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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 Hemispherical 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 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 • A - is freestanding height • B - is closed valve spring height • C - is open valve spring height 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 ... 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... 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... 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