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PostPosted: Fri Nov 20, 2020 8:38 pm 
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Saab Nut

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Am I correct in assuming all torque figures given by Saab are dry torque. I am reusing many fasteners after cleaning the threads with a wire wheel in an angle grinder. I then grease or oil before assembly. What should I do about the torque?


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PostPosted: Fri Nov 20, 2020 8:45 pm 
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razani67 wrote:
Am I correct in assuming all torque figures given by Saab are dry torque. I am reusing many fasteners after cleaning the threads with a wire wheel in an angle grinder. I then grease or oil before assembly. What should I do about the torque?

All torque figures are dry unless otherwise stated. By all means reuse fasteners but clean them in a solvent before tightening them. It is almost impossible to say what torque to apply to a “wet” fastener if the manufacturer only specifies a dry torque.


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PostPosted: Fri Nov 20, 2020 9:17 pm 
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The Saab workshop system refers to torque values on "lightly oiled bolts".

The risk of over oiling bolts that go into blind holes can lead to excess oil in the bottom of the hole which gets trapped and pressurised. The hydraulic pressure can, in extreme cases, crack a casting.

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PostPosted: Sat Nov 21, 2020 11:06 pm 
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Tightening by torque is a very inaccurate method. Bolted joints work by clamping the components together and using the pre-load (or stretch) in the bolt to keep them in contact. Depending on the type of fastener, this preload can either be below or just above the elastic limit. For most of the fasteners on a car it is below the elastic limit, which means there is no permanent stretch in the bolts.
The 'stretch' is very small, but it requires a good deal of force to stretch the bolt. This force becomes the clamping force.
Typically, in a bolted connection, the torque you put in is consumed roughly, on a dry faster by 90% overcoming friction and 10% actually stretching the bolt.
The friction is both under the head of the bolt and in the threads themselves, the exact split can depend on the style of the bolt and the material it is made from
So if we oil the bolt, on the thread and under the head we might perhaps redce the 90% to, let's say 50%. (These figures are random, but probably representative)
Putting the same amount of torque in means that the effort needed to overcome friction has almost halved but the stretch on the bolt has gone from 10% to 50%, which will probably take it way past the elastic (and probably plastic) limit, and into failure.
Tightening by angle (e.g. 90º ) is much more accurate and independent of friction. For example an M12 x 1,5mm bolt tightening by (say) 120º (one third of a turn) will stretch the bolt by 0,5mm. If the length of the bolt is known then the preload can be calculated. It will always be the same preload.

If the bolt is bone dry the friction will be broadly the same, but there will be scatter and from above, you can see that a small amount of scatter can make a vast difference on preload.

Putting washers on further varies the preload a set amount of torque can deliver.

If the SAAB manual says lightly oiled, I'd go with that.


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PostPosted: Sun Nov 22, 2020 1:08 am 
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Robert wrote:
If the SAAB manual says lightly oiled, I'd go with that.

I don't think I've ever seen oiling fasteners mentioned in the 9000 workshop manual, other than for cylinder head bolts (I think).

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PostPosted: Sun Nov 22, 2020 6:39 pm 
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BillJ wrote:
Robert wrote:
If the SAAB manual says lightly oiled, I'd go with that.

I don't think I've ever seen oiling fasteners mentioned in the 9000 workshop manual, other than for cylinder head bolts (I think).


I'll re-phrase.

I'd go with whatever method the manual tells you.


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PostPosted: Sun Nov 22, 2020 7:08 pm 
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I accept what has been said about oiling or not on bolt threads but in reality does it make a lot of difference on the cars? I do not recall the manual saying anything about it. I do tend to use copperslip on bolts on the basis that I have fought to get them undone and might need to undo them again one day. They are torqued up as spec which naturally wipes most of the lubricant off the threads, maybe leaving molecules in the base of the valleys and keeping damp out. Is this so wrong?

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PostPosted: Sun Nov 22, 2020 8:14 pm 
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If the lubricant gets wiped off, what use is it a a lubricant?

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PostPosted: Sun Nov 22, 2020 8:17 pm 
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Think I read/watched a discussion on this and for wheel bolts the guidance was 80% of the stated torque figure if they were fitted with grease ...

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PostPosted: Sun Nov 22, 2020 9:19 pm 
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Robert wrote:
BillJ wrote:
Robert wrote:
If the SAAB manual says lightly oiled, I'd go with that.

I don't think I've ever seen oiling fasteners mentioned in the 9000 workshop manual, other than for cylinder head bolts (I think).

I'll re-phrase.
I'd go with whatever method the manual tells you.

I understood what you meant, Robert. I was questioning whether sgould's advice (based on the 9-3 and 9-5?) is transferrable to the 9000, which Razani is asking about.

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PostPosted: Sun Nov 22, 2020 10:09 pm 
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Robert wrote:
Tightening by torque is a very inaccurate method. Bolted joints work by clamping the components together and using the pre-load (or stretch) in the bolt to keep them in contact. Depending on the type of fastener, this preload can either be below or just above the elastic limit. For most of the fasteners on a car it is below the elastic limit, which means there is no permanent stretch in the bolts.
The 'stretch' is very small, but it requires a good deal of force to stretch the bolt. This force becomes the clamping force.
Typically, in a bolted connection, the torque you put in is consumed roughly, on a dry faster by 90% overcoming friction and 10% actually stretching the bolt.
The friction is both under the head of the bolt and in the threads themselves, the exact split can depend on the style of the bolt and the material it is made from
So if we oil the bolt, on the thread and under the head we might perhaps redce the 90% to, let's say 50%. (These figures are random, but probably representative)
Putting the same amount of torque in means that the effort needed to overcome friction has almost halved but the stretch on the bolt has gone from 10% to 50%, which will probably take it way past the elastic (and probably plastic) limit, and into failure.
Tightening by angle (e.g. 90º ) is much more accurate and independent of friction. For example an M12 x 1,5mm bolt tightening by (say) 120º (one third of a turn) will stretch the bolt by 0,5mm. If the length of the bolt is known then the preload can be calculated. It will always be the same preload.

If the bolt is bone dry the friction will be broadly the same, but there will be scatter and from above, you can see that a small amount of scatter can make a vast difference on preload.

Putting washers on further varies the preload a set amount of torque can deliver.

If the SAAB manual says lightly oiled, I'd go with that.

Great stuff but if the book of words says torque to 110 ft/lb then I would read that as “applying this torque will ensure an acceptable preload”. On occasions I have seen torque settings in nn ft/lb then back off and tighten to some percentage of nn plus some number of degrees. In practice, applying the manufacturer’s specified torque, either dry or lubricated as required, will be perfectly adequate. I suggest that RB211 engine mount bolts are rather more critical than anything on a car. (No, I can’t remember the torque setting or the tightening process and it may include a final torque plus a number of degrees, the last engine change I was involved in was more than 25 years ago.)


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PostPosted: Sun Nov 22, 2020 10:41 pm 
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I can't find the reference at the moment. Still looking! :park:

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PostPosted: Sun Nov 22, 2020 10:51 pm 
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:)

Found this...

Attachment:
IMG_3569.JPG
IMG_3569.JPG [ 53.63 KiB | Viewed 100 times ]

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PostPosted: Mon Nov 23, 2020 1:11 pm 
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Saab Nut

Joined: Tue Sep 14, 2010 9:11 am
Posts: 1711
Thanks for all the input. My view is that unless specifically stated, all torque figures refer to dry fasteners. I have only found one ref. to lubed joints and that gives figs for both dry and lubed. The lubed value is 30% less than dry. My conclusion is:
1.Fasteners should be assembled dry
2.If you oil the fastener reduce the torque by at least 30%
In the real world when a car rolls off the line most fasteners are electroplated and are assembled dry. We are dealing with 20yr old fasteners that have frequently lost their plating and corroded. So, we clean them up and oil them on assembly. If possible, assemble dry, if oiled just be aware, [url]Image[/url]


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