Check Valves; Piston And Piston Rings - Mercury 40 Service Manual

Measure cylinder bore inside diameter (with an
inside micrometer) of each cylinder, as shown
below. Check for tapered, out-of-round ("egg-
shaped") and oversize bore.
If a cylinder bore is tapered, out-of-round or
worn more than .003 in. (0.08 mm) from stan-
dard "Cylinder Block Finish Hone" diameter (re-
fer to chart), it will be necessary to re-bore that
cylinder(s) to designated oversize bore and in-
stall oversize piston(s) and piston rings during
reassembly.
NOTE: The weight of an oversize piston is approxi-
mately the same as a standard size piston; therefore,
it is not necessary to re-bore all cylinders in a block
just because one cylinder requires re-boring.
IMPORTANT: Ports must be de-burred after hon-
ing.
After honing and thoroughly cleaning cylinder
bores, apply 2 cycle outboard oil to cylinder
walls to prevent rusting.

Check Valves

NOTE: Check valves can be replaced by removing
intake manifold when powerhead is attached to dri-
veshaft housing.
The check valves are a 3/16 in. (4.76 mm) diameter
brass casing containing a nylon ball-valve. These
can be damaged by hot combustion blow-by into
crankcase (ie.: backfire, or a hole in top of piston,
etc.).
4-24 - POWERHEAD
TO CHECK: Inspect check valves by looking through
hole. If light can be seen the nylon ball is bad (prob-
ably melted); replace valve. If you see no light, insert
fine wire into check valve hole to see if there is slight
movement of nylon ball. If ball moves, valve is O.K..
Replace valve if ball does not move.
Install check valve into check valve holder as shown.
TOP
CENTER
BOTTOM
a - Check Valve
b - Holder

Piston and Piston Rings

IMPORTANT: If engine was submerged while en-
gine was running, piston pin and connecting rod
may be bent. If piston pin is bent, piston must be
replaced. Piston pins are not sold separately be-
cause of matched fit into piston. If piston pin is
bent, connecting rod must be checked for
straightness (refer to "Connecting Rods" for
checking straightness).
Inspect piston for scoring and excessive piston
skirt wear.
Check tightness of piston ring locating pins. Lo-
cating pins must be tight.
Thoroughly clean pistons. Carefully remove car-
bon deposits, with a soft wire brush or carbon re-
mover solution. DO NOT burr or round off ma-
chined edges.
Inspect piston grooves for wear and carbon accu-
mulation. If necessary, scrape carbon from piston
ring grooves being careful not to scratch sides of
grooves. Refer to procedure for cleaning piston
ring grooves.
Air
a
b
Air
PE-51080
90-852572R1 JANUARY 1998