When it comes to drilling holes, to provide an outlet for the excess oil in the cylinders and so to reduce smoking, small holes. ¼-inch for example, are sufficient and may be drilled in on any spiral plan, simply beginning near the bottom and working up close to the piston pin bosses along a spiral track. The advantage of the spiral arrangement is that no hole is above another; the dripping from each hole is therefore distinct and the quantity which runs down is greater.
Another method of curing the excessive lubrication to which the older cars - particularly those with splash lubrication - are subject, is to turn a deep groove in the bottom of the piston, about like a piston ring groove but with the lower edge beveled off. When this is done, much as shown in Fig. 41, a series of small holes - made with about a No. 30 drill - are put in at the angle of the bevel; 6 or 8 holes, equally distributed around the circumference, are probably enough. The sharp upper edge acts as a wiper and removes the oil from the cylinder walls into the groove, whence it passes through the holes to the piston interior, and there drops back into the crankcase. No ring is placed in the slot as it would prevent the free passage of the oil. This device stops the smoking immediately.
Fig. 41. Method of Grooving and Drilling Piston to Overcome Excessive Lubrication and Smoking.