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