Pistons And Piston Rings - Mercury 150 EFI Service Manual

POWERHEAD
Standard Piston
0.015 in. (0.381 mm) Oversize Piston
4. If a cylinder bore is tapered, out-of-round or worn more than 0.003 in. (0.076 mm) from
standard "Cylinder Block Finish Hone" diameter (refer to chart, preceding), it will be nec-
essary to re-bore that cylinder(s) to 0.015 in. (0.381 mm) oversize or re-sleeve and in-
stall oversize piston(s) and piston rings during reassembly.
NOTE: The weight of an oversize piston is approximately the same as a standard size pis-
ton; therefore, it is not necessary to re-bore all cylinders in a block just because one cylinder
requires re-boring.
5. After honing and thoroughly cleaning cylinder bores, apply light oil to cylinder walls to
prevent rusting .

Pistons and Piston Rings

IMPORTANT: If engine was submerged while engine was running, piston pin and/or
connecting rod may be bent. If piston pin is bent, piston must be replaced. (Piston
pins are not sold separately because of matched fit into piston.) If piston pin is bent,
connecting rod must be checked for straightness (refer to "Connecting Rods," fol-
lowing, for checking straightness).
1. Inspect pistons for scoring and excessive piston skirt wear.
2. Check tightness of piston ring locating pins. Locating pins must be tight.
3. Thoroughly clean pistons. Carefully remove carbon deposits from pistons, with a soft
wire brush or carbon removal solution. Do not burr or round off machined edges.
Inspect piston ring grooves for wear and carbon accumulation. If necessary, scrape carbon
from piston ring grooves being careful not to scratch sides of grooves. Refer to proce-
dure following for cleaning piston ring grooves.
CLEANING PISTON RING GROOVES
NOTE: Cleaning instructions differ between the rectangular ring groove and keystone (ta-
pered) ring groove. Pistons may have two keystone ring grooves or one keystone ring
groove and one rectangular ring groove as shown.
Rectangular ring grooves
1. A broken rectangular piston ring can be used as a tool for scraping carbon from ring
grooves. Carefully scrape carbon from ring grooves without scratching the side surfaces
of grooves.
Keystone (tapered) ring grooves
Care must be taken not to scratch the side surfaces of the ring groove. Scratching
the side surface of the ring groove will damage the ring groove.
1. Use a bristle brush and carbon remover solution to remove carbon from side surfaces.
2. A tool can be made for cleaning the inner diameter of the tapered ring grooves. The tool
can be made from a broken tapered piston ring with the side taper removed to enable
the inside edge of the ring to reach the inner diameter of the groove. Carefully scrape
carbon from inner diameter of ring grooves. Care must be taken not to damage the
grooves by scratching the side surfaces of the grooves.
Page 4A-44
Models 150/175/200 2.5 liter Cylinder Block Finish Hone
3.501 in. (88.93 mm)
3.516 in. (89.31 mm)
CAUTION
90-883728 JULY 2001