(c) Insert compression tester in spark plug hole
(using adapter if necessary) (figure 6-28) and crank
engine for at least four compression strokes. Note
a. General. The engine is a six-cylinder, L-head,
reading on first full stroke as well as on final stroke. (See
water cooled unit of generally conventional design.
test indications (2) below.)
Overhaul can be
accomplished with tools and
equipment ordinarily available at the overhaul facility.
Detailed instruction for complete overhaul is given;
however, if preliminary diagnosis has shown a specific
defect to -be corrected, do not disassemble the engine
All cylinders must be tested the same
further than necessary to restore the engine to normal
number of compression strokes to
operating condition. The instructions in this manual
assure accurate readings. Slightly
emphasize that which is unique or special in repairing
higher compression readings will be
this particular engine. In all cases follow good general
obtained if throttle and choke are
shop practice in performance of these instructions.
opened during test.
Certain specialized shop equipment will be required to
perform a complete overhaul, such as a crankshaft
(d) Refer to manufacturer's specifications for proper
grinder and a boring bar. Detailed instructions for
cranking compression pressure.
operation of such equipment is not within the scope of
this manual. Refer to the publication covering the
(e) If readings are below normal or uneven, place a
equipment being used for this information.
small quantity of OE 30 oil on top of each piston through
spark plug hole and retest compression.
b. Engine test.
(2) Test indications.
(1) Test compression as follows:
(a) Normal. Compression builds up quickly and
(a) With engine warmed up, remove plug cables
evenly to specified compression on each cylinder, varies
from spark plugs, loosen plugs 1/2 to one turn, reinstall
less than 10 pounds between highest and lowest reading
cables and start and run engine for at least thirty
seconds, to get rid of carbon loosened by plug loosening.
(b) Ring trouble. Compression low on first stroke,
(b) Turn ignition switch OFF (or ground coil high
tends to build up on following strokes but does not reach
tension lead) and remove all spark plugs.
normal. Improves considerably with addition of oil.
transmission in NEUTRAL.
Figure 6-28. Compression Test