ASE A9 Engine Block Diagnosis and Repair — Questions and Answers
Question 1: A technician is installing wet cylinder liners in a heavy-duty diesel block. Each liner must be measured for protrusion above the block deck before the cylinder head is installed. What is the main purpose of the specified liner protrusion?
- It sets piston-to-head clearance at top dead center
- It compensates for liner wear so the bores can be rebored later
- It lets the head gasket clamp the liner flange so the liner seals against coolant and combustion leakage (Correct answer)
- It keeps the liner from rotating in the block
Correct answer: It lets the head gasket clamp the liner flange so the liner seals against coolant and combustion leakage
The liner flange must stand slightly above the deck so head clamping load seals the liner and gasket. Too little protrusion lets combustion gases and coolant leak, and too much can crush the gasket or distort the liner.
Wet liners are sealed at the top by the head gasket and at the bottom by O-rings. The manufacturer specifies a small protrusion, usually measured in thousandths of an inch, so head bolt clamping load acts on the liner flange. Insufficient protrusion causes gasket blow-by and coolant or combustion leaks. Excessive or uneven protrusion can crack the flange or deform the liner. Protrusion is measured at several points around each liner with a dial indicator and a bridge or straightedge. Shims or a different flange thickness correct out-of-spec readings.
Question 2: A diesel engine with wet cylinder liners has coolant contaminating the engine oil. The head gasket and cylinder head passed inspection, and a pressure test shows coolant leaking into the crankcase. Which is the most likely cause?
- Failed lower liner O-rings or seals (Correct answer)
- A worn turbocharger oil seal
- Excessive piston ring end gap
- A weak oil pressure relief valve spring
Correct answer: Failed lower liner O-rings or seals
On a wet-liner engine, the lower O-rings seal coolant from the crankcase. When they fail, coolant runs past the liner into the oil. The other choices do not connect coolant passages to the crankcase.
A wet liner is in direct contact with coolant around most of its length. The top is sealed by the head gasket and the liner flange, and the bottom by one or more elastomer O-rings in the block. If those O-rings harden, nick during installation, or are attacked by incorrect coolant, coolant can leak into the crankcase. Typical signs are milky oil, rising coolant loss with no external leak, and coolant seen at the block weep holes. The repair is to remove the liners and install new seals. Bores and liner seating surfaces should be checked for pitting first.
Question 3: Technicians find pitting and erosion on the outside of removed wet cylinder liners, mostly on the thrust side. What is the best way to prevent this problem in service?
- Run higher oil pressure to cool the liners
- Increase the thermostat opening temperature
- Use a harder piston ring set
- Keep supplemental coolant additive (SCA) at the specified concentration (Correct answer)
Correct answer: Keep supplemental coolant additive (SCA) at the specified concentration
The pitting is cavitation erosion, caused by liner vibration that forms and collapses vapor bubbles in the coolant. Proper SCA levels form a protective film on the liner surface and prevent it.
Piston side thrust makes thin wet liners vibrate. This creates low-pressure zones in the coolant where vapor bubbles form and then collapse against the liner wall. The collapse erodes the metal and can eventually perforate the liner. Nitrite-based SCA protects the surface and reduces the damage. Technicians should test coolant with test strips and add SCA as the engine manufacturer specifies. Too little or too much SCA both cause problems, so concentration must be checked.
Question 4: During block inspection, a technician finds that the main bearing bores are out of alignment after the caps are torqued. Which repair restores correct bore alignment?
- Install thicker main bearing shells in the misaligned bores
- Align-bore or align-hone the main bearing bores (Correct answer)
- Polish the crankshaft journals with crocus cloth
- Install oversize main bearing cap bolts
Correct answer: Align-bore or align-hone the main bearing bores
Main bore misalignment is corrected by machining the bores in a single line, either align boring or align honing. Thicker shells or polished journals do not fix the geometry of the block.
The main bores must lie on one straight centerline so the crankshaft turns freely with even bearing loads. Misalignment can come from heat damage, a spun bearing, or block distortion. It is found by a bind when turning the crank, uneven clearance readings, or a straightedge or gauge bar check. Align boring or honing removes a small amount of material from the caps and bores to restore the line. This usually means the main bearing caps get machined. The crank centerline then sits slightly closer to the deck, which the machinist must account for.
Question 5: A technician checks main bearing oil clearance with Plastigage. After the cap is torqued and removed, the flattened strip is noticeably wider at one end than at the other. What does this most likely indicate?
- The bearing cap was installed backward
- The crankshaft was rotated during the measurement
- The journal or bore is tapered (Correct answer)
- The oil clearance is within specification
Correct answer: The journal or bore is tapered
A strip that varies in width across the bearing shows uneven clearance, which points to a tapered journal or bore. Check the journal with a micrometer to confirm.
Plastigage is a crushable thread whose flattened width corresponds to bearing clearance. The width should be uniform across the bearing. If it is wider at one end, clearance is greater there, which indicates taper in the journal, a distorted bore, or a bent crankshaft. The crankshaft must not be turned while the strip is in place, because that smears the strip and ruins the reading. A micrometer measurement of the journal at several points confirms the taper. The crankshaft then needs to be reground, or the bore corrected if it is the cause.
Question 6: A service procedure for a diesel engine specifies torque-to-yield (torque-angle) cylinder head bolts. What should the technician do with them?
- Replace them, or reuse them only if the service information allows and they pass the length check (Correct answer)
- Reuse them and torque to the final value without lubricant
- Reuse them any number of times if the threads look clean
- Retorque them to the original value after the first heat cycle
Correct answer: Replace them, or reuse them only if the service information allows and they pass the length check
Torque-to-yield bolts are stretched into their plastic range during tightening, so they lose clamp consistency. Follow the manufacturer's reuse rules and length limits, otherwise replace them.
A torque-to-yield bolt is tightened to a snug torque and then turned a specified angle. This stretches the bolt permanently to give uniform clamp load. A stretched bolt is weaker and may not hold the same load again. Many manufacturers require new bolts every time. Others allow reuse only if the bolt length stays under a maximum. Reusing stretched bolts can cause head gasket failure or cracked bolts. Bolt threads and block holes should be cleaned, and the specified lubricant and tightening sequence should be followed.
Question 7: A technician measures a cylinder bore with a dial bore gauge. At the top of ring travel, thrust-direction = 4.7250 in. and axial-direction = 4.7254 in. At the bottom, thrust-direction = 4.7236 in. and axial-direction = 4.7240 in. What is the cylinder taper?
- 0.0004 in.
- 0.0014 in. (Correct answer)
- 0.0018 in.
- 0.0010 in.
Correct answer: 0.0014 in.
Taper is the difference between the top and bottom diameters measured in the same direction: 4.7250 − 4.7236 = 0.0014 in. The axial readings give the same value (4.7254 − 4.7240).
Taper is the change in bore diameter from the top to the bottom of the cylinder, with wear usually greatest near the top where ring pressure and heat are highest. It is found by subtracting the bottom reading from the top reading in the same direction. Out-of-round is the difference between thrust and axial readings at the same height. Here the out-of-round is only 0.0004 in. at either level. The 0.0014 in. taper should be compared with the manufacturer's limit to decide whether to hone, bore, or replace the liner.
Question 8: After installing new piston rings, a technician hones a cylinder. What is the purpose of the cross-hatch finish left on the bore wall?
- To reduce the bore diameter for tighter ring end gap
- To remove the need for break-in oil
- To increase compression by raising friction
- To hold oil on the wall so the rings can seat and lubricate properly (Correct answer)
Correct answer: To hold oil on the wall so the rings can seat and lubricate properly
The fine cross-hatch grooves hold a film of oil and give the rings a surface to seat against. A mirror-smooth bore does not retain oil, and rings may not seat.
Honing removes glaze and leaves intersecting scratches that retain oil. The oil film lubricates the rings and piston skirt, and the surface lets the rings wear in so they seal. A bore that is too smooth lets rings skate with poor sealing and high oil consumption. A bore that is too rough wears the rings quickly. The correct finish grade depends on the ring material, so follow the ring manufacturer's recommendation. After honing, the bore should be washed thoroughly with hot soapy water to remove abrasive particles.
Question 9: Which inspection method is best for finding a suspected crack in a cast iron diesel engine block?
- A visual check with a straightedge
- Dye penetrant only, because it works only on iron
- Magnetic particle inspection (Correct answer)
- A micrometer check of the main bores
Correct answer: Magnetic particle inspection
Cast iron is ferromagnetic, so magnetic particle inspection reveals cracks as particles gather at the flux leakage. Dye penetrant works on nonferrous parts too but magnetic particle is the standard method for iron.
Magnetic particle inspection magnetizes the block and applies iron particles. A crack disrupts the magnetic field and gathers the particles, making the crack visible. It works only on ferrous metals such as cast iron and steel. Aluminum parts need dye penetrant or pressure testing. Blocks can also be pressure tested with water passages sealed, which shows leaks that are not visible. Cracks often form around main bearing webs, lifter bores, and head bolt holes, and should be checked there.
Question 10: A dial indicator is used to measure crankshaft end play. The reading is above specification. Which part is most likely worn?
- The thrust bearing surfaces (Correct answer)
- The rod bearing inserts
- The camshaft journals
- The flywheel ring gear
Correct answer: The thrust bearing surfaces
Crankshaft end play is controlled by the thrust bearing, so excessive end play means the thrust faces are worn. Rod bearings and cam journals do not limit axial movement of the crankshaft.
One main bearing has thrust flanges that limit the crankshaft's forward and rearward movement. To measure end play, a dial indicator is mounted on the block with its tip on the crank, and the crank is pried back and forth. Readings above spec show worn thrust faces or worn crank thrust surfaces. The repair is a new thrust bearing set, and sometimes crankshaft regrinding. Excessive end play can lead to clutch problems, seal wear, and damage to the crankshaft.
ASE A9 Engine Block Diagnosis and Repair
A technician is installing wet cylinder liners in a heavy-duty diesel block.
Each liner must be measured for protrusion above the block deck before the cylinder head is installed.
What is the main purpose of the specified liner protrusion?
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