8 cylinder front engine iconic vehicle
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By worf
#48865
N_Jay wrote: Fri Jan 08, 2021 4:04 pm If it were me, I would do the 20nm in two steps (Maybe 10nm and then 20 nm) to make sure everything seats evenly.
This is a fine idea. For super important big things, like heads, I always progressively "creep up" on the torque value.

N_Jay wrote: Fri Jan 08, 2021 4:04 pm The fact you are tightening by angle and not by torque eliminates some of the variability due to thread lubrication.

Wet (oiled) threads put more force (stretch) on a bolt or stud than dry threads at the same torque.
This is correct but the explanation doesn't sit well with me in the context of head bolts/studs.

Torque - as far as nuts and bolts are concerned - is a direct measure of friction of the thread surfaces sliding over each other. It is only an indirect measurement of clamping force.

Friction, as we know from our first physics book, is the normal (clamping) force times the coefficient of friction. The last can vary significantly based upon condition of threads. New, shiny, electroplated threads will have a low coefficient, whereas old crufty, much-used threads will have a higher-than-new coefficient.

The actual clamping force of the fastener has to do with elongation of the fastener. The number of turns of the bolt or nut is what directly determines clamping force. So, an old crusty bolt turned to X N-m will have a lower clamping force than a new bolt turned to X N-m because the friction is higher. A lubricated bolt will have a higher clamping force because the coefficient is lower thereby allowing more turns before X N-m is reached. In applications where the only purpose of the fastener is to keep to pieces together the specific clamping force is not as important as the friction force of the threads that thereby keep vibration from loosening the fastener.

However, when we get to heads, actual clamping force within a small range is critical. That clamping force can be calculated from the material properties, thread pitch, and the number of turns a fastener is given. So, the directive to Torque to 20 N-m and then do 90° turns serves to first, ensure thread engagement and to then elongate the fastener by a calculated amount so as to provide a specific clamping force.

The lubrication on "torque-to-angle" serves the purpose of minimizing the bad effects of the thread surfaces sliding against one another: reduces chance of galling or otherwise damaging threads, makes the fastener easier to turn smoothly, etc.

Edit, Addition: So, in a torque-to-angle application the final torque is almost irrelevant.
Last edited by worf on Fri Jan 08, 2021 8:31 pm, edited 1 time in total.
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By worf
#48867
Scott at Team Harco wrote: Fri Jan 08, 2021 8:19 pm Please note: the S2 spec is only three stages. There is only one at 90 degrees. Much different from three stages of 90 degrees.
Oops. Sorry. Misread what you wrote.
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By worf
#48869
Scott at Team Harco wrote: Fri Jan 08, 2021 2:26 pm The torque spec is quite clear. Step 1: 20 Nm, Step 2: 60 degrees, Step 3: 90 degrees.
Ok. I really didn't read this well. So, unlike every 928 head that gets/got two or three 90° turns (two with bolts, three with factory-original studs) the 944 S2 head gets a 60° and then a 90°.

That's fascinating. Assuming the same materials for studs and block, the 944 head has about 1/2 of the clamping force as the 928.

Did the Turbo or 968 go to bolts? Or did they use studs to the bitter end?
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By Scott at Team Harco
#48885
worf wrote: Fri Jan 08, 2021 8:36 pm
Scott at Team Harco wrote: Fri Jan 08, 2021 2:26 pm The torque spec is quite clear. Step 1: 20 Nm, Step 2: 60 degrees, Step 3: 90 degrees.
Ok. I really didn't read this well. So, unlike every 928 head that gets/got two or three 90° turns (two with bolts, three with factory-original studs) the 944 S2 head gets a 60° and then a 90°.

That's fascinating. Assuming the same materials for studs and block, the 944 head has about 1/2 of the clamping force as the 928.

Did the Turbo or 968 go to bolts? Or did they use studs to the bitter end?
Actually, the early 16v heads had torque specs at 1: 20 Nm, 2: 90 degrees, 3:90 degrees. The addition of 20mm more material in the area of the head studs changed that spec to the one I quote for my engine.

They all use studs. Includes 944, 951 and 968. For what it's worth the 968 engine is almost identical to the 3.0 S2 engine.
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By N_Jay
#48892
Sometimes old threads torque easier than new because the surfaces of the threads are work polished and hardened.
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By worf
#48930
N_Jay wrote: Fri Jan 08, 2021 9:21 pm Sometimes old threads torque easier than new because the surfaces of the threads are work polished and hardened.
And then you end up with more clamping force and more fastener elongation at the same torque!
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By N_Jay
#48944
worf wrote: Fri Jan 08, 2021 10:45 pm
N_Jay wrote: Fri Jan 08, 2021 9:21 pm Sometimes old threads torque easier than new because the surfaces of the threads are work polished and hardened.
And then you end up with more clamping force and more fastener elongation at the same torque!

Hence the use of degrees of rotation.
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By worf
#48962
N_Jay wrote: Fri Jan 08, 2021 11:28 pm
worf wrote: Fri Jan 08, 2021 10:45 pm And then you end up with more clamping force and more fastener elongation at the same torque!
Hence the use of degrees of rotation.
The new Porsches have a lot of 5, 6, and 8mm aluminum bolts that are torque + angle.

And a lot of snapped bolts...
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By N_Jay
#49059
worf wrote: Sat Jan 09, 2021 1:09 am
N_Jay wrote: Fri Jan 08, 2021 11:28 pm
worf wrote: Fri Jan 08, 2021 10:45 pm And then you end up with more clamping force and more fastener elongation at the same torque!
Hence the use of degrees of rotation.
The new Porsches have a lot of 5, 6, and 8mm aluminum bolts that are torque + angle.

And a lot of snapped bolts...

Are people reusing stretch bolts?
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By worf
#49127
N_Jay wrote: Sat Jan 09, 2021 12:40 pm Are people reusing stretch bolts?
LoL. They are breaking before they get to be ‘reused.’
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By N_Jay
#49175
worf wrote: Sat Jan 09, 2021 5:07 pm
N_Jay wrote: Sat Jan 09, 2021 12:40 pm Are people reusing stretch bolts?
LoL. They are breaking before they get to be ‘reused.’
Then someone didn't design in enough margin.
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By worf
#49217
N_Jay wrote: Sat Jan 09, 2021 7:04 pm Then someone didn't design in enough margin.
Or a supplier is substituting materials.
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By N_Jay
#49219
worf wrote: Sat Jan 09, 2021 8:35 pm
N_Jay wrote: Sat Jan 09, 2021 7:04 pm Then someone didn't design in enough margin.
Or a supplier is substituting materials.
Been there.

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