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post #1 of 9 (permalink) Old Jan 16th, 2006, 9:50 pm Thread Starter
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Lightbulb Torque and Degrees

I've seen references to torque values for fasteners that say something like 60 nm, 60 degrees.

Anyone care to say what the 60 degrees means?

Velomaxx
97 Bakker Dakar K1200RS
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post #2 of 9 (permalink) Old Jan 16th, 2006, 10:43 pm
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Quote:
Originally Posted by Velomaxx
I've seen references to torque values for fasteners that say something like 60 nm, 60 degrees.

Anyone care to say what the 60 degrees means?

The big end con rod bolts in my old K100 are spec'd like this. They are special bolts that are INTENDED to stretch when tightening.

First you tighten them to a specified torque to get everything properly seated and preloaded, then you tighten the bolt XX degrees (and EXACTLY XX degrees!!) more to stretch the bolt an exact amount.

The reason for specifying the last stage in degrees, is that once a bolt begins to stretch, the torque actually drops slightly; it does not keep rising as you tighten. Therefore it is impossible to specify the end tightness as a torque, because it stays the same or even drops slightly in the last stages of turning.

Actually, this is exactly what happens when a normal bolt is overtightened, which is something many people are familiar with. When the bolt starts stretching, it immediately doesn't "feel" right, because you are turning it yet the force stays the same.

Bob.
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post #3 of 9 (permalink) Old Jan 17th, 2006, 3:41 pm
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You can buy circular degree scales in UK which slip on the socket hex. You could probably make something with cardboard, a protractor and a short length of weld wire for a pointer.

It's just as Bob says, school of thought is that torque due to friction on the bolt head is not consistent tension in the bolt which they are after specifying.



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post #4 of 9 (permalink) Old Feb 19th, 2006, 8:28 pm
 
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Quote:
Originally Posted by RFWILSON
The big end con rod bolts in my old K100 are spec'd like this. They are special bolts that are INTENDED to stretch when tightening.

First you tighten them to a specified torque to get everything properly seated and preloaded, then you tighten the bolt XX degrees (and EXACTLY XX degrees!!) more to stretch the bolt an exact amount.

The reason for specifying the last stage in degrees, is that once a bolt begins to stretch, the torque actually drops slightly; it does not keep rising as you tighten. Therefore it is impossible to specify the end tightness as a torque, because it stays the same or even drops slightly in the last stages of turning.

Actually, this is exactly what happens when a normal bolt is overtightened, which is something many people are familiar with. When the bolt starts stretching, it immediately doesn't "feel" right, because you are turning it yet the force stays the same.

Bob.
Sorry Bob bur you're slighly off here.

1. It isn't important to get it exactly xx degrees. Important torques are never given in degrees. The place you'll see that spec given most often is in structural steel and they usually give it in flats, i.e., 1/6 of a turn. It is an approximation and stretches the bolt differnt ammounts depending on how long it is. I don't like to use this spec and will generally calculate a torque to replace it. It's only advantage is that it doesn't depend on the condition of the threads to achieve correct tension in the bolt.

2. ALL bolts stretch, not just special bolts. That's why you torque them. Stretch, called preload, is what keeps bolts from backing out.

3. Torque goes up lineraly with stretch unless you reach the plastic region, i.e, past yield point. That will not happen on any motorcycle bolt. I've heard of aircraft parts spec'd this way but have never seen it myself. So your 3rd paragraph above it totally incorrect.

My advice is to always clean and lubricate your bolts well and use the torque wrench.

--Jerry
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post #5 of 9 (permalink) Old Feb 20th, 2006, 3:45 pm
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Quote:
Originally Posted by Stephejl
1. It isn't important to get it exactly xx degrees. Important torques are never given in degrees. The place you'll see that spec given most often is in structural steel and they usually give it in flats, i.e., 1/6 of a turn. It is an approximation and stretches the bolt differnt ammounts depending on how long it is. I don't like to use this spec and will generally calculate a torque to replace it. It's only advantage is that it doesn't depend on the condition of the threads to achieve correct tension in the bolt.

2. ALL bolts stretch, not just special bolts. That's why you torque them. Stretch, called preload, is what keeps bolts from backing out.

3. Torque goes up lineraly with stretch unless you reach the plastic region, i.e, past yield point. That will not happen on any motorcycle bolt. I've heard of aircraft parts spec'd this way but have never seen it myself. So your 3rd paragraph above it totally incorrect.

My advice is to always clean and lubricate your bolts well and use the torque wrench.
I'm not "off" at all. The BMW factory specification for tightening the big end bolts in a K100 (for example), states SPECIFICALLY that the bolts in question can only be used once, because they are designed to stretch PERMINENTLY when tightening. This specification also SPECIFICALLY states that it is necessary to tighten to a specified torque, AND THEN to tighten EXACTLY a specified number degrees further.

This is also stated very clearly in the Haynes manual for the bike, where they take care to explain why all this is necessary.

For bolts such as these (which are not uncommon for big-end bolts in many engines), it is absolutely impossible to get the required amount of stretch using a torque wrench. Once the bolt starts undergoing this required permenent deformation, torque no longer increases as you turn the bolts more. In fact, torque actually starts to decrease a bit when you pass the yield point, so there is no way to tell when you have stretched the bolt to the correct degree, except to measure the number of degrees you have turned.

Of course, torque is relatively linear until you reach the yeild point. Anyone knows that! The point here is that these bolts are DESIGNED to undergo a carefully defined amount of plastic deformation, meaning that a torque wrench is useless in this case for accurate tightening. This has nothing to do with the way "normal" fastenies are preloaded, and torqued.

So, for those few, highly stressed bolts that are INTENDED to be stretched past their yield point when tightening, your "advice" of using a torque wrench alone contradicts the manufacturer's requirements and can lead to failure.

Bob.
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post #6 of 9 (permalink) Old Feb 20th, 2006, 7:45 pm
 
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post #7 of 9 (permalink) Old Feb 21st, 2006, 5:58 am
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I had a diesel cage some years back. That was when I first came across the degree angle thing and bought the gizmo that fits my torque wrench. Angular tightening was specified for cylinder head bolts. Here's a photo:



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post #8 of 9 (permalink) Old Feb 21st, 2006, 3:45 pm
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Quote:
Originally Posted by voxmagna
I had a diesel cage some years back. That was when I first came across the degree angle thing and bought the gizmo that fits my torque wrench. Angular tightening was specified for cylinder head bolts. Here's a photo:
That is a real nice angle gauge. Is it available on the net?
Since our K1200's require the head bolts and rod bolts to be torqued to a value and then tightened an additional number of specified degrees. I would need one someday.

Bruce C
'04 K1200RS Capri Blue(totaled)
2008 Triumph Sprint ST

Last edited by brucecha; Feb 21st, 2006 at 3:47 pm. Reason: add wording
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post #9 of 9 (permalink) Old Feb 21st, 2006, 4:43 pm
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I've had mine about 8 years! There's been a lot said here already about these guages.

My cage had a history of manufacturer service updates to check and stop head gasket blow outs every 20K miles (serious design defect). They didn't specify this service method on the cyl. heads for no reason. Small head leaks into a sealed cooling system are not a good thing to have on any motor, or to figure out what's going on if you haven't had the joy of coolant overflowing your reservoir and the temperature guage wanging up and down like a yo-yo!

I did a Google search on ' Angular Guage' and came up with a load in UK. Your search syntax may be slightly different.

In fact, this one on Ebay looks smarter than mine:

http://cgi.ebay.co.uk/SEALEY-ANGULAR...QQcmdZViewItem



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