e:

The _Flask Trap_, Fig. 62. This trap gets its name from its
shape. There is an inside wall upon which the seal depends.
This trap is like the bag trap, only the two inside walls of
the pipe are combined into one. This wall should be of heavy
cast brass, free from sand holes.

_Clean Sweep Trap_, Fig. 63. Some clean sweep traps are
dependent upon an inside wall for their seals. They are made
of 1/2-S, 3/4-S, and full S.

[Illustration: FIG. 64.--Mechanical-seal trap.]

[Illustration: FIG. 65.--Standard "S" trap.]

[Illustration: FIG. 66.--Bag trap.]

_Sure Seal Trap._ The sure seal trap is designed to be
non-syphoning. This trap also has an interior waterway. If
this waterway leaks, the trap is unfit for use. If these traps
are made as shown in the second sketch with the way inside of
a larger pipe, it can be detected if the interior wall leaks.

_Centrifugal Trap._ The centrifugal trap is made similar to
the clean sweep, except that the wall of the inlet pipe is
entirely separate from the body of the trap. The inlet enters
the body of the trap on a tangent, thus making the trap
self-scouring which is a good feature.

CHAPTER XII

PIPE THREADING

The proper cutting of threads on pipe is overlooked by some
mechanics. There are many different kinds of dies and different
kinds of pipe to contend with. Steel pipe threads very hard and the
adjustable dies should be used on it. These dies cut more easily
and leave a cleaner thread than other dies when used on steel pipe.
When threads are cut on wrought-iron pipe the adjustable dies
should be used as they cut a better and cleaner thread than other
dies. To preserve the life of the dies and the quality of the
thread, oil is used freely while the dies are cutting.

THREADS.--The standard thread on pipe and fittings is a
right-handed thread. Left threads can be cut on the pipe and the
fitting can be tapped with a left thread. When a fitting is tapped
with a left thread it is marked so. The following table gives the
standard number of threads that a die will or should be allowed to
cut on the pipe:

--------+----------------+-------------------+----------------
Size | Length, inches | Threads per inch | Threads per end
--------+----------------+-------------------+----------------
3/8 | 9/16 | 18 | 10.825