C H A P T E R 18 Camshaft and Valve Train Service.

C H A P T E R 18

Camshaft and

Valve Train Service

Permission granted to reproduce for educational use only.© Goodheart-Willcox Co., Inc.

• Explain how to inspect and service the camshaft.• Summarize service procedures for in-block and

overhead valve assemblies.• Describe the steps in inspecting and reconditioning

valve seats.• Explain how to inspect and recondition valve lifters

and valve guides.

Learning Objectives


Introduction to Valve Train Service

• Valve train performs several vital functions– Allows air-fuel mixture into cylinder– Seals combustion chamber during compression– Allows exhaust gases to exit cylinder

• Damaged parts must be reconditioned or replaced– Also, parts in contact with damaged parts


Camshaft Service

• Inspect camshaft for wear or damage– Bearing surfaces, lobes, timing gear– Timing gears need to be replaced in pairs

• Measure journals and lobes for wear– Measure bearing bores in crankcase and cover

• Check compression release mechanism for proper operation


Measuring a Camshaft Bearing Journal


Measuring a Camshaft Bearing Bore


Measuring a Camshaft Lobe


Valve Service

• 4-cycle engines have poppet valves• Exhaust valves subjected to high temps

– Made from high-quality, heat-resistant steel– Proper operating clearance must be maintained


Inspecting Valves and Seats

• Clean valves with power wire brush• Inspect valves

– Damaged faces, heads, or stems– Warped valve head– Worn, improperly ground, or bent stems– Thin margins or partial seating

• Replace or refinish damaged valves


Inspecting Valve Springs

• Overheating and wear damage springs– Lose elasticity– Become warped or distorted

• Check squareness and length with square• Replace if improper length or warped


Valve Guides

• Align valves with valve seats• Clearance must be correct

– Allows for heat expansion and lubrication– Valve tipping causes improper seating– Proper clearance is typically .002 to .003

• Replaceable valve guide inserts• Integral valve guides


Types of Valve Guides


Inspecting Valve Guides

• Must be cleaned before inspection– Cylindrical wire brush

• Determining clearance– Measure valve guide with small hole gauge– Measure valve stem with micrometer– Guide diameter minus valve stem diameter

• Some guides measured with plug gauge


Correcting Worn Integral Valve Guides

• Ream guide and install replacement valve– Valves with oversize stems must be available

• Ream valve and install thin-walled bushing– Bushing must be reamed with finish reamer– Wash all chips away with penetrating oil


Valve Guide Bushing Replacement

• Refer to manufacturer’s service manual• Depth of guide is measured• Iron guides driven out through the bottom• Brass guides pulled out through top• Replacement guide pressed into bore

– Same depth as old bushing


Pulling a Soft Metal Valve Guide

(Briggs & Stratton Corp.)


Refacing Valves

• Can be ground with abrasive stone• Can be cut on special lathe• Can be cut with manual cutter• Cut to 30 or 45


Valve Cutting Lathe

(Neway Mfg. Co.)


Manual Valve Refacer

(Neway Mfg. Co.)


Valve Lifter Service

• Inspect valve lifter contact surface– Concavity, scoring, deformities

• Insert lifter in guide and check for play– Should move freely in guide– Lifter should not wiggle in its guide– Side play can be measured with a dial indicator


Measuring Valve Lifter Side Play with a Dial Indicator


Valve Seat Service

• Should always be reconditioned or replaced• Inspect seat for looseness or damage• Some seats are replaceable• Some seats are an integral part of block

– Must be bored out and seat insert installed– May be more practical to replace block


Peening a Valve Seat Insert

• Peening secures loose seat• Place insert squarely in its bore• Seat insert using centerpunch and hammer

– Three points equidistant around rim

• Use drift punch to indent block material outside insert’s rim– Work around rim in a star pattern


Peening a Valve Seat Insert



Valve Seat Angle and Width

• Valve seats generally cut to 45 or 30– Some seats cut with 1 interference angle

• Valve seat width must be correct– Wide enough to prevent cutting into valve face– Enough contact surface for heat dissipation– Not so wide as to cause carbon buildup– Typically .030 to .060


Refacing (Cutting) Valve Seats

• Special cutting tool with carbide blades• Seat condition determines amount of cutting• Cover seat area in ink before cutting

– Makes progress easier to see


Refacing (Cutting) Valve Seats

• Place pilot in valve guide• Slide cutter over pilot• Install T-handle and use it to turn cutter

– Moderate downward pressure– Clockwise direction only

• Remove cutter and check progress• After cutting seat, wash away chips


Valve Seat Cutters

(Neway Mfg. Co.)


Replacing Valve Seat Inserts

• Damaged seat inserts pulled– Use special puller, slide hammer, or pry bar

• Replacement insert is installed– Chamfered edge down– Peened in place

• Some integral seats replaced with insert• Reface newly installed seat inserts


Driving In a Valve Seat Insert



Lapping Valves

• Lapping compound is grease and abrasives– Course and fine grades

• Procedure:– Apply lapping compound to valve face– Attach lapping stick to valve head– Place valve in guide and twirl– Finished when gray band appears on valve face

• Clean valve and block after lapping


Lapping Valves


Valve Lifter-to-Stem Clearance

• Space between lifter and end of valve stem– Exhaust valve typically has greater clearance

• Too little clearance– May keep valve open, burning valve and seat– Advances valve timing

• Too much clearance– Reduces valve lift– Rapid lifter wear


Proper and Improper Valve Clearance


Adjusting Valve Lifter-to-Stem Clearance

• Valve refacing reduces clearance• Measuring clearance

– Insert valve in guide– Rotate camshaft so lobe is opposite stem– Hold valve against seat– Slide feeler gauge between lifter and stem

• Adjust lifter or grind end of valve stem


Measuring Valve Clearance


Servicing Overhead Valves, Seats, and Guides

• Serviced in same way as on L-head engines• Valves cleaned and resurfaced• Seats reconditioned• Measure and recondition guides as needed


Review

What parts are measured to check a camshaft for wear?

The camshaft’s bearing journals, the camshaft bearing bores in the crankcase and cover, and the camshaft lobes.


Review

What are the two most common angles for valve faces?

45 and 30


Review

How are valve seat inserts secured in their bores?

Peening


Review

What problems are associated with insufficient valve lifter-to-stem clearance?

Advanced valve timing, improper valve seating, burned valves and seats

C H A P T E R 18 Camshaft and Valve Train Service.

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