How To Rebuild Your Ford V-8 351C-351M-400-429-460.pdf - Index of

UPPER
COMPRESSION
RING
PISTON
PIN
BOLT -
-
LOWER
COMPRESSION
G
L RlNG
SEMBLY
I
Complete piston and connecting-rod
assembly including bearing inserts and
piston rings. Wrist pin is prevented from
moving laterally by an interference fit between
it and its connecting-rod bore. Photo
courtesy Ford.
PISTONS & CONNECTING RODS
Replace the Pistons-If your block has
to be rebored for one reason or another,
you'll have to replace your pistons to fit
the larger holes. However, if you determine
from checking that most of your
pistons should be replaced, then the block
should have been bored in the iirst place.
The cost of pistons is the major expense,
so you may as well give your engine a
fresh start with new pistons and straight
bores. Also, its durability will be as good
or better than new, depending on the care
you take during the rebuild.
CHECKING THE PISTONS
If your engine doesn't need reboring,
your next step is to check the pistons to
establish whether or not they are reusable
before disassembling them from the rods.
Remove the old rings and toss them away.
Be careful when doing this so you don't
scratch the pistons in the process. A ring
expander will help. Don't remove the
rings down over the skirt, remove the top
ring first, then the second followed by
the oil ring. If you do this by hand, make
sure the ends of tlze rings don't gouge the
piston. For convenience, support the
piston-and-rod assembly so it doesn't
flip-flop while you're trying to remove
the rings. Clamp the rod lightly in a vise
with the bottom of the piston against
the vise. If you don't have a vise, clamp
the rod to the edge of your workbench
with a C-clamp-again not too tightly.
Four items should be checked before
a piston is given the OK: general damage
to the dome, skirt or ring lands; ringgroove
wear; piston-skirt and pin-bore
wear. If any one proves unsatisfactory,
replace the piston.
General Damage-Not so obvious damage
that can render a piston useless: skirt
scuffing or scoring, skirt collapse, ring-land
damage and dome burning. Obvious damage
can be done by something such as a
valve dropping into the cylinder.
Scuffing and scoring is caused by lack
of lubrication, excessively high operating
temperatures or a bent connecting rod.
All cause high pressure or temperature between
the piston and cylinder wall. If
there are visible scuffing or scoring
marks-linear marks in the direction of
piston travel-replace the piston. Scuffmarking
indicates the engine was excessively
overheated. If the piston has
unsymmetrical worn surfaces on the skirt
thrust faces, a twisted or bent connecting
rod is the likely culprit and should be
checked and corrected. An engine machine
shop has the equipment to do this and it
should be a normal part of their routine
when rebuilding an engine to check all
the connecting rods for alignment and
bearing bore, regardless of what the old
pistons look like. However, if you are on
a tight budget, tlze wear pattern on the
pistons will tell you what you want to
know. Otherwise, it's not a bad idea to
have your rods checked.
Damage to a piston's dome usually
comes in the form of material being removed
as a result of being overheated
from detonation or preignition. The
edges of tlze dome will be rounded off
or porous or spongy-looking areas will
show high heat concentration. To get a
good look at the dome, clean off any
carbon deposits. A good tool is a worn
screwdriver which has rounded corners at
its tip. The normal toolbox is usually well
equipped with these. Be careful when
POSlTlONl NG
NOTCH
WRIST-PIN BORE
MEASURE
In addition to piston nomenclature, note
positioning notch or arrow in top of piston.
It is important for assembling a piston to its
connecting rod, then for installing complete
assembly in its bore.
scraping the carbon so you don't damage
the piston by digging into the aluminum.
Also, don't use a sharp or hard tool like a
chisel or gasket scraper for this very
reason-or be extra careful.
Detonation or explosion of the fuel
charge, can also cause broken or distorted
ring lands through impact loading. Check
the top ring land for this condition. It
receives the brunt of the compression
loading. Consequently, if it's not damaged,
the others will be OK. Broken ring
lands are readily visible, but a bent one
may not be, particularly without a ring
in the groove - to use as a reference. ~ein;
stall a ring in the top groove and use a
feeler gage which fits snuggly between the
ring and the groove. slide the ring and
feeler gage around the groove to check
for any ring side-clearance changes which
may indicate a distorted ring land. The
top ring land is also the one that gets the
wear from an engine inhaling dirt. The
wear will be on its upper surface and
even all the way around.
Any of the types of damage I have just
mentioned are causes for discarding a
piston
Piston-Skirt Diameter-It's micrometer
time again. Mike each piston 90' to its
connecting-rod pin axis in the plane of
the pin and compare this figure-to what
the piston mikes across the bottom of
its skirt. If the skirt is not wider by at
least 0.0005 inch than at the pin, the piston
should be replaced because this indicates
the skirt is partially collapsed. Skirt
collapse is usually accompanied by heavy
scoring or scuff marks on the skirt-sure
signs that the engine was severely over-