Bore Assessment on a Seized Four-Cylinder Block
A 1.6-litre four-cylinder petrol engine is stripped on the bench after the No. 3 piston seized following a coolant loss. The crankshaft has gone to the machine shop for grinding, so the block must be assessed now. Manual data: standard bore 79.00 mm; maximum bore increase before reboring is required 0.08 mm; maximum taper 0.10 mm; maximum out-of-round 0.05 mm; piston-to-bore clearance at the skirt, measured 90° to the gudgeon pin, 0.025–0.045 mm.
Bore readings taken with a 75–100 mm micrometer, axial (A) and thrust (T) at each level, in mm:
No. 1 - top A 79.02, T 79.01; middle 79.01/79.01; bottom 79.00/79.00. No. 2 - top 79.03/79.02; middle 79.02/79.01; bottom 79.01/79.00. No. 3 - top 79.14/79.04; middle 79.10/79.03; bottom 79.02/79.01. No. 4 - top 79.02/79.02; middle 79.01/79.01; bottom 79.00/79.00.
The customer wants the cheapest repair that will last.
1. Calculate the taper and the maximum out-of-round for each bore, and decide whether the block can be re-rung as-is, honed, or must be rebored.
Read the worked answer
Taper = top axial − bottom axial. Out-of-round = largest difference between A and T at the same level.
| Bore | Taper | Max out-of-round | Max diameter | Verdict |
|---|---|---|---|---|
| 1 | 0.02 | 0.01 | 79.02 | Pass |
| 2 | 0.02 | 0.01 | 79.03 | Pass |
| 3 | 0.12 | 0.10 (top) | 79.14 | Fails all three limits |
| 4 | 0.02 | 0.00 | 79.02 | Pass |
No. 3 breaks every limit: taper 0.12 > 0.10, out-of-round 0.10 > 0.05, and it is already 0.14 mm over standard against an allowable 0.08 mm.
Decision: No. 3 cannot be re-rung and cannot be salvaged by honing. A rigid hone follows the shape it is given, so it will not straighten a 0.12 mm taper, and the bore is already past the point where a standard-size ring can seal. The block must be rebored. In practice all four bores are taken to the same oversize so that the block stays uniform and one piston/ring size is ordered. Bores 1, 2 and 4 are still serviceable, but mixing one fresh bore with three worn ones leaves uneven compression and a repeat strip-down later.
2. The block is to be rebored 0.50 mm oversize. The new pistons each measure 79.462 mm. What finished bore should the machinist aim for? Show the calculation and state what must be verified after honing.
Read the worked answer
The finished bore is set by the piston, not by the nominal oversize figure, because the clearance must be built in.
- Choose the mid-point of the clearance range: (0.025 + 0.045) / 2 = 0.035 mm.
- Target bore = 79.462 + 0.035 = 79.497 mm, so hone to 79.50 mm (nominal 0.50 oversize).
- Actual clearance then obtained = 79.500 − 79.462 = 0.038 mm, inside 0.025–0.045.
Each piston should be measured and its own bore finished to suit; do not assume all four pistons are identical.
After honing, verify:
- Final clearance for each piston at the skirt, 90° to the pin.
- Ring end gap of every ring, checked squarely in its own finished bore.
- Roundness and taper within about 0.01 mm, and a correct 45° plateau crosshatch for oil retention.
- Block washed thoroughly - honing grit destroys new bearings.
- Main bearing bore alignment checked at the machine shop, since the engine seized and the block has been hot.
3. A trainee suggests honing only No. 3 and fitting a new piston and rings to save the customer money. Explain why you would refuse, and what would happen if the job were done that way.
Read the worked answer
Refuse it, and put the reason in writing on the job card.
A hone cannot remove a 0.12 mm taper - it follows the existing shape. A new ring set would be asked to seal against a bore that is 0.12 mm tighter at the bottom than at the top and 0.10 mm out of round at the top. As the ring travels down the bore its gap opens and closes, so it never beds in: the result is blow-by, crankcase pressure pushing oil past the seals, heavy oil consumption and a fresh crosshatch that glazes and scores within a few thousand kilometres.
The bottom of No. 3 is also the worn end, so a standard piston would run with excessive skirt clearance - piston slap when cold, and a skirt that can crack. Meanwhile the other three bores are still at their original size, so compression is uneven across the engine and the new cylinder runs hotter than the rest.
The realistic outcome is a second strip-down within months, a scored bore too far gone to rebore, and the workshop paying for a replacement block. The cheapest repair that lasts is a full rebore to one oversize with matched pistons, done once.