Metrology
Engine metrology guide
The measurements and factory tolerances a pro checks on a removed crankshaft.
Confirming internal wear isn't done by ear: it's measured, to the hundredth of a millimetre, with calibrated instruments and the manufacturer's exact figures. Here are the three reference metrology protocols applied to a bottom end: crankshaft runout with a dial indicator, bearing running clearance with Plastigage, and big-end axial play with a feeler gauge. For each, you'll find the instrument, what is measured, the typical tolerance, what exceeding it means, and what to ask the mechanic. The ranges given are common orders of magnitude: the figure that counts is always the one in the model's workshop manual.
Dial indicator protocol (runout / TIR)
- Instrument
- Dial indicator on a magnetic stand + two V-blocks
- What is measured
- Total Indicated Runout (TIR) of the crankshaft: its straightness, checked at the bearing journals
What the professional does
- 1.The crankshaft rests on two V-blocks at the end main journals, clean and dry.
- 2.The indicator is brought perpendicular onto a centre journal, zeroed.
- 3.The crankshaft is turned slowly one full revolution; the needle sweeps a total deviation (the TIR).
- 4.The measurement is repeated on each journal; the largest deviation is kept.
A TIR beyond tolerance reveals a bent crankshaft: on a 2T built-up crank it can sometimes be trued in a press; on a monolithic forged 4T it is replaced. A bent crank prematurely wears bearings and seals and generates vibration.
Ask for the measured runout value and the manufacturer's limit, and whether the main-bearing line (bearings, journals) was checked at the same time.
The TIR read on the indicator is roughly double the actual eccentricity. Exact value and service limit per the workshop manual.
Plastigage protocol (bearing clearance)
- Instrument
- Plastigage (crushable calibrated thread) + torque wrench
- What is measured
- Hydrodynamic running clearance of the big-end bearing (radial clearance between the plain shell and the crankpin)
What the professional does
- 1.Crankpin and shell perfectly clean and dry (no trace of oil, which would falsify the reading).
- 2.A strand of Plastigage is laid on the crankpin, along the axis.
- 3.The rod cap is refitted and torqued to the exact figure, without turning the rod.
- 4.The cap is removed again: the width of the crushed strand, compared to the packet scale, gives the clearance.
A clearance above tolerance signals a worn shell or a marked crankpin: it's the direct cause of rod knock and a drop in oil pressure. The shells are replaced (at the appropriate size) and, if the crankpin is scored or oval, the crank is reground or replaced.
Ask for the clearance measured with Plastigage, the state of the crankpins, and whether the shells were fitted to the correct selection size.
Order of magnitude for a plain big-end shell; the exact value and wear limit are in the model's manual.
Feeler gauge protocol (big-end side-play)
- Instrument
- Feeler gauge set
- What is measured
- Axial (side) play of the big-end on the crankpin: the rod's travel along the crankshaft axis
What the professional does
- 1.The rod is pushed to one side on its crankpin.
- 2.A feeler gauge is slid between the side of the big-end and the crank web, on the opposite side.
- 3.You look for the thickest blade that passes without forcing: that's the side-play.
- 4.The measurement is compared to the new value and the service limit.
Excessive side-play indicates wear of the rod flanks or the crank; near-zero play signals too tight an assembly or a distortion. Both degrade lubrication and can generate noise and heat. Correction in the workshop (rod and/or crankshaft).
Ask for the side-play recorded and whether it stays within the service limit, on each rod of a multi-cylinder engine.
Common range for big-end side-play; service limit per the manual.