Car Mechanics (UK)

Torque ad­vice

- Keith Stoker Cape Verde · Belgium

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.

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