Borrowed Torque Wrench on an Aluminium Cylinder Head
A four-cylinder 1.2-litre petrol engine is being reassembled after a head gasket replacement in a small workshop. The workshop manual states: clean and dry the bolt threads and blind holes; use new head bolts; tighten in three stages to 40 Nm in the specified sequence, then turn a further 90 degrees. A borrowed click-type torque wrench is the only one available. Its calibration sticker expired 16 months ago, and it was dropped on the floor the previous week. The wrench scale is 20–200 Nm. One technician has already dipped the head bolts in engine oil 'for smooth tightening' and left about 5 ml of oil in one blind bolt hole. The angle gauge is missing, but an apprentice suggests 'counting one flat and a bit' on the hex head. The cylinder head is aluminium and the block is cast iron. The supervisor is asked to approve the next step.
1. Should the supervisor allow the borrowed torque wrench to be used for the 40 Nm stages? Explain the judgement.
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The wrench should not be trusted for final tightening without verification. Two facts matter: it was dropped, and its calibration is 16 months old. A click-type wrench can be damaged internally by a drop, and the error may be large enough to affect clamp load. At 40 Nm on a 20–200 Nm scale, the setting is only 20% of full scale, near the lower end where some click wrenches are least reliable.
The supervisor should either use a known-good calibrated wrench or check the borrowed wrench against a calibrated reference such as a beam-type tester or load cell. If neither is available, the correct decision is to stop and arrange a calibrated wrench, not to guess. The 90-degree stage does not need a torque value; it needs an angle gauge. A torque wrench is not a substitute for the angle stage, and the final angle should not be rechecked with a torque wrench.
2. What is wrong with oiling the bolts and leaving oil in a blind hole? What preparation is required?
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Oiling the bolts changes the friction between threads and between the bolt head and seating face. Torque is only an indirect way of producing clamp load; if friction is lower than the manual assumes, the same 40 Nm can produce too much clamp load. More seriously, oil trapped in a blind bolt hole is almost incompressible. When the bolt is driven in, the oil can hydraulic-lock the bolt, give a false torque reading, or crack the casting as pressure builds.
Preparation should follow the manual: clean the threads and blind holes, remove oil and debris, and dry them. Compressed air can be used with eye protection, but the hole should be wiped and checked. Threads should be chased if damaged. New head bolts should be used if the manual specifies torque-to-yield or single-use bolts; reused bolts may have stretched. Bolts should not be lubricated unless the manual explicitly states it. The supervisor should reject the oily hole before any bolt is run down.
3. Describe the correct tightening sequence and angle method, including how to avoid the 'one flat and a bit' error.
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The sequence should start at the centre of the head and work outward in a spiral or the manual's numbered pattern, so the gasket is compressed evenly. The three 40 Nm stages should be applied in the same sequence, increasing gradually rather than fully tightening one bolt at a time. Each stage should be completed across all bolts before the next stage begins.
For the 90-degree stage, the supervisor should use a torque-angle gauge or a marked socket and protractor. Paint a reference mark on the bolt head and on the head casting, then turn exactly 90 degrees. 'One flat and a bit' is unreliable: on a hex head one flat is 60 degrees, so one flat plus 30 degrees would be 90, but the 'bit' is an estimate and can vary with viewing angle. The angle stage should not be followed by re-torquing to 40 Nm, because that would alter the designed clamp load. The aluminium head and cast-iron block also mean the manual's cold-torque procedure should be followed exactly, and the final assembly should be recorded with the wrench identification and angle method used.