatively speedy wear. Several bimetal pistons have already been marketed satisfactorily and the Lancaster patent hi-metal piston, as the new Bobbies. product is called, is one that lends itself to quantity production.
The head and gudgeon-pin hnsses form an aluminium die-casting, whilst the skirt is made from weldless steel tube. The arrangement for securing the skirt to the head is made fairly clear by the accompanying sketch. In manufacture the skirt is first waisted and then the semi-circular portions, through which the bushes pass, are punched upwards. Phosphor-bronze gudgeon-pin bushes, which are drilled and grooved for lubrication, serve to hold the skirt to the crown, and additional security is afforded by four cylindrical steel nuts which are inserted into the sides of the bosses and into which bolts from the underside of the waist flange on the skirt are screwed. A steel locking washer is fitted to each bolt.
The claim of the designer is that the piston crown, which is of conventional design but is strengthened by two deep-sectioned ribs that are carried down to the bosses, is able to expand away from the skirt, and it will be noticed that there is a space of about -116. in. between the upper portion of the skirt and the side of the crown. This is said also to facilitate the conveyance of surplus MI back to the crankcase. A Tower scraper ring can be fitted if necessary, but satisfactory results have been
obtained without it. The renewable bushes will be appreciated, as pins of standard size can always be refitted.
The piston, as above described, is suitable for most classes of petrol engine for commercial vehicles. The clearance recommended is .001 in. For compression-ignition oil engines the head of the Lancaster piston is reinforced by a central circular trunk.
The patents provide for a heavier type of piston having the same kind a head but a skirt which is a nickel-iron casting. In this type the bushes are dispensed with, the pin fitting through both head and skirt.
Pistons for many popular makes of commercial vehicle will shortly be available.