Mercury 305 CID, Service Manual, Page: 258 / 967

3A-66 - ENGINES 90-823225--1 1096
2. Establish a reference point on the block (on taut
side of chain) and measure from this point to the
chain.
3. Rotate camshaft in the opposite direction to
slacken the chain, then force chain out with fin-
gers and again measure the distance between
reference point and timing chain.
4. The deflection is the difference between these
two measurements. If the deflection exceeds 3/4
in. (19 mm), timing chain should be replaced.
72376
a - Reference Point
a
Checking Timing Gear Backlash and
Runout
1. Remove fuel pump and fuel pump push rod to re-
lieve tension on camshaft.
2. Loosen rocker arm nuts to relieve tension on hy-
draulic valve lifters.
3. With timing gears installed, check cam-
shaft-to-crankshaft gear backlash, as follows:
a. Mount a dial indicator on engine so that indi-
cator stem contacts one of the teeth on cam-
shaft gear. Indicator stem should be as per-
pendicular to gear tooth surface as possible.
b. Check the backlash between the camshaft
gear and crankshaft gear while applying in-
ward pressure on camshaft gear.
c. If backlash is not within .004-.006 in. (0.102-
0.152 mm) for new gears or 0.004-0.008 in.
(0.102-0.203 mm) for used gears, check for
improperly machined parts or worn camshaft
or crankshaft bearings. If bearings check out
good, replace both timing gears.
72379
Checking Backlash
4. With timing gear installed, check camshaft and
crankshaft timing gear runout, as follows:
a. Mount dial indicator on block so that indicator
stem is perpendicular to camshaft timing gear
and contacts gear surface just adjacent to
teeth.
b. Apply inward pressure on timing gear and
zero indicator, then turn crankshaft as shown.
Check gear runout through one complete rev-
olution of camshaft gear.
c. If not within .004 in. (0.102 mm), check for
burrs and foreign material between gear and
camshaft joining surfaces. If none is found,
replace both timing gears.