Operating instructions

6-7
FIGURE 6-1. INSPECTING RING SIDE CLEARANCE.
cam-ground contour should not be altered. If the
piston is dimensionally serviceable in other respects
and apparently sound, measure side clearances of new
rings (after measuring their gaps while squared in the
cylinder barrel) by installing the slotted oil control
ring assembly in the third groove, the two
compression rings in the top and second grooves and
the scraper ring in the fourth groove, with part
numbers toward the piston head, and inserting various
thickness gauges on either side of each ring (See
Figure 6-1). The gaps of rings in the barrel should be
measured first so that those selected may be left in the
piston grooves, if the grooves are not excessively
worn or distorted. When installing rings, take care not
to allow their sharp ends to scratch the piston lands. If
the cylinder barrel has not been ground oversize and
fits the piston within the allowable clearance limit, it
is permissible to install either standard or 0.005 inch
oversize rings, whichever have the specified gap, as
measured with the ring pushed up by the piston head
to a point in line with the base flange.
6-23. CYLINDERS. Measure the barrel bore near
the top of the ring travel limit and at the 5-1/4 inch
station from the open end in the thrust direction and at
right angles to that in order to detect out-of-roundness
and wear-in taper. There should be little or no wear at
the open end. Look for bent barrel fins and broken
head fins. Barrel fins can be straightened if not badly
bent or cracked. A reduction of not over 10% in area
of head fins due to breakage is allowable. Look for
cracked head fins, and specify repair of any radial
crack by drilling a vee notch to remove it. If a radial
crack extends to the root of a fin it may have
penetrated the wall; hence, the cylinder should be
rejected. If the cylinder base nuts were loose at
disassembly, or if the base studs were loose or bent,
test the machined side of the cylinder flange for
bending, which is cause for rejection. Measure valve
guides for wear, and look for scoring in their bores.
Valve seats should be inspected after refacing to make
sure that their outside diameters are still less than the
valve head diameters. Exhaust valves should be
checked for warpage before refacing, and all valves
should be measured in length if the stem tips were
ground. Inspect the spark plug hole and intake flange
screw hole helical coil inserts for looseness,
deformation and position. The outer ends should lie in
the first full thread of the tapped holes in which they
are installed. The spark plug hole helical coil has
teeth at the outer end which are forced into the head
metal and should not be visible. If there was any
evidence of overheating of cylinder or piston, check as
well as possible for turning of the head in relation to
the barrel flange. Security between cylinder head and
barrel is dependent on metal to metal contact of
cylinder barrel top threads within the head shoulder.
Dark stains at this area on both new and rebarreled
cylinders is generally due to emission of thread
lubricant used upon factory assembly of head to
barrel. Also, a very slight gas leakage in this area
causing the same condition generally stops of its own
accord when the gap fills with carbon during service.
Neither condition is detrimental to engine
performance or operation.
Due to IO-520 engine series cylinder design, however,
a persistent oil leak (not sealing compound) may
indicate the required pre-load at the head/ barrel
junction has been relieved and should be investigated
for possible cylinder replacement (See Service
Bulletin M69-7).
6-24. HYDRAULIC VALVE LIFTERS. During
examination of each part, look for sludge and carbon
residues. Also check for obstructed oil holes. Inspect
face of cam follower on body for any type of damage
and look for deep scoring and corrosion on exterior of
tubular portion. Discard any lifter body which
exhibits any of these faults. To test roughly for
excessive diametrical clearance between hydraulic
unit plunger and cylinder and to check valve wear in
cylinder, start dry plunger into dry cylinder. While
holding cylinder between thumb and middle finger,
depress plunger with index finger and release it
quickly. Compression of air in cylinder should make
plunger kick back instantly. If plunger does not return