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 TM 55-1905-219-14-8
POWER HEAD AND RECEIVER ASSEMBLY - MAINTENANCE INSTRUCTIONS.
a.
Power head, crankshaft, and pistons. Combustible vapor from the carburetor is inducted into the
crankcase as described. As one piston reaches the bottom of its power stroke, this vapor charge is
compressed and forced through the inlet ports into the cylinders. This action drives out the burned gases which
were previously ignited during the last power stroke, and leaves the cylinder charged with a new supply of
combustible vapor. The burned gases are driven through the exhaust ports into the receiver, where they are
cooled by a spray of water.
b.
As the piston starts its upward stroke, the inlet and exhaust ports are closed, and the vapor trapped
within the cylinder is compressed in preparation for the next power stroke. Engine cooling is accomplished by
the circulation of part of the water being pumped by the fire pump.
c.
The power head consists of the cylinder head, cylinder block, pistons, connecting rods, crankshaft
and crankcase. The cylinder head is attached to the top of the cylinder block and forms the combustion
chamber. The cylinder block contains the cylinder bores, water jacket, intake and exhaust ports and the upper
bearing supports. The pistons are connected to the connecting rod by full floating piston pins and are attached
to the crankshaft by means of cap screws. The crankcase is attached to the bottom of the cylinder block and
provides the lower crankshaft bearing supports.
d.
When major repairs on the power head (engine) such as installation of new piston rings, pistons,
connecting rods, crankshaft, etc, is required, a disassembly operation becomes necessary, which should be
carefully performed in clean surroundings with clean tools and on a clean and orderly bench top with sufficient
space to temporarily store the various parts as they are removed for inspection, corrective measures or
replacement.
4-180
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