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newSchoolWorks.com › VoTechWorks › Transportation › Diesel & Heavy Truck › Diesel Engines: Theory, Diagnosis, and Overhaul › Lesson 2
TRN-DIES20-02 · Lesson 2 · 16 clock hours · demo

Engine Block, Crankshaft, and Bearing Systems

Objective. Students will identify all major block and lower-end components, state their function, and describe the consequences of bearing clearance out of specification.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a certified examiner or master technician in the trade or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Engine Block, Crankshaft, and Bearing Systems

The Engine Block

The engine block is the structural foundation of a diesel engine. It is typically cast from gray iron or compacted graphite iron (CGI), materials chosen for their strength, vibration damping, and machinability. The block houses the cylinder bores, coolant passages, oil galleries, and the main bearing saddles. Cylinder bores are precision-machined to accept piston assemblies, and their surfaces are cross-hatch honed to retain oil and seat piston rings properly. Main bearing saddles are machined as a matched set with their caps and must never be swapped between engines or even between positions within the same engine.

Block integrity is critical. Cracks, most common between cylinders or around head bolt bosses, can allow coolant into the oil or combustion gases into the cooling system. Cracks are detected by pressure testing, dye-penetrant inspection, or magnetic particle inspection. A cracked block typically means replacement, though some repairs are possible depending on location and severity.

The Crankshaft

The crankshaft converts the reciprocating motion of the pistons into rotational power delivered to the drivetrain. Heavy-duty diesel crankshafts are forged from steel alloy for maximum fatigue resistance. Key features include:

Journal surfaces are induction-hardened and ground to precise diameter specifications. Wear beyond OEM limits requires grinding undersize and fitting undersize bearing shells, or crankshaft replacement.

Bearing Systems

Plain (sleeve) bearings support both main and rod journals. They consist of a steel backing bonded to a softer bearing material — typically aluminum-tin, copper-lead, or tri-metal overlays — that sacrifices itself to protect the expensive crankshaft. Bearings are precision-engineered to create a specific oil clearance (the gap between the journal OD and the bearing ID) through which pressurized oil flows to form a hydrodynamic wedge. This oil film prevents metal-to-metal contact under normal operating conditions.

Oil clearance is measured with Plastigage or with micrometers and bore gauges. OEM specifications vary by engine family, but clearance must fall within a tight range. Too little clearance restricts oil flow and causes overheating; too much clearance drops oil pressure and allows the bearing to pound and fail.

Bearing shells are positioned by a locating tang and must never be filed, shimmed, or interchanged. They are available in standard and undersize dimensions.

Bearing Wear Patterns and Failure Modes

Reading a worn bearing tells you what went wrong. The most common patterns are:

Common Failure Scenarios

Spun bearing: The bearing seizes to the rotating journal and spins in its bore, destroying the saddle. Root causes include oil starvation (low level, failed pump, clogged pickup), extremely low oil pressure, or an improperly installed shell that loses its crush fit.

Journal scoring: Deep scratches or grooves machined into the journal surface by abrasive particles or direct metal contact. Dirt ingestion from a failed air filter or improper assembly (un-cleaned parts) is a frequent cause. A scored journal requires grinding or replacement.

Block crack: Cracks develop from thermal fatigue (overheating, rapid cool-down), freeze damage (insufficient coolant protection), or chronic detonation. Overheating is the most common single root cause in service.

Safety During Lower-End Work

Key terms8

Main bearing saddleThe machined bore in the engine block (and its mating cap) that holds the main bearing shells and supports the crankshaft main journals.
Oil clearanceThe precise gap between a bearing shell's inner diameter and the journal's outer diameter, through which pressurized oil flows to form a hydrodynamic film preventing metal contact.
Hydrodynamic lubricationA condition where a pressurized oil wedge completely separates two moving surfaces so that no metal-to-metal contact occurs.
Fatigue spallingA bearing failure mode characterized by cratering and flaking of the bearing overlay material due to repeated cyclic overloading.
WipingA bearing failure where soft overlay material is smeared or transferred because the oil film collapsed and surfaces made contact.
PlastigageA plastic thread gauge placed between a bearing shell and its journal; when the cap is torqued and removed, the crushed width of the material indicates oil clearance.
Spun bearingA catastrophic failure in which the bearing shell rotates with the journal rather than remaining stationary in its bore, destroying both the saddle and journal.
Bearing crushThe slight extra height of a bearing shell above the parting line of its bore; when the cap is torqued, this crush presses the shell firmly against the bore to prevent rotation and promote heat transfer.

Worked examples3

1. An OEM service manual specifies main bearing oil clearance at 0.002–0.004 in. You measure the main journal OD with a micrometer and get 3.4978 in. You measure the bearing bore ID (with shell installed, cap torqued) at 3.5014 in. Is clearance in spec?
  1. Calculate oil clearance: bore ID minus journal OD = 3.5014 in − 3.4978 in = 0.0036 in.
  2. Compare to specification: 0.0036 in falls between 0.002 in and 0.004 in.
Answer: Clearance is 0.0036 in, which is within the 0.002–0.004 in specification. The bearing is acceptable for use.
2. After measuring, you find the same journal is 3.4958 in OD. The standard bearing shell gives a bore ID of 3.5014 in. What is the clearance, and what does this indicate?
  1. Calculate oil clearance: 3.5014 in − 3.4958 in = 0.0056 in.
  2. Compare to spec (0.002–0.004 in): 0.0056 in exceeds the maximum of 0.004 in.
  3. The journal is worn undersize. An undersize bearing shell (e.g., 0.010 in undersize) must be fitted after the journal is ground to the corresponding undersize dimension, or the crankshaft must be replaced.
Answer: Clearance is 0.0056 in — 0.0016 in beyond the maximum limit. Standard bearings cannot correct this; the journal must be ground undersize and matched with the appropriate undersize bearing set.
3. Using Plastigage, you install a strip between a rod journal and rod bearing, torque the cap, then remove it. The Plastigage is crushed to a width that the envelope scale reads as 0.001 in. The OEM spec is 0.001–0.003 in. What action do you take?
  1. The measured clearance (0.001 in) equals the minimum of the specification (0.001 in).
  2. This is technically at the lower limit. The bearing will restrict oil flow slightly but is within spec for a new or freshly ground journal.
  3. Check the journal for taper and out-of-round with a micrometer before accepting it — a reading at the minimum limit can indicate a slightly out-of-round journal where the Plastigage caught the largest diameter.
Answer: Clearance is at the low end of spec. The bearing is acceptable if the journal passes micrometer checks for taper and out-of-round. Document the reading and monitor during break-in.
In class: Instructor demonstration: disassemble lower end of a bench engine, naming each component and its material; students complete a labeled-parts worksheet as the demo proceeds. · Students handle main bearing shells and crankshaft journals, identifying wear patterns (fatigue spalling, wiping, cavitation erosion) using a reference wear-pattern chart. · Guided discussion: instructor presents three real-world failure scenarios (spun bearing, journal scoring, block crack) and students trace each failure back to likely root cause using the OEM service manual.
Materials: Bench diesel engine (teardown unit), main and rod bearing sets, crankshaft, OEM service manual, bearing wear-pattern reference chart, parts washer, nitrile gloves, safety glasses
Check: Parts-identification checklist: students must correctly identify and state the function of 15 lower-end components; pass = 13 of 15 correct.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

ASE T-Series: Engine Systems
1. What is the PRIMARY purpose of oil clearance between a crankshaft journal and its bearing shell?
ASE T-Series: Engine Systems
2. A technician finds a main bearing shell with a smeared, polished surface where the overlay material has been displaced. Which wear pattern is this, and what is the most likely root cause?
ASE T-Series: Engine Systems
3. During a lower-end teardown, a student notices one main bearing saddle bore is severely damaged and discolored, while the crankshaft journal at that location is also deeply scored. The most likely cause is:
ASE T-Series: Engine Systems
4. Which tool is used at the workbench to directly measure oil clearance between a rod bearing shell and crankpin journal after cap installation?
Safety/OSHA General Industry
5. Before beginning disassembly of a diesel engine mounted on an electrically powered test stand, the FIRST safety action a technician must take is:
ASE T-Series: Engine Systems
6. An OEM manual states rod bearing oil clearance should be 0.001–0.003 in. A technician measures 0.005 in. What is the correct course of action?
ASE T-Series: Engine Systems
7. Main bearing caps are marked and must be reinstalled in their original position and direction. Why?
ASE T-Series: Engine Systems
8. A diesel engine was overheated severely and then shut down quickly. The technician suspects a block crack. Which inspection method is MOST appropriate for detecting surface cracks in a cast iron block?
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