THE INTERNAL -COMBUSTION TRACTOR 85 



with a water jacket through which a liquid cooling medium 

 circulates. The cylinder may be provided with a removable 

 head or the head and cylinder may be cast in one piece. The 

 removable head permits easy inspection of the interior of the 

 cylinder, while the cast-on head obviates difficulty from im- 

 perfect joints between the cylinder and the head. Owing to 

 the inaccessibility of the one-piece cylinder and the higher 

 manufacturing cost, the great majority of tractor cylinders 

 are provided with the removable heads. One tractor has a 

 separate liner for the cylinder so that in case it becomes badly 

 worn or warped it may be removed without substituting an 

 entire new cylinder. Cylinders are usually cast separately 

 and bolted to the base or crankcase, though in four-cylinder 

 engines they are frequently cast in pairs. 



VALVES 



The valves of gas tractors are of the poppet, or mushroom, 

 type. They may be set in the cylinder head, in the cylinder 

 walls, or in a vertical engine, in a chamber projecting to one 

 side of the cylinder. They are set to act vertically, wherever 

 other features of design will permit, in order to avoid unequal 

 wear on the valve stem guide, followed by imperfect seating 

 of the valve. By having the valves in the cylinder head they 

 may be removed with the head for examination. Where the 

 valves are placed opposite each other on the sides of the cylin- 

 der they are usually provided with removable seats or cages, 

 so that a valve may be examined without taking off the cylin- 

 der head. The valve cage contains a seat for the valve and a 

 guide for the valve stem, which, especially in case of the 

 exhaust valve, is usually surrounded by a water jacket. 

 The intense heat of the exhaust is otherwise apt to cause the 

 valve stem to warp and stick in its guide. The inlet valve is 

 also frequently jacketed to prevent too early expansion of the 

 incoming charge and consequent decrease in the heating value 



