joining rubber, D, the new tread, E, dark rubber forming the new tread, F, light rubber and canvas forming the base of the new tread. After the cover has been stitched as shown at Fig. 450, A, the next stage is to treat it to a generous layer of rubber compound on the outer side and the prepared jointless tread is carefully fixed in place. The cover is then filled with molds which are hammered into place to stretch the cover base into its original shape and approximating as near as possible the shape it assumes under working inflation pressure. The next process is the wrapping and binding of the cover. This is then followed by treatment in a vulcanizer from whence it emerges with the tread attached to the original canvas base by a thick layer of resilient black rubber. It is said that the life of a reconstructed tire is often equal to that of a new one whereas the cost of reconstruction is but 50% of that of new tires in most cases.

Repairing Punctures with Mechanical Plugs.-Repairmen who have had experience in bicycle work are thoroughly familiar with the advantages as well as the limitations of the screw down plug which has been widely sold for repairing single tube bicycle tires. A modification of this form of plug has been introduced for use on inner tubes and has the advantage of being very easily handled though its use is limited to the repair of small punctures. The plug itself consists of two threaded discs of metal which are firmly vulcanized in a surrounding mushroom shaped mass of rubber. The lower one of these has a stem attached to it on which the upper head is threaded. The plug is clearly shown at Fig. 451. For motorist's use these are sold as a kit with a set of special pliers to facilitate manipulation. The first operation after the puncture is located is to use the conical punch end or cutter as indicated at C, which makes a smooth round hole that is not apt to tear. The next operation is to stretch the hole as shown at D in order to permit the insertion of the lower portion of the plug. After this is in place the upper part is kept from turning as shown at Fig. 451, E, by pressure of a finger, while the lower portion is brought tightly to bear against the inner tube which is sandwiched between the two parts of the plug by turning the bent part of the stem which acts as a lever and makes possible the secure retention of the inner tube

between the upper and lower portions of the plug. After these parts have been tightly screwed together, the projecting end of the threaded stem is broken off or cut with the pliers and if any projects it is smoothed down with a file. In order to prevent the plug cutting through as would be the case with metal plugs the edges of the rubber pieces are very flexible and soft which, of course, prevents them cutting into the inner tube.

AIR PRESSURE AND CARRYING CAPACITY PER WHEEL OF STANDARD SIZED PNEUMATIC TIRES

For weights in excess of 1,000 pounds per wheel, 5-inch tires or larger are recommended.

The figures given by tire manufacturers as the most suitable for initial inflation generally take into account the increase in temperature and pressure created by prolonged running. It, however, is useful to know what this increase is. The figures in the following table are given by a French authority and are averages computed on tires from 3 to 41⁄2 inches diameter under usual touring car weight and speed conditions. For larger tires the increase is greater on account of the greater rigidity of the cover walls, resulting in greater internal strains in the fabric at the points of bending.

CHAPTER XI

MISCELLANEOUS REPAIR PROCESSES

Oxy-acetylene or Autogenous Welding-Torches for Welding-Sources of Gas-Cost of Autogenous Welding-Instructions for Operating-Welding Cast Iron-Method of Preheating-Welding Aluminum-Welding Malleable Iron-Welding Brass and Bronze-General Hints-Treatment of Steel, Annealing-Box Annealing-Hardening-Pack Hardening-Tempering-Case Hardening-Distinguishing Steel from Iron-Hardening Steel Tools-Temperatures for Tempering-Molten Metals to Produce Desired Heat-Working Iron and Steel-Annealing Chilled Cast Iron-Bending Pipe and Tubing-Filling the Tubing-Pipe Bending Fixture-Straightening Out Bent Fenders-Removing Dents in Tanks-Soldering and Brazing Processes-Fluxes for Soldering-Solders and Spelter for Different Purposes-Lead Burning-Soldering Aluminum-How to Braze Iron and Steel-Testing Lubricating Oils-Evils of Exhausting in Closed ShopInstructions for Repairing Storage Battery-Care of Grinding WheelsSpeeds for Grinding Wheels-Grading of Grinding Wheels.

MANY men are engaged in the automobile repair business who have been specialists in some particular branch of mechanical work before becoming interested in the automobile. Wood workers, blacksmiths and carriage smiths are especially noted owing to the decrease in carriage and wagon work and increase in automobile repairing. The review of various mechanical processes which follows cannot fail to be of value to all those not thoroughly familiar with all branches of mechanical work. Even the automobile mechanic will find the material useful for review.

Autogenous Welding.-Autogenous welding is the process of uniting metal surfaces by heat without the aid of solder or compression. High temperature, full control and easy application of the heat are necessary requisities. The most satisfactory method ist termed the oxy-acetylene process, the flame having a temperature