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Cylinder Heads and Valves
Cylinder Heads
Purpose – regulates the air/fuel in/out of the engine
Construction Cast Iron Cast Aluminum
Overhead valve heads incorporate: Valves @ related components Coolant passages Valve operation mechanism(s)
Cylinder Heads Overhead camshaft heads will also incorporate: Camshaft(s) Rocker arms or followers
Hemispherical Cylinder Heads
Hemi – a Chrysler term for a symmetrical cylinder design.
Typically valves would be positioned directly opposite in the head with (ideally) a spark-plug positioned between them. Modern designs my incorporate two spark-
plugs. NOT exclusive to Chrysler!
Hemi Head
Cylinder Heads
Cross flow head design – the practice of placing the intake port and the exhaust port on opposite sides of the cylinder head.
Traditional Arrangement
Traditionally, combustion chambers would have one exhaust valve and one intake valve.
Four valves per cylinder – two exhaust and two intake valves. Pentroof
design – each pair of valves are inline
Multiple Valves
Intake - Exhaust Ports
The passageways in the cylinder head that lead to/from the combustion area.
Intake: Larger ports = more airflow Smaller ports = better
velocity for low RPM operation
Longer ports = better atomization on carb and TBI
Shorter ports = denser A/F charge
Gasket Matching
•Using an intake gasket as a template to “port” the heads
Coolant Passages
Coolant travels through the cylinder head from the engine block.
Cylinder head gaskets may be designed to restrict coolant flow rate. Often a source for
corrosion and leakage.
Blown Head Gasket
Cylinder Head Removal
All aluminum cylinder heads should be removed with a reverse torque procedure.
Cylinder Head Resurfacing
Heads should be checked in five places for warpage, distortion, bends or twists.
Check manufacturers specifications, maximum tolerances usually around .004”.
Valve Guides
The “bore” in the cylinder head that supports and controls lateral valve movement. Often integral on
cast iron heads Always an insert
on aluminum heads
Valve Guides
Steel insert on aluminum heads
Valve Stem To Guide Clearance
Always check manufacturers specs Intake valve will typically be .001
to .003” Exhaust valve will typically be .002 to
.004” The exhaust valve stem clearance
will generally be greater due to the higher operating temperatures.
Valve Guide Wear
Guides are checked in 3 locations With a small-hole
gauge then measured with a micrometer
Or checked with a small bore gauge
Valve Stem Wear
Measured with a micrometer at three separate locations.
Valve Stem To Guide Clearance Correction
Oversized Valve Stems – the guide is reamed to accept a larger stem. Must use a valve
with an oversized stem.
Reduced flow rate
Valve Stem To Guide Clearance Correction
Valve guide Knurling – a tool is driven into the guide that displaces metal thus reducing the inside diameter of the guide. (p. 340-341) The guide is then reamed to attain proper
clearance Not recommended for clearances +.006
Valve Stem To Guide Clearance Correction
Valve guide replacement – (insert) the old guide is driven out and a replacement guide is driven in.
The guide may require reaming to achieve proper stem to guide clearance.
Valve Guide Inserts – (integral) the old guide is drilled oversized and inserts are installed. Pressed fit May be steel or
bronze
Valve Stem To Guide Clearance Correction
Valve & Seat Service
Intake & Exhaust Valves
Automotive valves are of a poppet valve design.
Valve Materials
Stainless steel May be aluminized to
prevent corrosion Aluminum
Hardened valve tips and faces
Stellite (nickle, chromium and tungsten) valve tips and faces
Stellite is non-magnetic
Valve Materials Sodium-filled – a hollow
stem filled with a metallic sodium that turns to liquid when hot (heat dissipation).
Exhaust valves are largely comprised of a chromium material (anti-oxidant) with nickel, manganese and nitrogen added. May be heat-treated May be of a two-piece
design
Intake & Exhaust Valves
Valves are held into place by a retainer and keeper.
Aluminum heads will have a separate spring seat (iron heads will have integral seats)
Valve Seats
Integral seats – cast iron heads – induction-hardened to prevent wear
Valve seat inserts – typically aluminum heads – hardened seats are pressed into the heads
Valve Inspection
Valve tips should not be mushroomed
Most valve damage is due to excessive heat or is debris “forged”.
Replace any valve that appears Burnt Cracked Stressed Necked
Valve Springs
A spring “winds-up” as it is compressed – this causes the valve to rotate.
May have inside dampers to control vibration.
Springs are camshaft specific. Squareness (+ (-) .060) Spring free height (+ (-) .060) Compressed force (+ (-) 10%)
Valve open height Valve closed height
Valve Spring Tester
Valve Seat Reconditioning
The angle of the valve seat is reconditioned.
Often 3 stage (triple-angle) to promote flow and overhang. May be done with “seat
stones” May also be done with a
SERDI type set-up where the 3 angles are cut with one cutting tip.
Valve Reconditioning
The stem is lightly chamfered to insure proper fit in the valve grinder.
The face of the valve is reground using a valve grinder. (45 or 30 degrees typical).
Interference angle – the practice of grinding the face 1degree less than the seat angle.
The valve must retain its “margin” area.
the stem should be ground ½ the value that the face was ground with nonadjustable rockers.
Valve Lapping
The use of valve compound and a suction cup stick to establish a pattern
May be done to “freshen” the seat and face areas
Valve Lapping
The use of valve compound and a suction cup stick to establish a pattern
May be done to “freshen” the seat and face areas
Also used to check the contact pattern while cutting valve seats
All compound must be removed prior to
service
Valve Seals Valve Seals are
designed to allow sufficient lubrication of the valve stem/guide and also control oil consumption.
Umbrella seals – hold tightly onto the valve stem
Positive valve stem seals – hold tightly onto the guide
O-rings – controls oil between the spring and retainer
Checking Installed Height
If a valve seat and face are cut the valve will sit lower in the head.
The result is that the stem will sit higher on the top of the head.
This will cause the springs to have improper tension.
Installed height is measured and shims are added under the spring to compensate.
Camshafts
CamshaftThe camshaft rotates ½ times
the crankshaft – or – once per four-cycle stroke.
The camshaft may operate the: Valve train Mechanical fuel pump Oil pump Distributor
Camshaft Major function - operate the valve
train. The lobes on the cam open the
valves against the pressure of the valve springs.
Bearing journal can be internally or externally lubricated (oiled).
When installing externally oiled cam bearings it is essential that the holes in the bearings lineup with the oil passages in the block
Camshaft
Pushrod engines have the cam located in the block.
•Cam is supported by the block and the cam bearings.
•Cam may or may not be held in place by a thrust plate.•Most roller camshafts are held in by a thrust plate.
Camshaft
Overhead Camshafts
Overhead camshafts are either belt or chain driven and are located in the cylinder heads.
Overhead Camshafts Will use one of the following:
Cam followers Rocker arms
May have a one piece lifter – rocker design
A bucket design
Camshaft Operation
Bucket Design
Camshaft Followers
Rocker Arms
Design
A cam casting will include Lobes Bearing
journals Drive flange
(gear)
Design
A cam casting may include Oil pump drive
gear(s) Fuel pump
eccentric (mechanical fuel pump)
The fuel pump plunger rides on the camshaft eccentric.
Classification Camshafts are of one of four types:
Hydraulic flat-tappet Hydraulic roller Solid flat-tappet Solid roller
This designation is actually determined by the lifter design.
Hydraulic flat-tappet
The lifter is “spring” and oil loaded to allow for compensation.
Traditional O.E. style (1950’s – mid 90’s)
Used with flat or convex-faced lifters Generally cast iron or hardened steel Requires a “break-in” period to
establish a wear pattern
Cutaway view of a hydraulic lifter. TECH
TIP
Flat tappet Lifters
Hydraulic flat-tappet
Most cams are coated at the factory with manganese phosphate . This gives the cam a dull black appearance. This coating is to absorb and hold oil during the “break-in period”.
Hydraulic flat-tappet Most late model designs use a convex bottom (.002”)
to encourage lifter rotation. This rotation helps reduce lifter and (or) bore
wear. The Cam lobe will also be slightly tapered (.0007”
- .002”). This provides for a wider contact pattern.
Hydraulic flat-tappet
Camshaft “break-in” The lobes of the cam and the bottom of
the lifters must be coated with a molydisulfide lubricant often called “cam lube”. This insures that the cam is properly
lubricated during “break-in”.
Hydraulic flat-tappet
Camshaft “break-in” Typical procedure –
Maintain 1,500 RPM for 10 - 20 minutes Drain the engine oil a immediately
afterwards
Check the recommended procedure and lube for your
particular cam!
Hydraulic Roller The lifter is “spring”
and oil loaded to allow for compensation.
The contact between the cam and lifters are separated by a steel roller. This roller reduces
friction. Lifters cannot be
allowed to rotate within the lifter bore.
Hydraulic Roller A roller camshaft is generally
made of non-hardened steel. The lobes must be “finished” by
the manufacturer prior to assembly there is no “break-in period”.
Hydraulic Lifters (tappets) Hollow cylinders
fitted with a plunger, check valve, spring and push-rod seat.
Hydraulic Lifters (tappets)
Hydraulic Lifters
The oil passed through the check valve exits through the hole in the push rod seat.
The oil then passes through the pushrod to lubricate the rocker arms.
A cross-sectional view of a typical flat-bottomed hydraulic lifter.
•Engine oil pressure forces oil into the lifter through the oil inlet holes.•A check valve and ball hold most of the oil inside the lifter “hydro-locking” the plunger inside the cylinder.
Hydraulic Lifter Preload Also called valve lash. The distance between the pushrod
seat and snap-ring when the lifter is resting on its base circle.
Typical values range from .020 to .045”.
Check manufacturers specifications.
To correctly adjust the valve clearance (lash), position the camshaft on the base circle of the camshaft lobe for the valve being adjusted.
Remove all clearance by spinning the pushrod and tightening the nut unit all clearance is removed.
The adjusting nut is then tightened one complete revolution. This is what is meant by the term “zero lash plus 1 turn.”
End of show
Hydraulic Lifter Preload Adjusted by:
Adjustable rocker arms Often referenced by “turns past
zero lash” Non-adjustable rocker arms
Longer or shorter pushrods Shim or grind rocker stands
Hydraulic Lifter Preload Necessary if:
Cylinder head has been decked Cam has been changed Altered head gaskets Camshaft is worn An engine rebuild
Hydraulic Lifter Valve-floatNOT GOOD
The lifter fills with oil faster than it can purge it. This raises the lift of the camshaft.
Usually caused by excessive RPM.
May damage valves, pushrods, pistons etc.
Solid Flat-tappet and Roller No internal hydraulic absorption.
Allows for a more consistent valve lift, especially at high RPM.
Noisy when cold, more frequent and precise valve-lash adjustments required.
Solid Flat-tappet and Roller
Oil is diverted through the
pushrods via a
pushrod seat.
Solid Flat-tappet and Roller No lifter preload – valve lash only. Lash values may be given hot or
cold Typical values range from .002
- .005”.
Cam Specifications Lift Duration Valve overlap Lobe center (separation angle or
lobe spread)
Lobe Lift The amount the cam lobe lifts
the lifter Expressed in decimal inches
As lift increases the forces on the entire valve train
also increase.
The lobe lift is the amount the cam lobe lifts the lifter.
Because the rocker arm adds to this amount, the entire valve train has to be considered when selecting a camshaft that has the desired lift and duration.
A 1.5:1 ratio rocker arm means that dimension A is 1.5 times the length of B.
Therefore, if the pushrod is moved up 0.400 inch by the camshaft lobe, the valve will be pushed down (opened) 0.400 inch X 1.5, or 0.600 inch.
Lobe Lift Asymmetrical design – the amount
of lift between the intake and exhaust lobes is different.
Symmetrical design - the amount of lift between the intake and exhaust lobes is the same.
Duration The number of degrees of
crankshaft rotation for which the valve is lifted off of the seat. If the amount of degrees that the
intake and exhaust valve are open differ – it is of an asymmetrical design.
DurationUsually expressed as one of two
values
Duration (at zero lash) Duration at .050” lift – preferred
method Compensates for tappet styles and
clearances
Duration More duration = rougher idle and
better high RPM performance Less duration = smoother idle and
better low RPM performance
Valve Overlap The number of degrees of
crankshaft rotation that both valves are off of their seat (between the exhaust and intake strokes). Lower overlap = a smoother idle and
better low RPM operation Higher overlap = better high RPM
operation
Valve Overlap Having the exhaust
valve still open when the intake starts to open uses the exhaust "pull" out the exhaust port to help start the intake charge entering the chamber -- before the piston has started down and has generated it's own vacuum.
Lobe Separation Angle The difference, in degrees,
between the center of the intake lobe and the center of the exhaust valve. The smaller the angle the
greater the valve overlap The larger the angle the
less the overlap Link to LSA effects
Camshaft (Valve) Timing
It is crucial that the crankshaft, camshaft and balancing shaft (if equipped) are timed correctly.
This is often achieved by aligning “timing marks” on the gears
Pushrod- Type Engine
Modern DOHC motors may incorporate chains and belts on the same motor
Some of these designs are quite elaborate
Camshaft (Valve) TimingV-type DOHC Design
Some designs do not provide “alignment marks” and require special tools for proper timing
Camshaft (Valve) Timing
Camshaft Degreeing Advanced cam timing
The camshaft is slightly ahead of the crankshaft
More low speed torque less high RPM power
Retarded timing – The camshaft is slightly behind the
crankshaft More high RPM power Reduced low RPM torque
Adjustable Camshaft Gear