Car Mechanics (UK)
Armed with a driveshaft CV boot repair kit plus a new driveshaft to wheel hub bolt from my local VW garage, I removed the passenger side driveshaft from my 2008 VW Golf Plus with a DSG gearbox. The torn inner CV joint boot was removed, the joint cleaned up, inspected and the new boot put on with new grease applied to the joint.
The new driveshaft to wheel hub bolt was without ribbing; the torque value for this bolt is 200Nm + 180°. The old driveshaft to wheel hub bolt that I took out was with ribbing; the torque value for this bolt is 70Nm + 90°.
The huge difference between torque values for the same size of bolt had me confused and worried about changing from a bolt with ribbing to a bolt without ribbing with such an increase in torque value. I took the easy decision and replaced the old, original bolt with ribbing and used the torque value of 70Nm + 90°. The new driveshaft to wheel hub bolt without ribbing was stamped with a steel strength of 8.8 and, in my opinion, 200Nm + 180° would take the bolt to its limit or possibly over.
Please can you advise why such a torque difference exists and if I would have done any damage if I had used the new bolt without ribbing and applied torque of 200Nm + 180°?
The difference between the two torque settings for what appears to be the same bolt is, as you have seen, very dramatic. VW makes a note that the only visible difference is that on one bolt the contact face is ribbed and on the other the face is flat.
When calculating the optimum torque values for any bolt there are a few different factors to consider. Along with the tensile strength of the bolt, the thread pitch and friction from the bolt under-face and thread must be considered. Different materials will cause different amounts of friction on the thread face, along with a different yield rate. From the engineer’s viewpoint, the higher the preload the better as the bolt will resist loosening due to vibration or fatigue.
The setting of 200Nm + 180° is the setting for the later bolt with a tensile strength of 8.8 and this should have a shear strength of 800MPA, bringing it well within the limits of the high torque value. The old bolt should not be used, but I can find no reason for VW’S dramatic change in specifications.