A piece to which the eccentric strap, eccentric rod, and pump rod are attached is cut out of 5/16-inch brass.  Its shape is indicated in Fig. 53.  The side next the eccentric must be shaped as accurately as possible to the radius of the eccentric.  The strap, of strip brass, is fastened to the piece by four screws, the eccentric rod by two screws.

Crosshead and Guides.—­The crosshead (Figs. 53 and 54) is built up by soldering together a flat foot of steel, a brass upright, and a tubular top fitting the piston rod.  The guides, which consist of a bed, covers, and distance-pieces united by screws (Fig. 64), have to withstand a lot of wear, and should preferably be of steel.  The importance of having them quite flat and straight is, of course, obvious.

[Illustration:  Fig. 64.—­Cross section of crosshead and guide.]

The last 1-3/8 inches of the piston rod has a screw thread cut on it to engage with a threaded hole in the fork (cut out of thick brass plate), to which the rear end of the connecting rod is pinned, and to take the lock nut which presses the crosshead against this fork.

Assuming that all the parts mentioned have been prepared, the cylinder should be arranged in its proper place on the bed, the piston rod centrally over its centre line.  Mark and drill the screw holes in the bed.

The Valve Gear.—­We may now attend to the valve gear.  A fork must be made for the end of the valve rod, and soldered to it with its slot at right angles to the slots which engage with the valve lugs.  Slip the rod into the steam chest, put the valve on the rod, and attach the chest (without the cover) to the valve plate by a bolt at each corner.  Pull the valve forward till the rear port is just uncovered, and turn the eccentric full forward.  You will now be able to measure off exactly the distance between the centres of the valve-rod fork pin and the rear screw of the eccentric.  The valve connecting rod (Fig. 53, VCR) should now be made and placed in position.  If the two forward holes are filed somewhat slot-shaped, any necessary adjustment of the valve is made easier.  If the adjustment of VCR and the throw of the eccentric are correct, the valve will just expose both end ports alternately when the crank is revolved.  If one port is more exposed than the other, adjust by means of the eccentric screws till a balance is obtained.  Should the ports still not be fully uncovered, the throw of the eccentric is too small, and you must either make a new eccentric or reduce the width of the valve. (The second course has the disadvantage of reducing the expansive working of the steam.) Excess movement, on the other hand, implies too great an eccentric throw.

Setting the Eccentric.—­Turn the crank full forward, so that a line through the crank pin and shaft centres is parallel to the bed.  Holding it in this position, revolve the eccentric (the screw of which should be slackened off sufficiently to allow the eccentric to move stiffly) round the shaft in a clockwise direction, until it is in that position below the shaft at which the front steam port just begins to show.  Then tighten up the eccentric lock screw.[1]