Valve Guides
The valve
guides are the parts that support the valves in the cylinder head. They are machined to
fit a few thousandths of an inch clearance with a valve stem, This close clearance is
important for the following reasons:
- It
keeps lubricating oil from getting sucked into the combustion chamber past the intake
valve stem during the intake stroke.
- It
keeps exhaust gases from getting into the crankcase area past the exhaust valve stem
during the exhaust stroke.
- It
keeps the valve face in perfect alignment with the valve seat.
Valve guides
may be cast integrally with the head, or they may be removable (fig. 3-52). Removable guides are press-fit into
the cylinder head.
Valve
Springs, Retainers, Seals, and Valve Rotators
The valve assembly is completed by the spring, the retainer, the seal, and the valve
rotator (fig. 3-53). The spring,
which keeps the valve in a normally closed position, is basically the same for all
engines; however, the number and types of coils can vary. Most valves have only one
spring, but, in some cases, there may be two—an inner spring and an outer spring. The
second spring increases the pressure holding the valve closed. Low-spring tension can
cause valve float (spring too weak to close the valve at high rpm).
A valve
retainer and keepers lock the valve spring on the valve. The retainer is a specially
shaped washer that fits over the top of the valve spring. The keepers, or locks, fit into
the valve stem grooves, holding the retainer and spring in place.
The seal keeps
the valve operating mechanism oil from running down the valve stem and into the combustion
chamber. Valve seals come in two basic types—umbrella and O ring. Both are common on
modern engines. The umbrella valve seal is shaped like a cup and can be made of neoprene
plastic or rubber. An umbrella valve seal slides down over the valve stem before the
spring and retainer. It covers the small clearance between the valve stem and guide. The O
ring is a small, round seal that fits into an extra groove cut into the valve stem. It
fits on the valve stem after the spring and retainer. Unlike the umbrella type, it seals
the gap between the retainer and the valve stem, not the guide and stem. It stops oil from
flowing through the retainer down the stem and into the guide.
A valve
rotator (fig. 3-54) turns the valve
to prevent a carbon buildup and hot spots on the valve face. There are two types of
retainers—the release type and the positive type. The release type of rotator
releases the spring tension from the valve while open; this allows the valve to rotate
from engine vibration The positive rotator is a two-piece valve retainer with a flexible
washer between the two pieces. A series of balls between the retainer pieces roll on
machined ramps, as pressure is applied and released from the opening and closing of the
valve. The movement of the balls up and down the ramps translates into rotations of the
valve.
Reconditioning
Valves
Valve reconditioning includes grinding valves and valve seats, adjusting valve tappet
clearances, installing new valve seat inserts, and timing the valves. Together, these
operations constitute the valve service necessary for smooth engine performance and
maximum power output.
To recondition
valves and valve seats, first remove the cylinder head from the engine. Once the cylinder
head is off, remove the carbon from the head, the cylinder block, and the pistons. In
cleaning the top of the piston, you must exercise care to prevent gouging and scratching,
as rough spots collect carbon readily and lead to preignition and detonation during
operation.Remove the valves using a valve spring compressor. Next, clean the valves with a
wire brush or buffing wheel (fig. 3-55). When the buffing wheel is being used,
make sure you wear proper eye protection to prevent wire and other foreign matter from
flying into your eyes.
Be careful not
to interchange the valves. Bach valve must be replaced in the same valve port from which
it was removed. The valve stem moving up and down in the valve guide develops a wear
pattern. And, if the valves are interchanged, a new wear pattern is developed. This causes
excessive wear on the valve stem and guide.
To eliminate
confusion, you should devise a system to identify a valve with the cylinder from which it
was taken. The most common way to identify valves is to place them on a piece of board
with holes drilled and numbered to correspond with the cylinder each valve came from. The
next step is to resurface the valve face. This is done by using a valve grinding or
refacing machine.
VALVE
GRINDING is done by machining a fresh, smooth surface on the face and stem tips. Valve
faces suffer from burning, pitting, and wear caused by opening and closing millions of
times during the service life of the engine. Valve stem tips wear because of friction from
the rocker arms.
Although there
are some variations in design, most valve grinding machines (fig. 3-56) are basically the same. They use a
grinding stone and a precision chuck to remove a thin layer of metal from the valve and
stem tip. The following steps are used in preparing to reface a valve:
- DRESS
THE STONE by using a diamond cutter to true stone surface (fig. 3-57). Do this before grinding the valves. A
diamond-tipped cutting attachment is provided with the machine for truing the stone.
Follow the equipment manufacturer’s instructions for that specific piece equipment.
CAUTION
Be careful when using a diamond tool to dress a stone. Wear eye
protection and feed the diamond into the stone SLOWLY. If fed too fast, tool or
stone breakage may result.
- SET
THE CHUCK ANGLE by rotating the valve grinding machine chuck assembly. An interference
angle (normally 1 degree difference in valve face angle and valve seat angle)
is set on the machine. If the valve seat is 45 degrees, the chuck is set to 44 degrees.
This allows for reduced break-in and sealing time.
- CHUCK
THE VALVE in the valve grinding machine by inserting the valve stem into the chuck
Make sure the stem is inserted so the chuck grasps the machine surface nearest the valve
head.
WARNING
The chuck must NOT clamp onto an unmachined surface or runout
will occur.
Before
grinding, inspect each valve face for burning and each stem for wear. Replace valves that
are badly worn or burned. Grind a new valve along with the old, used valves.
WARNING
Wear a face shield when grinding
valves. The stone could shatter, throwing debris into your face.
To grind the
valve face, turn on the machine and cooling fluid slowly feed the valve into the
stone. While feeding, slowly move the valve back and forth in front of the stone. Use the
full face of the stone but do not let the valve face move out of contact with the
stone while cutting. Grind the valve only long enough to clean up its face. When the full
valve face looks shiny with no darken pits, shut the machine off and inspect the face.
Grinding, by
removing metal from the face, makes the valve stem extend through the head more. This
affects spring tension and rocker-arm geometry. Grind the face of the valve as little as
possible. A sharp valve margin (fig. 3-58) indicates excessive valve face removal
and requires valve replacement. If the margin is too thin, the valve can burn when
returned to service. It may not be thick enough to dissipate heat fast enough. The head of
the valve can actually begin to melt, burn, and blow out the exhaust port. Refer to the
manufacturer’s manual for specifications about minimum valve margin of thickness.
If the head of
the valve wobbles as it turns on the valve grinding machine, the valve is either bent or
chucked improperly. Turn off the machine and check for causes. If the valve is bent,
replace it with a new one.
If a burned
valve is not noticed during initial inspection, it will show up when excess grinding is
required to clean up the valve face. A normal amount of grinding does not remove a deep
pit or groove. Replace the valve if it is burned.
Another area
on the valve that must be attended to is the valve stem. This is due to wear from the
valve operating mechanisms. When the tip end of the valve stems is rough, smooth them by
grinding lightly with a special attachment furnished with the valve grinding machine.
Grind as little off the stem as possible. Many stems are hardened and too much grinding
results in rapid wear when the valve is returned to service. Generally, cut the same
amount of metal off the face and stem. This helps to keep the valve train geometry
correct. |
