The Micrometer That Reads High: Measurement Errors on a Crankshaft Job
You are rebuilding a 4-cylinder gasoline engine. The service manual specifies crankshaft main journal diameter 51.980–52.000 mm, wear limit 51.950 mm, maximum taper 0.020 mm and maximum out-of-round 0.015 mm. On the bench you have a 0–25 mm outside micrometer, a 0–150 mm vernier caliper with 0.02 mm graduations, and a dial bore gauge with a 50–75 mm setting ring. Before use you check the tools: the micrometer reads 0.02 mm when the anvils are closed on a clean gauge block, and the vernier caliper reads 0.10 mm with the jaws closed. Your micrometer readings are: journal No. 1 - 51.972 mm at the front, 51.966 mm at the rear; journal No. 2 - 51.968 mm; journal No. 3 - 51.971 mm; journal No. 4 - 51.966 mm. The shop's only torque wrench was dropped on the floor last week, and its calibration sticker expired 14 months ago.
1. Correct the measurements and decide what to do with the crankshaft. Show the arithmetic.
Read the worked answer
Correct the readings first. A micrometer that reads 0.02 mm with the anvils closed is reading high, so true size = reading − 0.020 mm:
| Journal | Reading | Corrected |
|---|---|---|
| 1 front | 51.972 | 51.952 |
| 1 rear | 51.966 | 51.946 |
| 2 | 51.968 | 51.948 |
| 3 | 51.971 | 51.951 |
| 4 | 51.966 | 51.946 |
Taper, journal 1: 51.952 − 51.946 = 0.006 mm, within the 0.020 mm limit.
Against the wear limit of 51.950 mm: journals 2 and 4 are already below it. Journals 1 and 3 sit only 0.002 mm and 0.001 mm above it - that is smaller than the realistic uncertainty of the tool and the operator, so treat them as at the limit, not as 'good'.
Decision: do not reinstall with standard bearings. Grind all four journals 0.25 mm undersize (target 51.750 mm) and fit matching 0.25 mm undersize bearings, then verify oil clearance with Plastigauge or a bore gauge plus micrometer, checking the manual's clearance range.
Also required: out-of-round must be measured at two planes 90° apart on each journal, which the single readings above do not provide - measure before authorising the grind.
2. Which instrument do you use for each measurement, and what do you do about the two faulty tools?
Read the worked answer
Instrument selection:
- Crankshaft journals: outside micrometer. It resolves 0.01 mm or better and has the right range; this is its job.
- Cylinder bores: dial bore gauge, zeroed on the 50–75 mm setting ring, measuring taper and out-of-round at several heights and two planes.
- Crank end play / thrust: dial indicator on a magnetic base, not a caliper.
- Vernier caliper: only for coarse work - hose outside diameters, bolt lengths, filter dimensions. It is not a bearing-journal instrument.
The micrometer: the 0.02 mm zero error can be corrected by adjustment on most micrometers - zero it on the gauge block, re-check, and record that it was zeroed. Re-check at mid-range against a gauge block, since a zero error at closed anvils can coexist with a spindle error further up the range.
The vernier caliper: a 0.10 mm error is five graduations - far too large to compensate by mental arithmetic, and it signals a damaged jaw or bent beam. Tag it out of service, do not use it, and send it for repair and calibration. Do not 'allow for it'. A caliper that reads 0.10 mm out is also telling you someone has dropped it - check the other tools in the same drawer.
3. How do you handle the torque wrench situation for the main bearing caps and connecting rod bolts?
Read the worked answer
Do not use it on these fasteners. It has two independent problems: a calibration sticker 14 months out of date, and a drop, which can bend the beam, shift the pivot, or damage the ratchet and leave the wrench reading anywhere from 10% low to completely wrong. Main bearing cap and rod bolt torque directly controls bearing crush, oil clearance and joint integrity; a wrong value means a spun bearing or a thrown rod, not a rework.
What to do:
- Tag the wrench 'out of calibration - do not use' and remove it from the bench.
- Use the shop's calibrated backup and record its tool ID and calibration date on the job sheet.
- If no calibrated wrench exists, borrow or rent one, or stop the assembly. Torque is not 'close enough'.
- Send the dropped wrench for calibration; if the drop also damaged it, condemn it - calibration of a bent beam is a waste of money.
- Verify the wrench at the start of the job against a torque tester at the values you will use (e.g., cap torque and rod bolt torque), because a wrench can be accurate at mid-scale and wrong at the low end.
Torque-to-yield rod bolts are a separate trap: they are tightened to a torque plus a specified angle (using an angle gauge) and are single-use. If the manual specifies TTY, the bolts get replaced regardless of how good the wrench is, and the thread and under-head surfaces must be clean and lightly oiled exactly as specified.