torque and rotation angles

[todwic]

Alrighty then there. The factory manual sez for early model head nut torques use 3 steps:

1. 20 NM
2. 90&deg torque angle
3. 90&deg torque angle

And for the later models it goes something like

1. 20 NM
2. 60&deg rotation angle
3. 60&deg rotation angle

Lucy, 'splain plez.

[ae1969]

http://www.clarks-garage.com/shop-manual/cyl-03.htm


loookie .... loookie

[todwic]

I want to know what the difference is between torque angle and rotation angle is, what the inst. hope to accomplish, or the desired result that differs with just a NM number vs. angles.

[todwic]

bump!

[justin]

I think they mean the same thing.

[todwic]

Then why would the manual differentiate between the two? German engineers just don't go throwing terms around willy-nilly when they talk mechanical stuff.

[ae1969]

its the same....

The nice thing is that on a hex nut......... every corner is 60 degrees.

I usually go over a couple of degrees to be safe.

[profssr2017]

I'm not sure why they used torque angle versus rotation angle, but it appears that they are using the "turn of the nut" method to establish bolt tension.

What I thing they are saying is to torque the bolts down to an initial setting of 20 NM, then repeat the bolt torque pattern with a 90 degree rotation of the bolt, and then repeat the torque bolt pattern with an additional 90 degree of rotation.

For the later models, Porsche appears to have reduce the final torque values, by only using a 60 degree rotation of the bolt, which as "ae1969" correctly states, is one flat of the bolt head.

I believe (and I'm sure you guys will advise if you think otherwise) what Porsche is trying to do, was to set an initial base value to seat the head, and then gain a certain amount of bolt stretch (tension), that could be observed by the degree of rotation. This may because Porsche was looking for a certain amount of bolt elongation (tension), which is not always directly related to torque, due to friction between the bolt face and the cylinder head. Bolt tension can be measured by a device called a Skidmore-Wilhelm device, and is often used for calibrating torque wrenches for structural steel (buildings and bridges), in order to achieve a specific amount of tension in the bolt. We don't have these devices in our garages, so an equivalent torque value is often used to achieve the required tension.

I've never taken the head of my car, so I don't know if you are allowed to reuse Porsche head bolts (haven't looked it up either). But I can say that if you are required to use new head bolts when doing head work, than what I have guessed at should be true.

There are other ways to evaluate bolt tension, such as load-indicating washers and bolts that will shear off a piece of the the top when the right amount of tension is applied, but they wouldn't be appropriate for this type of application.

I believe our Mechanical Engineer cohorts could provide more enlightenment.

[todwic]

I guess 90&deg is torque value,
60&deg is rotation value,
so 30&deg must me twist value and 15&deg is a spin value.

I agree with you, prof., and I think it's something to do with the washer. The manual sez you can reuse the old washers if you scuff em up (not verbatim) with some sandpaper.

Thank you for your answeres.