CH12 upper end theory and service STUDENT VERSION rev

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Chapter 12

Upper End Theory and Service

Cylinder Heads

• Made of cast iron or aluminum

• Contains valves, valve seats, valve guides, springs, rocker arm supports, and the upper portion of the combustion chamber

• Contains coolant and oil return passages

• Head design is one of the most influential factors in overall engine performance

Cylinder Heads (cont’d)

• Intake and exhaust ports are cast into the head

• Most heads are aluminum to reduce weight

• Heads can have crossflow or Siamese port

Wedge Combustion Chamber

Combustion Chamber

Pentroof Combustion Chamber

Intake and Exhaust Valves

• Commonly called poppet valves

• Allow air to flow into and exhaust out of

the combustion chamber

• Seal the upper portion of combustion

chamber

• Intake valves are larger than exhaust valves

Valve Terminology

Valve Seats

• May be cast into or inserted into head

• Seals the valve and transfers heat

Valve Guides

• Support the valve and allow only up and down motion

• Close clearance to valve stem

• Keeps valve aligned with valve seat

• Can be cast into or insert into the head

Valve Guides (cont’d)

Valve Spring Retainers and Oil Seals

Rocker Arms and Pushrods

Cylinder Head Inspection

• Replace a head showing severe damage to valve sealing areas

• Check for dents, scratches, cracks, and

corrosion damage

• Inspect for warpage

• Check intake and exhaust manifold

mounting surface flatness

Inspect the Stem for Wear

Valves Seats

• Inspect for damage, burning, cracks, and deterioration

• Check for loose seats

• Inspect for sunken valve seats

• Seats should be replaced if the valve was broken or bent

Retainers and Keepers

• Worn retainers will

allow the spring to

move away from the

valve centerline

• Uneven wear on the

stem tip indicators

rotators not working

properly

Valve Face

• Valve face angles and dimensions must be kept within specifications

Valve Stem

• Grinding the face sets the valve lower into the port – extending the stem height

Valve Stem (cont’d)

• If stem height is excessive, the valve stem tip may be ground

• Chamfer valve tip after grinding

• Valve and/or seat may need to be replaced

to correct incorrect stem height

Valve Guide Clearance

• Measure the guide diameter and stem diameter

Valve Guide Problems

• Inadequate Lubrication

– Is caused by oil starvation in the upper valve train.

• Valve Geometry Problems

– Lead to increased guide clearance, poor valve seating, and premature wear.

• Wrong Stem to Guide Clearance

– Can cause increased oil consumption.

Valve Guide Reconditioning

Measuring Seat Width

Valve Stem Seals

• Control the amount of oil between the stem and guide

Assembling the Cylinder Head

• Installed stem height and installed spring height must be checked

• Stem height affected by valve and seat

• Spring height may be adjusted using shims