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newSchoolWorks.com › VoTechWorks › Transportation › Automotive Technology › Engine Systems: Theory, Diagnosis, and Overhaul › Lesson 6
TRN-AUTO19-02 · Lesson 6 · 22 clock hours · lab

Block, Crankshaft, and Piston/Rod Assembly Inspection

Objective. Students will measure cylinder bore dimensions, crankshaft journal diameters, and connecting-rod side clearance, use Plastigage to verify bearing clearance, and produce a complete block inspection data sheet with correct size-class recommendations.

⚠ 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.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Block and Crankshaft Inspection Matters

An engine block that leaves the shop with a worn or out-of-round bore, or a crankshaft with an undersize journal, will fail early—often taking expensive new bearings with it. Precise measurement and honest documentation are the two skills that separate a professional engine overhaul from an expensive comeback. Every dimension you record on the data sheet becomes a permanent record that justifies your machining recommendations to the customer and protects the shop.

Safety Before You Touch the Engine

Wear safety glasses at all times in the engine lab. Sharp casting edges on the block deck and crankshaft counterweights can cut deeply. Use nitrile gloves when handling the crankshaft—skin oils accelerate surface rust on precision ground journals. Never leave a crankshaft balanced on its nose or flange; lay it on V-blocks or purpose-made cradles. If you must move the block, use the designated engine stand and verify the stand bolts are fully torqued before rotating the block. Dispose of used Plastigage and shop rags in the labeled waste containers.

Cylinder Bore Measurement

A bore that is no longer perfectly round or perfectly cylindrical causes poor ring seal, increased oil consumption, and blow-by. You will measure each bore at three depths—near the top (just below the ridge), mid-stroke, and near the bottom—in two perpendicular axes: one parallel to the crankshaft centerline and one perpendicular to it. That gives you six readings per cylinder.

Taper is the difference between the largest diameter reading (usually near the top, where combustion heat and ring pressure are greatest) and the smallest reading (usually near the bottom). Out-of-round is the difference between the two perpendicular readings at the same depth. OEM specifications list maximum allowable values for both; exceeding either means the bore must be honed to the next oversize or sleeved.

Zero your dial bore gauge against a known standard or use a telescoping gauge set to the bore and read it with an outside micrometer. Either way, handle the gauge gently—a dropped bore gauge is a ruined bore gauge.

Crankshaft Journal Measurement

Each main journal and rod journal must be measured for diameter, taper, and out-of-round using an outside micrometer. Measure each journal at two positions 90 degrees apart (out-of-round) and at two positions along the journal's length (taper). Compare every reading to the OEM standard diameter and the OEM wear limit.

If any journal is below the minimum diameter, or if taper or out-of-round exceeds the limit, the crankshaft must be sent to a machine shop for regrinding to the next undersize. Document your regrind recommendation on the data sheet with the exact journal number and measured value. Never assume a journal is good by appearance—scoring too fine to feel by fingernail can still destroy a bearing.

Plastigage Bearing Clearance Check

Plastigage is a precision-calibrated strip of soft plastic used to measure the oil clearance between a journal and its bearing shell without removing the crankshaft from the block. The color-coded packages correspond to different clearance ranges; select the package whose range includes your expected specification.

Procedure:

1. Clean and dry the journal and bearing surface completely—oil will give a false reading.

2. Lay a strip of Plastigage across the full width of the journal, parallel to the crankshaft centerline.

3. Install the bearing cap and torque the bolts to OEM specification, following the correct sequence, in the correct number of passes.

4. Do not rotate the crankshaft—this will smear the Plastigage and destroy the reading.

5. Remove the cap carefully. The Plastigage will be crushed and widened.

6. Hold the printed scale on the Plastigage package next to the widest part of the crushed strip and read the clearance. A wider crushed strip means less clearance; a narrower strip means more clearance.

7. Compare the result to the OEM specification. Typical rod bearing clearance is in the range of 0.001–0.003 inch, but always confirm with the OEM service manual.

Remove all Plastigage residue before final assembly.

Connecting-Rod Side Clearance

After the rod caps are torqued onto the crankshaft, use a feeler gauge between the side of the rod and the crankshaft cheek to measure side clearance. Slide the feeler gauge blade into the gap and find the thickest blade that passes with light drag. Excessive side clearance causes a knocking noise and indicates rod or crank wear; insufficient clearance can restrict oil flow to the bearing.

Completing the Data Sheet

Every measurement gets recorded immediately—never from memory. For each bore and journal, write down all individual readings, calculate taper and out-of-round, state the OEM limit, and circle your disposition: within spec, hone to oversize, regrind, or replace. A complete, legible data sheet is the deliverable for this lab. Your instructor will verify three of your measurements with a calibrated reference instrument; all three must fall within ±0.001 inch of the reference to pass.

Key terms8

TaperThe difference in bore or journal diameter measured at two positions along the length of the cylinder or journal; indicates uneven wear along the axis.
Out-of-roundThe difference in diameter measured at the same depth or position but in two perpendicular directions; indicates that a bore or journal has worn into an oval shape.
Dial bore gaugeA precision measuring instrument with a calibrated dial indicator and interchangeable anvils used to measure the inside diameter of a cylinder bore.
PlastigageA brand of calibrated soft plastic strips used to measure oil clearance between a bearing journal and bearing shell by measuring how much the strip is crushed when the cap is torqued.
Oil clearanceThe designed gap between a rotating journal and its bearing shell that allows a film of pressurized oil to separate the two surfaces and prevent metal-to-metal contact.
Connecting-rod side clearanceThe measured gap between the side face of a connecting rod and the adjacent crankshaft cheek, which controls axial movement of the rod on the journal.
RegrindA machining process in which a worn crankshaft journal is precision ground to a smaller, standardized undersize diameter so that an undersize bearing shell can restore proper oil clearance.
Oversize boreA cylinder bore that has been honed to a diameter larger than the standard specification to restore roundness and correct surface finish, requiring a correspondingly larger oversize piston.

Worked examples4

1. A cylinder bore is measured at the top in two perpendicular axes and gives readings of 3.5028 in. and 3.5022 in. At the bottom, the readings are 3.5008 in. and 3.5005 in. Calculate taper and out-of-round at the top. The OEM limit for each is 0.0010 in. What is your disposition?
  1. Identify the largest reading in the bore: 3.5028 in. (top, axis 1).
  2. Identify the smallest reading in the bore: 3.5005 in. (bottom, axis 2).
  3. Calculate taper: compare the two top readings to the two bottom readings to find the worst depth-to-depth difference along the same axis. Axis 1: 3.5028 − 3.5008 = 0.0020 in. taper. Axis 2: 3.5022 − 3.5005 = 0.0017 in. taper. Worst taper = 0.0020 in.
  4. Calculate out-of-round at the top: 3.5028 − 3.5022 = 0.0006 in.
  5. Compare to OEM limits: taper limit = 0.0010 in.; measured taper = 0.0020 in. — EXCEEDS LIMIT. Out-of-round limit = 0.0010 in.; measured out-of-round = 0.0006 in. — within limit.
  6. Because taper exceeds the limit, the disposition is: hone to next oversize.
Answer: Taper = 0.0020 in. (exceeds 0.0010 in. limit); out-of-round at top = 0.0006 in. (within limit). Disposition: hone to oversize.
2. A rod journal is measured in two perpendicular positions at its center and reads 2.0985 in. and 2.0978 in. The OEM standard diameter is 2.1000 in. and the wear limit (minimum diameter) is 2.0990 in. Calculate out-of-round and determine whether the journal requires regrinding.
  1. Calculate out-of-round: 2.0985 − 2.0978 = 0.0007 in.
  2. Compare each reading to the OEM minimum diameter of 2.0990 in. Reading 1: 2.0985 in. is below 2.0990 in. Reading 2: 2.0978 in. is below 2.0990 in.
  3. Both readings are below the wear limit, so the journal is undersize regardless of out-of-round.
  4. Disposition: regrind to the next available undersize (e.g., 0.010 in. undersize) and use matching undersize bearing shells.
Answer: Out-of-round = 0.0007 in. Both diameter readings are below the 2.0990 in. wear limit. Disposition: regrind required.
3. After torquing a rod cap, a Plastigage strip is crushed to a width of 0.060 in. The Plastigage package scale shows that 0.060 in. width corresponds to a clearance of 0.0015 in. The OEM specification for rod bearing clearance is 0.0010–0.0025 in. Is this clearance acceptable?
  1. Read the measured clearance from the Plastigage scale: 0.0015 in.
  2. Compare to OEM specification: 0.0010–0.0025 in.
  3. 0.0015 in. falls within the specified range of 0.0010–0.0025 in.
  4. Disposition: clearance is within specification; bearings are acceptable for assembly.
Answer: Measured clearance = 0.0015 in., which is within the OEM specification of 0.0010–0.0025 in. Acceptable — no corrective action needed.
4. A connecting-rod side clearance is checked with feeler gauges. The 0.008 in. blade passes with light drag; the 0.010 in. blade will not enter the gap. The OEM specification is 0.005–0.015 in. What is the measured side clearance and is it within specification?
  1. The thickest blade that passes with light drag is 0.008 in.; therefore the side clearance is approximately 0.008 in.
  2. Compare to OEM specification: 0.005–0.015 in.
  3. 0.008 in. is within the 0.005–0.015 in. range.
Answer: Side clearance ≈ 0.008 in. — within the OEM specification of 0.005–0.015 in. Acceptable.
In class: Cylinder-bore measurement circuit: using a dial bore gauge, students measure each bore at top, middle, and bottom in two perpendicular axes, calculate taper and out-of-round, and determine whether bores are within standard or oversize specification. · Crankshaft journal measurement: students measure each main and rod journal for diameter, taper, and out-of-round with an outside micrometer; results are compared to OEM wear limits and a regrind decision is documented. · Plastigage bearing clearance check: students install a rod cap with a Plastigage strip, torque to spec using the OEM sequence, remove the cap, measure the crushed Plastigage width against the reference scale, and record the actual clearance compared to specification.
Materials: Engine block and crankshaft from disassembly, dial bore gauge, 2–3 in. outside micrometer set, Plastigage (correct clearance range), torque wrench, OEM torque specifications, block inspection data sheet, feeler gauges
Check: Instructor spot-checks three measurements from each student's data sheet using a calibrated reference instrument; all three must be within ±0.001 inch and dispositions must match specification—any discrepancy requires re-measurement and correction.

Practice check8

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

ASE A1 Engine Repair
1. A technician measures a cylinder bore and finds that the diameter near the top of the bore is 0.002 in. larger than the diameter near the bottom. This condition is called:
ASE A1 Engine Repair
2. When using a dial bore gauge, the technician should zero the gauge against:
ASE A1 Engine Repair
3. A Plastigage strip that is crushed wider than expected after torquing the rod cap indicates:
ASE A1 Engine Repair
4. Before applying Plastigage to a rod journal, the journal and bearing surface must be:
ASE A1 Engine Repair
5. A rod journal measures 0.0009 in. difference between two perpendicular diameter readings taken at the same position along the journal. This measurement describes:
ASE A1 Engine Repair
6. After measuring all main journals, a technician finds that two journals are below the OEM minimum diameter. The correct recommendation is to:
ASE A1 Engine Repair
7. Which measuring tool is used to check connecting-rod side clearance?
ASE A1 Engine Repair
8. A technician is performing a Plastigage check and torques the rod cap bolts, then rotates the crankshaft to verify it turns freely. What error has been made?
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