3

The torque specification for tightening a crank bolt in some car is

Tighten bolt 95 Nm + 45° + 15°

How does tightening 45 degrees and then another 15 degrees differ from tightening 60 degrees? When dealing with multiple bolts I can see how you could get a more even clamping force by first tightening all of them 45 degrees, followed by another 15 degrees. But when dealing with a single bolt, what's the point?

Improve this question
5
  • Which car are we talking about? – Pᴀᴜʟsᴛᴇʀ2♦ Jul 16 '17 at 13:21
  • Car in question is an Opel Astra H 2008 Z14XEP. The torque spec I'm quoting is from the (official?) workshop manual. – Ives Jul 16 '17 at 13:22
  • Thanks. This gives a point of reference so we are all on the same sheet if music. – Pᴀᴜʟsᴛᴇʀ2♦ Jul 16 '17 at 13:23
  • Is the official one or not? Ie is the information correct or is there a misprint? – Solar Mike Jul 16 '17 at 14:35
  • 1
    It's the service manual on workshop-manuals.com. It looks like the official workshop manual but I'm sure that's not exactly freely available. The same scheme (1 torque spec followed by two subsequent degree specifications) is repeated for other bolts, such as flywheel to crankshaft, tie rod to steering knuckle, etc. It's not a misprint. – Ives Jul 16 '17 at 14:40
3

I would suggest that the difference is down to how the torque is applied to the pulley (I assume that is the part with the crank bolt you mention). Some pulleys have a taper fit and using a three-stage torquing regime allows things to settle between stages.

If the full torque is applied in one hit then the taper may "lock" and give the appearance / feel of being tight while it is still loose leading to a catastrophic failure.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.