87 “Titan” 10-20 oiling diagram 198 88 “International” oiling diagram 199 89 Grinding valve in engine with fixed head 231 90 Grinding valve in detachable head 233 91 Grinding valve in detachable seat 234 92 Valve seat cutter 235 93 “Holt” valve arrangement 236 94 Valve timing, using marks on flywheel 238 95 Valve timing 239 TRACTOR PRINCIPLES CHAPTER I TRACTOR PRINCIPLES While tractors and automobiles are the same in general principles, there is a wide difference between them in design, construction, and handling, due to the differences in the work that they do and in the conditions under which they do it. An automobile is required to move only itself and the load that it carries. While it can run over rough roads, these must be hard enough to support it; on soft ground it will sink in and be unable to get itself out. It can make great speed over smooth, level roads; but only rarely do road and police conditions permit it to run its fastest for more than a few minutes at a time. For the greater part of its life it develops only a portion of the power of which it is capable. A tractor, on the other hand, is intended not to carry, but to haul. It must run and do its work on rough hillsides, soft bottoms, or any other land where it is required to go. Instead of developing speed it develops pulling power, and must be able to develop its full power continuously. Appearance and comfort count for a great deal in an automobile, and much attention is devoted to making it noiseless and simple to manage. These things do not apply in a tractor, which is a labor-saving and money-making machine, valuable only for the work that it can do. There is no question of upholstery or nickel-plating; all that is wanted is a machine that will do the required work with the least possible cost of operation. As is the case with any kind of machine that is purchased as a money-maker, its cost should be as low as is consistent with its ability to do its work. Any extra cost for accessories, or finish, or other detail, is wasted unless it permits the machine to do more work, or, by making the operator more comfortable, allows him to run the machine for a longer stretch of time or with greater efficiency. It may be taken for granted that any tractor will run and will do its work with satisfaction, provided it is sensibly handled and cared for. Far more troubles and breakdowns come from careless handling and from neglect than from faulty design and material. A tractor that is running and doing its work is earning a return on the money invested in it; when it is laid up for repairs there is not only a loss of interest on the investment, but a loss of the value of the work that it might be doing.