made for the lead at the opposite end, when the crank is on back dead centre in the position B (Fig. 44). The lead at the back end will therefore be equal to the distance between the marks a₂b₂ on the spindle (Fig. 43–IV). In some works the leads are measured, not by marking the valve spindle, but by pressing thin wedges into the space A₁ B₁ between the edge of the valve and that of the port, so that these edges leave marks on the wedges which can be measured.
If the front lead a₁b₁ is equal to that a₂b₂ at the back the valves are properly set “to equal leads,” and no further adjustment is necessary. If, however, the lead a₁b₁ in fore gear at the front is say ¹⁄₁₆ in, whilst a₂b₂ at the back is say ³⁄₁₆ in., the valve requires adjusting. To do this the fore gear eccentric rod is taken down and “jumped” or shortened in the smiths’ shop by ¹⁄₁₆ in. This increases the opening A₁ B₁ or the distance a₁b₁ at the front end from ¹⁄₁₆ to ⅛ in., and decreases a₂b₂ from ³⁄₁₆ in. to ⅛ in. so that the leads are made equal. Similarly if the lead at the back end be smaller than that at the front, the eccentric rod has to be lengthened slightly. Some engines are arranged with valve spindles having a screw and nut, which can be adjusted to lengthen or shorten each spindle instead of altering the length of the eccentric rod.
The maximum port openings, when the valve is at the ends of its stroke, are also noted and marked on the valve spindle. The valve spindle is blackened by soot from a lamp, so that the marks
