connecting-rod bearing without taking down the motor; many motors are made in which the hand-hole plates are so small that they are absolutely useless. Of course,

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in four-cycle motors, the base being open and the entire side plates removable, there is no trouble in getting at the crank shaft.

In small-sized motors it is often easier to remove the

Fig. 48.-Base with One End Removable

cylinder to reach the piston and crank shaft than to attempt working through a hand-hole in the base, but in large-powered and heavy motors the hand-holes are

preferable, and in this class of engines removable cylinder heads are a great convenience as they permit the piston being withdrawn through the cylinder by simply removing the head and disconnecting the connecting-rod from the shaft through the hand-hole plates.

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Fig. 49.-Split Base

Water jackets are usually cast integral with the cylinders (Fig. 38), but in a number of light-weight motorsespecially those for vehicle use the jacket is formed from spun copper or similar material and is clamped over the cylinder, leaving a space between the two

(Fig. 52). This method is very satisfactory where fresh water is used for cooling, but for salt-water use it is not advisable, as the salts in the sea-water set up galvanic action between the copper jacket and the iron cylinder

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Fig. 50.-Both Ends of Base Removable

and this soon eats away the cylinder and causes rapid corrosion.

Crank shafts are perhaps too often neglected in the smaller and cheaper motors, and even in many of the

larger sizes they are not given as much care and attention as they should have. Crank shafts bear all the strain and pressure of the explosive impulses and serve to

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Fig. 51.-Cylinder with Separate Head

transmit all the power to the machinery; they should therefore be designed with a large excess of strength and should be forged from the highest grade of steel. Open

hearth .35 to 45 carbon steel is excellent material, and after forging into shape it should be annealed, which relieves the strain from forging and renders the steel tough and strong. They should then be machined to

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Fig. 52.-Water Jacket Clamped On

within 3/1000 of an inch in size, and finally ground to a mirror finish, where the bearings are to fit. Many otherwise good motors have cranks without counterbalances, but these are most essential to single-cylinder