Torque, Clamping and a Stripped Thread

The torque figure of 25 N·m is a bolt figure intended to produce a given clamp load in a suitable material. In an aluminium tapped hole the thread is much weaker than a grade 8.8 steel bolt, so the same torque can shear the aluminium threads before the bolt reaches its intended preload. Contributing factors:

• Thread engagement may be too short (rule of thumb is roughly 1.5–2 × diameter in aluminium).
• Reused or damaged threads, or a hole drilled and tapped slightly oversize.
• Lubrication/friction variation - a lubricated thread converts more torque into tension, raising the load on the aluminium.
• Torque wrench accuracy - the wrench is 6 months past its 12-month calibration, so 25 N·m indicated may not be 25 N·m applied.

The partner's suggestion is unsafe because a longer spanner increases leverage and gives no control of applied torque. "Feel" is not a measurement; on aluminium it will strip more threads and on steel it risks over-stretching or snapping bolts. Fasteners must be tightened with a calibrated tool to a specified torque.

Steps:

1. Stop assembly. Do not continue tightening the remaining bolts with an uncalibrated wrench or by feel.
2. Quarantine the torque wrench and tag it as out of calibration; arrange recalibration or borrow a calibrated wrench covering 25 N·m (a 5–50 N·m wrench gives better resolution than one whose range starts at 20 N·m).
3. Assess the stripped hole. Options are: drill out and fit a thread insert (helicoil or similar) to restore an M8 thread; drill and tap to the next size up with a matching bolt; or, if the housing allows, weld and re-drill - usually not practical on aluminium. A thread insert is the standard repair and should be recorded.
4. Re-check the first four bolts once a calibrated wrench is available, since they were torqued with a suspect tool.
5. Investigate the root cause - check thread depth, hole size, whether a lubricant was used, and whether the drawing torque is appropriate for aluminium. Raise it with the teacher before assembling the rest.

Record the non-conformance so the same fault is not repeated on the next housing.

A safer specification would be:

• Reduce torque for M8 into aluminium to roughly 15–18 N·m, or follow the bolt/insert manufacturer's data for the specific aluminium grade and engagement length - the exact figure should come from a table, not guessed.
• Specify thread engagement of at least 1.5 × diameter (about 12 mm) in aluminium, or fit thread inserts so the steel bolt bears on a steel thread.
• Specify the tightening method - calibrated torque wrench, correct range, within calibration date.
• Specify lubrication (dry or a defined lubricant) because friction strongly affects the torque–tension relationship.
• Use a tightening sequence (e.g. diagonal/star pattern in stages) for the housing so clamp load is even.

Justification: torque is only a proxy for clamp load, and the proxy fails when the materials differ in strength. Lowering torque protects the aluminium thread; inserts remove the weak link entirely; controlling friction and sequence makes the clamp load repeatable. The revision should be documented on the drawing so the next student does not repeat the failure.