Operating instructions

2-2 JANUARY 1985
base flanges. The governor mount pad is located at
the lower front corner. On the right permold
crankcase an alternator pad is located at the front.
c. The crankcase interior is ventilated by a breather
consisting of a tube and baffles assembly with a
side extension for hose attachment. The breather
assembly is pressed into the upper left crankcase.
2-4. CRANKSHAFT. The six throw, steel alloy
forging is machined allover except for some
portions of the crankcheeks. The main bearing
journals and crankpins are nitrided after grinding.
A flange is formed at the front for attaching a
propeller. An oil transfer collar, encompassing the
crankshaft between the front and rear halves of the
main thrust bearing, transfers the governor
controlled oil from the crankcase passage to the
crankshaft interior. Side blades projecting from the
crankcheeks 1 and 2, 3 and 4 are machined for the
installation of one 4th, one 5th and two 6th order
counterweights. IO-520-BA crankshafts use three
sixth order counterweights and one fourth order
counterweight. Oscillation of the counterweights
on their pins dampen crankshaft torsional vibration.
a. The crankshaft gear is heated prior to installation
to obtain a shrink fit. The gear is driven by a dowel
of uniform diameter. A cluster gear, typical of
permold engines, provides for direct drive of the
fuel pump.
b. The accessory drive gear, typical with the
permold crankcase, is heated and shrunk onto a
flange just behind the oil transfer collar at the front
of the crankshaft, and retained by four bolts.
c. A rubber oil seal, which is stretched over the
crankshaft flange and a split retainer ring are seated
between crankcase castings in front shaft exit, and
are sealed to the crankshaft by a helical spring
inside the seal's cavity.
2-5. CONNECTING RODS. The "I" beam type
connecting rods have split bronze piston pin
bushings and two identical precision inserts (of the
same type as the main bearings) at the crankpin end.
Weight variation of rods in anyone pair is limited to
1/2 ounce in opposite bays.
2-6. CAMSHAFT. A steel alloy forging is
machined on four journals, nine cam lobes and the
gear mount flange at the rear end. The lobes and
journals are hardened and ground. A groove around
the front journal passes engine oil from the right
crankcase cross passage to the left case passage.
The camshaft gear is attached by four unequally
spaced bolts to locate its timing mark in relation to
the cam lobes. On the sandcast crankcase, a cluster
gear is bolted with the camshaft gear and drives the
fuel pump gear.
2-7. PISTONS. Pistons are aluminum alloy
forgings. The skirts are solid and have cylindrical
relief cuts at the bottom to clear the crankshaft
counterweights. Pistons have three grooves above
the pin hole and one groove below. Compression
rings arc installed in the top, second and the groove
below the pin hole. A center grooved and slotted
oil ring is installed in the third groove, which has
six oil drain holes to the interior. Weights are
limited to 1/2 ounce in opposite bays. Piston pins
are full floating ground steel rubes with
permanently forged-in aluminum end plugs.
2-8. TAPPETS. The barrel type hydraulic tappets
may be removed and replaced without complete
disassembly of the engine as described in Section
IV. The construction and operation of the tappets
are described in paragraph 2-15, figure 2-6.
2-9. CYLINDERS. The externally finned
aluminum alloy head castings are heated and valve
seat inserts installed before the head is screwed and
shrunk onto an externally finned steel alloy barrel to
make the permanent head and barrel assembly.
Valve guides are pressed into the cold cylinder
assembly and reamed to slightly different
diameters. Special 18 mm helical coil thread inserts
are installed in upper and lower spark plug holes.
Smaller helical coils are installed in exhaust
manifold attaching stud holes. Both intake and
exhaust ports are on the bottom of the head when
the cylinder is installed. Exhaust valve faces are
Stellite No.6 and stem tips are hardened. Valve
stems are solid. Outer retainers of the two
concentric springs surrounding each valve are
locked to the stems by tapered, semi-circular keys
which engage grooves around the stems. Rotocaps
are installed on exhaust valves only. The rotating
action of this type retainer helps to prevent burning
and eroding of the valve and valve seat. Inner
spring retainers are pressed steel. Valve rocker
covers are aluminum alloy castings. Rocker shafts
are ground steel tubes with a hole drilled in one end
at a 90 degree angle to the longitudinal axis. The
two inside rocker shaft bosses are drilled and tapped
for the 5/16 inch rocker shaft retaining screws.
Valve rockers are steel forgings with hardened