Cylinder Head and Valve Train Flashcards

When checking a diesel cylinder head for warpage, which method is correct?

Answer: Lay a straightedge on the machined surface and use a feeler gauge to measure gaps lengthwise, crosswise and diagonally

Flatness is verified by placing a precision straightedge on the deck and sliding feeler gauges under it. Readings are taken along the length, across the width and diagonally.

Why must a diesel cylinder head's thickness be measured before it is resurfaced?

Answer: To make sure removing material will not drop it below the minimum thickness specification

Each resurfacing removes metal from the deck. Going below the minimum specified thickness can alter combustion chamber geometry, injector protrusion and valve-to-piston clearance, and can weaken the head.

A diesel engine has coolant in the oil but no external leaks, and the head gasket has been ruled out. What is the best way to test the cylinder head for cracks?

Answer: Pressure test the head's coolant passages after plugging all ports, and check for leaks

Plugging the water ports and applying air or water pressure to the coolant passages shows leaks through cracks into the oil, intake, exhaust or cylinder areas.

After a valve job on a diesel engine, which measurement most directly shows whether the valve was sunk too deeply into the head?

Answer: Valve stem height or valve recession measured from the deck or the spring seat

Valve stem height, or recession relative to the deck, shows how far the valve has moved into the head as the seat and face are ground. Excess recession can change spring installed height and compression.

Which valve spring measurement shows that a spring is bent or distorted rather than weak?

Answer: Squareness, checked against a square on a flat surface

Standing the spring on a flat surface next to a square and measuring the gap at the top shows if it is out of square. Free length and pressure show weakness or shortening, not leaning.

A diesel engine has tight valve lash on the exhaust valves, below specification. What is the most likely result if it is not corrected?

Answer: The valve will not close fully and can burn from lack of heat transfer to the seat

If lash is too tight, the valve can be held slightly off its seat as parts expand with heat. This lets hot gas leak past and prevents the valve from cooling through the seat, so it burns.

While inspecting a pushrod, a technician rolls it on a flat surface and sees it wobble. What does this indicate?

Answer: The pushrod is bent and must be replaced

A straight pushrod rolls smoothly on a flat surface. Wobble or a visible gap means it is bent, and a bent pushrod must be replaced.

A technician measures a valve stem with a micrometer and the valve guide bore with a small-hole gauge. Which condition does excessive stem-to-guide clearance most commonly cause?

Answer: Oil consumption and blue smoke as oil is drawn past the valve stem

Worn guides let oil pass between the stem and guide, particularly on the intake side, burning in the cylinder and producing blue smoke. It also causes the valve to seat unevenly.

What is the correct way to tighten diesel cylinder head bolts that are torque-to-yield?

Answer: Use new or approved bolts and follow the specified sequence and torque-plus-angle procedure

Torque-to-yield bolts stretch permanently when tightened, so they are generally replaced and the manufacturer's sequence, torque steps and angle procedure must be followed.

A technician installs a new injector sleeve and measures injector tip protrusion. Why is this measurement important?

Answer: It ensures the nozzle tip is positioned correctly in the combustion chamber for spray pattern and proper cooling

Protrusion affects where fuel is sprayed in the combustion chamber and how well the tip is cooled. Incorrect protrusion can cause poor combustion, smoke or a damaged injector or piston.