f a disc of
1-15/16-inch diameter and 5/16 inch thick. The centre of the shaft hole
must be exactly 9/32 inch from the centre of the eccentric to give the
proper valve-travel. Drill and tap the eccentric edgeways for a lock screw.

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