amount of pressure for which the regulator should be set depends upon the design of the torch and the amount of oxygen contained in the storage tank.

How Oxygen Is Produced.-While the best results are obtained by the use of the proper burner and compressed oxygen tank, it is possible to generate oxygen by very simple mechanism, and for the ingenious repairman to extemporize a burner that will give fairly good results in removing the carbon. The simplest method of producing oxygen is by heating a mixture of potassium chlorate and manganese dioxide. Although oxygen may be produced by heating potassium chlorate alone, the combination of the manganese dioxide acts as a stimulating agent which not only gives out oxygen, but which assists in breaking up the chlorate so that more of the gas will be evolved. At Fig. 108, A, an extremely simple oxygen generator that can be made by any repairman is outlined. The basis of the device consists of a 5 or 6-inch length of seamless iron pipe, one and one-half or two inches in diameter. The pipe is threaded at each end and standard pipe caps are fitted, one of these being drilled and tapped to receive a one-eighth inch brass pipe. This is bent in the form of an ell and a piece of flexible copper tubing is soldered into the end of the pipe. In order to operate this generator the unperforated cap is unscrewed and a mixture composed of two ounces of potassium chlorate and one tablespoonful of manganese dioxide is placed in the pipe. The cap is screwed back in position and the flame of a blow torch is used to heat the bottom of the iron pipe. In a short period oxygen gas will issue from the end of the copper tube joined to the generator by rubber hose, which should be placed inside of the combustion chamber against the carbonized area. A lighted match or taper thrown into the cylinder will start combustion of the carbon.

One of the latest forms of oxygen generator is shown at Fig. 108, B. The feature is that the oxygen is evolved by heating a special cartridge by an alcohol burner. The oxygen gas passes through a purifier which is provided with a safety valve to prevent accumulation of excess pressure. The purifier is connected with the oxygen generating chamber by a one-way check valve which

permits the gas to flow into the purifier, but not from that member into the generating chamber. The cartridges or charges weigh six pounds and sell for one dollar apiece. A cupful of water is placed on top of the cartridge to prevent the solder on that member from melting. When the flame plays on the bottom of the cartridge, which also acts as a generating chamber, the oxygen issues from the hole in the center into the purifying chamber.

Repairing Scored Cylinders. If the engine has been run at any time without adequate lubrication, one or more of the cylinders may be found to have vertical scratches running up and down the cylinder walls. The depth of these will vary according to the amount of time the cylinder was without lubrication, and if the grooves are very deep the only remedy is to purchase a new member. Of course, if sufficient stock is available in the cylinder walls, the cylinders may be rebored and new pistons which are oversize, i.e., larger than standard, may be fitted. Where the scratches are not deep they may be ground out with a high speed emery wheel or lapped out if that type of machine is not available. Wrist pins have been known to come loose, especially when these are retained by set screws that are not properly locked, and as wrist pins are usually of hardened steel it will be evident that the sharp edge of that member can act as a cutting tool and make a pronounced groove in the cylinder. Cylinders that have been damaged in this manner have sometimes been repaired by the autogenous welding process, the oxy-acetylene flame being used to fuse new cast iron into the groove, then grinding out the ridge of excess material in order to obtain a smooth bore. Cylinder grinding is a job that requires skilled mechanics, but may be accomplished on any lathe fitted with an internal grinding attachment.

When a car that has been used for a considerable length of time is overhauled it may be found that the cylinder bore will have worn enough so reboring will be necessary. The wear is commonly found about midway the length of the cylinder walls, as it is at this point that the connecting rod side thrust is greatest due to the angularity of that member. In order to remedy this defective condition some repairmen are content with merely fit

Fig. 109.-Appliances for Measuring Cylinder Bore to Determine Amount of Wear.

ting new piston rings of slightly larger circumference, but this plan is not wholly satisfactory, because if the rings are of sufficiently small diameter to enter the unworn portion of the bore, when they spring out into the worn part there must exist too wide a gap at the slots. For this reason most mechanics prefer

to fit either a new cylinder and piston or to rebore the cylinder and fit a larger piston with correspondingly larger rings.

If the car is an old one and it is not considered wise to spend the money necessary to restore the cylinder bore to correct shape, one can get considerably more service by a simple expedient of making new piston rings with long stepped ends. In this case they will pass the smaller portions of the bore and expand sufficiently to fill the worn parts. As there is no gap between the ends, even when fully expanded, there will be but little loss of compression, such as might be possible with either butt joint or diagonal cut rings under the same conditions. One precaution is essential, however, and that is to insure that the grooves are of sufficient width to take step cut rings having substantial steps. If the piston rings are narrow, the ends are apt to snap off at the steps. If the grooves are turned out to a greater width, new piston rings must be made to fit. The compression is much improved and power is increased as the rings with the long stepped joint will follow the worn bore, constricting as the bore is smaller and filling out as the wear increases.

It is not difficult to measure the bore to find out if the walls are worn. Different calipers have been devised for this purpose, some of which are illustrated at Fig. 109. These are usually constructed on the multiplying gauge principle so that the slightest inaccuracy will be magnified at the dial. In the device shown at A, a fixed block, A, carries a standard on which is fulcrumed the indicating needle, D. This indicating needle is in contact below the fulcrum with a plunger pin attached to the movable block D, which is normally kept pressed out by the coil spring C. If any irregularities are present the block B indicates them by its movement, which is translated to the short arm of the indicating lever D, which is fulcrumed at E. Another form of indicator working on practically the same principle except that the indicating lever E is actuated by the plungers DD, is shown at B. The use of the Ames Dial Indicator, carried by a special fixture for use in determining the truth of bore of automobile cylinders, is so clearly shown at Fig. 109 that it will not be necessary to mention the principle of operation any further than to state that the internal

mechanism of the indicator is such that the needle will indicate variations of less than .001 inch.

Where the grooves in the cylinder are not deep or where it has warped enough so the rings do not bear equally at all parts of the cylinder bore, it is possible to obtain a fairly accurate degree of finish by a lapping process in which an old piston is coated with a mixture of fine emery and oil and is reciprocated up and down in the cylinder as well as turned at the same time. This may be easily done by using a dummy connecting rod having only a wrist pin end boss, and of such size at the other end so that it can be held in the chuck of a drill press. The cylinder casting is firmly clamped on the drill press table by suitable clamping blocks, and a wooden block is placed in the combustion chamber to provide a stop for the piston at its lower extreme position. The back gears are put in and the drill chuck is revolved slowly. All the while that the piston is turning the drill chuck should be raised up and down by the hand feed lever, as the best results are obtained when the lapping member is given a combination of rotary and reciprocating motion.

Even if power is not available, the repairman need not be discouraged, because very good results may be obtained by hand. lapping. The same method is used as for lapping by power. The abrasive material is composed of very fine emery and light machine oil, and is renewed from time to time and the cylinder cleaned out to remove the old lapping compound before any new mixture is introduced. A cylinder may be easily supported in a box of sand, as indicated, the compression relief petcock in the top of the cylinder being provided with a piece of rubber hose which projects above the surface of the same, and which acts as a breather opening for escape of the air compressed by the downward stroke of the lap.

Where the cylinders are cast in block, as is now common practice on nearly all automobiles of recent development, it is not difficult to held the cylinder block, as this is of sufficient size and weight to stand upright on the bench without clamping it down. While the writer has used this method in repair work over ten years ago, he wishes to acknowledge his indebtedness to Motor Age