Engine overhaul, reconditioning and testing

Why it matters

Engines in trucks, buses, tractors and gensets are routinely overhauled rather than scrapped, and a well-reconditioned engine should give close to new-engine power, oil consumption and life. The workshop decides from measurements — bore taper and ovality, journal wear, bearing clearance — whether to rebore, regrind or simply re-ring, and it proves the result on a dynamometer. Measurement limits, oversize selection, compression-ratio change and dynamometer and Morse-test calculations are asked in university exams, GATE-style numericals and workshop interviews.

Key ideas

When an overhaul is due. Symptoms: low and uneven compression, high blow-by and crankcase pressure, high lubricating-oil consumption and blue smoke, low oil pressure from worn bearings, knocking, loss of power and poor fuel economy. Diagnosis before stripping: compression test (dry, then wet — if a little oil raises the reading, rings/bores are worn; if not, valves or gasket are leaking), cylinder leak-down test (percentage leakage and where the air escapes: intake, exhaust, crankcase or coolant), oil-consumption measurement and blow-by flow.

Dismantling, cleaning and crack detection. Parts are marked to keep their positions, decarbonised, cleaned (hot tank, ultrasonic) and inspected. Cracks in heads, blocks and crankshafts are found by dye-penetrant or magnetic-particle testing; the block and head water jackets are pressure-tested.

Measurements and limits (limits always come from the engine maker's service manual):

• Cylinder bore: measured with a bore (dial) gauge at top (just below the ring ridge), middle and bottom, in the thrust and axial directions. Taper = difference along the bore; ovality (out-of-round) = difference between perpendicular diameters at the same height. Wear is greatest at the top on the thrust side, where ring pressure, heat and poor lubrication combine.
• Crankshaft journals and crankpins: outside micrometer at two positions along and two directions across each journal for taper and ovality; run-out on V-blocks with a dial gauge; end float with a dial gauge or feeler.
• Bearing (oil) clearance: Plastigauge or bore-and-journal measurement.
• Piston-to-bore clearance, ring end gap (ring squared in the bore) and ring side clearance in the groove.
• Cylinder head and block deck flatness: straight-edge and feeler gauge in several directions.
• Valve train: stem diameter and guide clearance, valve-face run-out, spring free length and load, camshaft lobe lift and journal wear.

Reconditioning operations.

• Reboring and honing: the bore is machined to the next standard oversize (commonly +0.25, +0.50, +0.75, +1.00 mm — check the maker's list) that cleans up all wear, then honed to a cross-hatch finish with the correct piston clearance; matching oversize pistons and rings are fitted. When oversizes are exhausted, dry or wet liners (sleeves) are fitted.
• Crankshaft regrinding to the next undersize (commonly −0.25, −0.50, −0.75 mm), with fillet radii preserved, oil holes chamfered, journals polished, and the crank re-hardened (nitrided or induction) if the hard layer is ground through. Matching undersize bearing shells are fitted. Dynamic balancing of the crank, flywheel and clutch follows.
• Line boring of main-bearing housings if they are distorted.
• Cylinder head: surface grinding (skimming) to restore flatness, new valve guides, valve-seat cutting or grinding (typically 45° seats with narrowing cuts), valve refacing, lapping and a leak test of the valve seal. Skimming the head or block reduces clearance volume and raises the compression ratio, so maximum removal is limited.
• Connecting rods are checked for bend and twist and for big-end ovality; small-end bushes renewed.

Reassembly. Clean galleries, new gaskets and seals, correct ring-gap stagger, bolts torqued in sequence (often torque-plus-angle with new stretch bolts), timing set, oil system primed before first start.

Running-in and testing. A run-in schedule on the dynamometer (test bed) with progressively increasing load seats the rings. The engine is then tested for brake power and torque versus speed, fuel consumption (bsfc), oil pressure and temperature, blow-by, leaks and smoke/emissions. Frictional power can be found by the Morse test (multi-cylinder engines: cut one cylinder at a time at constant speed), Willan's line (diesel: extrapolate fuel rate versus BP to zero fuel), or motoring.

Formulas

r = (V_s + V_c) / V_c, V_s = (π/4)·D²·L

• r = compression ratio (–); V_s = swept volume per cylinder; V_c = clearance volume; D = bore; L = stroke (use cm and cm³ consistently).

Taper = d_max(along bore) − d_min(along bore), Ovality = d_thrust − d_axial (same height)

• In mm.

ΔV_c = (π/4)·D²·Δt

• Approximate reduction in clearance volume when Δt is removed from a flat head or deck over the bore area.

BP = 2π·N·T / 60

• BP = brake power (W); N = speed (rpm); T = brake torque (N·m).

bsfc = ṁ_f / BP, η_bth = BP / (ṁ_f · CV)

• ṁ_f = fuel mass flow (kg/h or kg/s); CV = calorific value (kJ/kg); bsfc in kg/kWh.

IP_k = BP − BP_(k cut), IP = Σ IP_k, FP = IP − BP, η_m = BP / IP

• Morse test at constant speed; IP = indicated power, FP = frictional power (kW).

Worked examples

Example 1 (standard) — bore wear, oversize and new compression ratio. Given: 4-cylinder engine, standard bore 80.00 mm, stroke 90 mm, clearance volume 50 cm³. Worst cylinder readings: top-thrust 80.18 mm, top-axial 80.10 mm, bottom-thrust 80.06 mm. Service limits: taper and ovality 0.05 mm. Oversizes: +0.25 and +0.50 mm; at least 0.04 mm must be left for honing clean-up.

1. Taper (thrust) = 80.18 − 80.06 = 0.12 mm; ovality (top) = 80.18 − 80.10 = 0.08 mm — both exceed 0.05 → rebore.
2. Minimum clean-up bore = 80.18 + 0.04 = 80.22 mm ≤ 80.25 mm → +0.25 mm oversize.
3. V_s (old) = (π/4) × 8.0² × 9.0 = 452.4 cm³ → r = 502.4/50 = 10.05
4. V_s (new) = (π/4) × 8.025² × 9.0 = 455.2 cm³ → r = 505.2/50 = 10.10

Answer: rebore to 80.25 mm; compression ratio rises slightly from 10.05 to 10.10.

Example 2 (GATE level) — Morse test. Given: 4-cylinder engine at constant speed; BP with all cylinders firing 40.0 kW; BP with cylinders 1–4 cut in turn: 28.5, 28.8, 28.3, 28.6 kW.

1. IP₁ = 40.0 − 28.5 = 11.5; IP₂ = 11.2; IP₃ = 11.7; IP₄ = 11.4 kW
2. IP = 11.5 + 11.2 + 11.7 + 11.4 = 45.8 kW
3. FP = 45.8 − 40.0 = 5.8 kW; η_m = 40.0 / 45.8 = 0.873

Answer: IP = 45.8 kW, FP = 5.8 kW, mechanical efficiency 87.3 %. Cylinder 2 is the weakest — check its injector or valves.

Example 3 — dynamometer acceptance test. Given: brake torque 150 N·m at 3,000 rpm; fuel 12 kg/h of CV 44,000 kJ/kg.

1. BP = 2π × 3,000 × 150 / 60 = 47,124 W = 47.1 kW
2. bsfc = 12 / 47.1 = 0.255 kg/kWh
3. η_bth = 47.1 / (12/3600 × 44,000) = 47.1 / 146.7 = 0.321

Answer: BP ≈ 47.1 kW, bsfc ≈ 0.255 kg/kWh, brake thermal efficiency ≈ 32 %.

Common mistakes

• Measuring bore wear only at the bottom or in one direction; maximum wear is at the top, thrust side.
• Choosing an oversize equal to the worn size with no allowance for honing — the bore will not clean up.
• Forgetting that head or block skimming raises the compression ratio (a 0.3 mm skim on an 80 mm bore removes about 1.5 cm³ — enough to raise r from about 10.0 to 10.3 here).
• Mixing m³ and cm³ in compression-ratio work; engine volumes are a few hundred cm³, never m³.
• In the Morse test, letting speed change when a cylinder is cut — speed must be restored by reducing the load.
• Using bearing shells of the wrong undersize after a regrind.

For GATE ME

This topic links to IC-engine questions: compression ratio from bore, stroke and clearance volume, brake power from torque and speed, bsfc and efficiencies, and the Morse test for frictional power. Practise unit consistency (cm³, kW, kg/h) and the logic of IP = Σ(BP − BP_cut).

Quick check

1. Bore readings 75.12 mm (thrust) and 75.05 mm (axial) at the same height. Ovality?
2. Swept volume 450 cm³, clearance 50 cm³. Compression ratio?
3. Torque 100 N·m at 3,000 rpm. Brake power?
4. A wet compression test raises a low reading markedly. What is worn?
5. Why must undersize bearings be fitted after crankshaft regrinding?

Answers: 1. 0.07 mm. 2. 10. 3. 31.4 kW. 4. Rings and/or bores. 5. The journals are smaller, so standard shells would give excessive oil clearance.