8mm DRIVESHAFT BOLTS

[Geoff Deakin]

Hi,

Does anybody know the correct torque for the 8mm Driveshaft to U/J bolts?

I'm replacing the old hex bolts with HT capscrews with Dowty washers.

Cheers,

Geoff.

 

[DARREN]

Hi Geoff,

According to the Unbrako technical data you are looking at 326 inch-lbs torque or 37 Nm in new money.

http://www.unbrako.com/docs/engguide.pdf its very useful for fastener selection.

D.

 

[Geoff Deakin]

Thanks Darren,

That's another job to cross off my list!

Cheers,

Geoff.

 

[Cliff Beer]

You're using 12.9s? Or grade 8?

Might want to put a small dab of locktite blue on the threads if there's no other locking mechanism being used.... I drilled the heads on my socket head CV bolts and wired them but that may be overkill for most situations.....

 

[Geoff Deakin]

Hi Cliff,

This also occured to me.

All the old bolts being removed are liberally covered in CV grease.

Should I strip down the joint, wash it out, and fit the new bolts dry with loctite, seeing as I'm also using Schnorr lock washers?

Any thoughts anybody?

Cheers,

Geoff.

 

[Pete T]

The bolts need to be dry or only have Loctite on them. Also, it is very important to have the mounting surfaces between the CV's and the flanges clean as well. You can loose a huge amount of clamping force with greasy flanges. Here is an excerpt from a 911 forum I found a while back:


100 mm CV joint with M8 x1.25 strength class 12.9; I recall the actual bolt circle was something like 85 mm in diameter or a 0.0425 m working radius for a bolt to resist the torque.

M8x1.25 12.9: .2% offset yield load is 40,000 newtons
ISO sets the "proof strength" as 88% of this. It is common tightening practice to only use 90% of the proof strength to provide some margin against twisting off the screw due to torsional and tensile stresses being combined.

The coefficient of static friction of lubricated steel on steel is about 0.16 and dry steel on steel is about 0.8.

Therefore per bolt and with greasy flanges: 40,000 N x .88 x .9 x .0425 m x .16 = 215 Nm; so six bolts would provide 1290 Nm or about 950 ft-lbs. If the flanges are clean and dry this increases to 6450 Nm or 4760 ft-lbs.

Every CV that I have seen that has come apart has had greasy surfaces.

 

[Cliff Beer]

Geoff,

Pete's definitely onto something there.

But if you're using schnorr lock washers then you should be OK I would think without any locktite.

Friendly suggestion would be to make sure you're hardware is of very good grade - 12.9 is best - particularly if you're putting some meaningful amount of tq through the CV's.

Cheers.

 

[Seymour Snerd]

Hi Geoff,

According to the Unbrako technical data you are looking at 326 inch-lbs torque or 37 Nm in new money.

http://www.unbrako.com/docs/engguide.pdf its very useful for fastener selection.

D.

Where exactly in that doc did you find that figure? (if that's where you found it)

 

[Seymour Snerd]

Friendly suggestion would be to make sure you're hardware is of very good grade - 12.9 is best - particularly if you're putting some meaningful amount of tq through the CV's.

The previous suggestion of 37 nM is about right for 10.9 fasteners dry.
To get any benefit from 12.9 fasteners you would need to raise the torque figure to about 41 nM.

 

[Cliff Beer]

The previous suggestion of 37 nM is about right for 10.9 fasteners dry.
To get any benefit from 12.9 fasteners you would need to raise the torque figure to about 41 nM.

Yup, got it, sounds about right Alan - 970 MPa v. 830 MPa or thereabouts.

So when one (like me, for example...), takes out that lowly Ford Racing 302 and puts in a 351 stroker (396??) then perhaps I'll be able to keep the CVs together...or, hopefully at least.

Right now, with my power plant I could probably get away with 4.6s.......

Cheers.