the differential gears and driving gearing at the rear axle it is imperative that only semi-fluid greases be used, because the rear construction as ordinarily built is not oil-tight and would not retain fluid lubricants. The more viscous or heavy bodied greases are also necessary in sliding gear transmissions and gear driving systems, because oil would not have sufficient cushioning properties and would soon be forced from between the working surfaces of the gear teeth. Greases reduce noise and wear where pressures are heavy, while oils are better adapted on rapidly moving, close fitting parts.

Q. What are the best lubricants for automobile use and why? A. Automobile lubricants should be derived from a crude petroleum base because oils of this nature are inorganic and are not apt to decompose by exposure to the air or by heat as are the organic lubricants derived from animal fats or vegetable sources.

Q. What is the most difficult part of the automobile to oil properly?

A. The cylinders of the automobile power plant are the most difficult portions to lubricate satisfactorily on account of the heat generated and great care is necessary in selecting oils for this purpose.

Q. What kind of lubricant should be used in the cylinder?

A. Gas engine cylinder oil should have sufficient body to maintain a positive film between the piston and cylinder walls and yet should not be so heavy that it will gum up the piston rings. It should have a high fire test which means that it should not decompose readily by heat and it should have a good cold test in order that it will not thicken too much at low temperatures. Its viscosity should be such that it will flow readily through pipes and so it can be pumped without difficulty by the various forms of gear and plunger pumps used to circulate oil. It should be free from acid and should be a pure mineral oil without adulterants or animal or vegetable fillers. It should be thoroughly filtered to remove the free carbon and other impurities which would be deposited in the combustion chamber when the oil is vaporized by the intense heat of the explosion.

Q. How can a lubricant be tested for acid?

A. A simple test for acid in oil can be made by dissolving a small quantity of the lubricant in alcohol which may be slightly warmed by immersing the test tube or flask in which the alcohol is contained in warm water. This will facilitate dissolving the lubricant, especially if heavy greases are tested. When the oil has been dissolved the solution may be tested with litmus paper which may be procured at any drug store. A piece of blue litmus paper will turn red when placed in the solution if any acid is present.

Another test which requires more time but which gives positive results can be made by soaking several strands of candle wicking or waste in the suspected lubricant and then wrapping these around a brightly polished steel shaft, placing this in the direct sunlight but not exposed to the elements. If pronounced etching or rusty spots are present under the strands of oil soaked material on the steel after the shaft has been exposed a few days it may be inferred that the oil contains acid and is not suitable for use on machines. Q. What is the effect of acid in oil?

A. The presence of acid in oil is not desired because this will produce rusting or etching of the brightly finished bearing surfaces and will roughen them so they have considerable friction. Oils that contain acid are much quicker to act injuriously when used in the cylinder because they are rapidly decomposed by the heat present at that point. Obviously, rusty, roughened surfaces will have more friction than smooth, bright, well-polished ones.

Q. What is viscosity and how is it determined?

A. Viscosity is a term used in speaking of the body of oils and is measured by allowing a given quantity of the oil to be tested to escape from a container, a drop at a time. When two oils are compared for viscosity the one that flows from the container quickest is the most fluid, and therefore has less viscosity than one of heavier body which cannot flow as fast. In some parts of automobiles it is necessary that oils have considerable body, these points being where shock and heavy pressures exist, as in gear boxes and axles. Tight fitting bearings that should be free running usually require thinner oils on account of the small space existing between the journals and the brasses.

Q.

What is flash test and how is it made?

A. The flash test is an indication of the amount of heating an oil will stand before the vapors given off will flash or catch fire. The method of performing the simple test is outlined at Fig. 113. A quantity of the oil to be tested is placed in a porcelain casserole which is heated by a Bunsen burner placed under the oil container which is carried on a tripod. A thermometer is placed in the oil to indicate the rise in temperature and when the vapors rising from

Lighted Taper

Casserole.

Thermometer

Bunsen Burner

1GasTube

Fig. 113.-Defining Method of Testing Cylinder Oil for Flashing

Point.

the surface can be flashed by a lighted wax taper the reading on the thermometer will indicate the flash point. In this connection it will be well to state that the thermometer is a special form which will indicate temperatures up to 650 degrees F. The bulb should be supported so that it will be kept from contact with the sides or bottom of the dish which are somewhat hotter than the oil and which would give a false reading. The flashpoint is made chiefly in connection with tests of cylinder oils, and it is desirable that these have

as high a flashpoint as possible so that they will not be vaporized from the cylinder walls as soon as they come in contact with the hot products of combustion.

Q. How is the fire test made?

A. The fire test of the oil is really a continuation of the flash test and is shown at Fig. 114. The fire test is the point indicated by the thermometer where the oil becomes sufficiently heated so not only the vapors flash momentarily but the entire mass of oil takes fire.

Fig. 114.-Simple Method of Making Fire Test of Lubricating Oil.

Q. What is the cold test and what does it indicate?

A. The cold test is made as indicated at Fig. 115 and is useful in showing the point at which oil will coagulate or become so stiff that it will not flow. This is easily performed by putting a flask of lubricant in a freezing mixture of ice and salt and inserting a thermometer to indicate the point at which the oil becomes solidified. An ordinary form of temperature indicating thermometer may be used in making this test. A suitable oil for winter use should

not stiffen enough at zero temperature to prevent its reaching the bearing points.

Q. Name simple ways to determine presence of adulterants. A. The presence of animal fats in greases can be quickly determined by exposing a quantity of the suspected grease to the air.

If the grease turns rancid and has a foul smell this may be considered a good indication that organic adulterants are present. The presence of fats in oils may be determined by the saponification test which consists of heating a quantity of lubricant under suspicion with potash and stirring briskly till the alkali is well absorbed by the oil. If the surface of the oil is covered with soapy matter after cooling it indicates the presence of animal fats in quantities proportionate to the amount of soap. In any mixed oil in which mineral and animal lubri

cants are combined, there is considerable

fat which will decompose liberating fatty acid if used in the cylinder and there is also the danger of charring the fat and augmenting the proportion of carbon deposits in the combustion chamber.

Q. What are specifications of a suitable cylinder oil?

A. The oils used for cylinder lubrication should preferably be