Bmwgirl

strange bolts for calipers?

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Does anyone know where these bolts go?  They were in a ziploc with my four caliper bolts and I had written left/right caliper bolts so I'm assuming I took them off the calipers long ago but I cannot figure out where they go on the calipers or even if they are related to the calipers.  I read a blog where someone said they messed up their struts not putting in a specific caliper bolt so if anyone has any insight/help I would be super grateful.  Trying to get the brakes done this weekend!! :)  Thank you for any help!!!

 

bmw bolt 001.JPG

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They may be the fillister bolts that hold the rotor to the wheel bearing housing.  There are four of them.  Those look too long though.

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Thank you for responding...I just figured out that they are two of the bolts that hold the caliper together!!  Mystery solved! :)

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Hi there,

 

better do not re-use those used bolts for putting the calipers back together again. According to a technical drawing from ATE I have they´re "Dehnschrauben" (I´m german and this is the german word for these special bolts - not sure but I think the english term is "stress bolt" maybe) and should only be used once. New ones are available at W&N but I´m pretty sure there should be suppliers in US, too.

 

Best regards, Lars.

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Always wondered about stretch bolts, & this post inspired me to look it up:

 

"All bolts stretch when they are tightened, but usually they stay within their 'elastic limit'. So, when they are loosened, they return elastically to their original length.

The normal tightening method is by torque wrench. The tightening torque is calculated to apply the correct stretch, and so put the correct load on the joint. It's not very accurate, and is affected particularly by thread condition, lubricant on the thread etc. Even using a bolt a second time when the thread is polished gives a different load on the bolt.

Stretch bolts try to eliminate some of these problems. A steel bolt will extend steadily in proportion to load up to its elastic limit. At that point it yields, and takes on permanent deformation. For a short while the material continues to extend steadily for very small increases in load (the load/extension curve being virtualy horizontal), and after that it takes only very small loads to keep stretching it to the point of failure. That is the load/extension curve drops rapidly. Stretch bolts are made of steel which has a relativly long horizontal section of the load/ extension curve past the yield point. In use they are tightened by stretching them a specific amount, by measuring the rotation after the joint is nipped. This is calculated to take them a short way past their yield point, to the point where the load extension curve is horizontal. The load in the bolt and hence the load on the joint is accurately known, as opposed to being at some ill-defined point on the elastic part of the load extension curve.

Because the deformation is permanent, and further extension would actually reduce the load applied, it is not possible to check the tightness of such bolts after a period of service. It is also unwise to reuse them, as they would not apply the correct load to the joint.

Regards
JS
"

 

From http://www.honestjohn.co.uk/forum/post/index.htm?t=24079

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Excellent information.  Thank you for adding it to the collective  :)

 

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Metric one use only stretch bolts have a circular depression on the top of the head sometimes with a triangle mark in side the circle (look at a 02 flywheel bolt) if you see this its a one use bolt, the factory manual will also spell out bolts that may not be so marked but also need to be replaced, may one use bolts also come with a thread locking compound on them.  I was unable to come up with the markings for metric allen bolts

Edited by Son of Marty
More imfo

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On Saturday, July 30, 2016 at 2:02 PM, Bmwgirl said:

Thank you for responding...I just figured out that they are two of the bolts that hold the caliper together!!  Mystery solved! :)

The answer above indicates yes. Most people don't split the calipers when rebuilding them, but if this was done, make sure to replace the O-rings that go between the two halves.

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Thank you so much to everyone for all the excellent and super helpful information!!!  I am so grateful for all of you who are helping me learn and get my car put back together right!!! :)

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Yes, it turned out that they were from one of my calipers, see picture below where bolts on right caliper are missing on upper left and lower right....The tops of the bolts are like a six pointed star!   :)

 

bmw 2002 July 001.JPG

 

 

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They're technically spline- drive bolts, and the driver you're 'supposed' to use on them costs more than a caliper, if you can find it.

 

However, the best fit Torx worked for me.  And yes, I have reused them several times, and I'm not dead yet. 

 

I'd be mildly suprised if tty bolts were in use in 1968.  Just mildly.

 

t

 

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JohnH did a great post on rebuilding calipers (where he identified that spline pattern as a 'Ribes'). Pretty sure he reused the bolts too after some research. 

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I should say that I am no expert on Stretch Bolts, but I do have a few ideas on the subject.

 

As far as I aware there are no markings on metric bolts to identify them as Stretch Bolts. As far as I'm aware the only way to visually identify them is if the shank of the bolt is necked down, as in these Rover k-series head bolts;

 

IMG_4148.JPG

 

The idea behind this necking down is to create a "weak" spot so that the bolt will stretch there and not in the theaded area.

 

These were the bolts that I removed from my calipers;

 

IMG_3250.JPG

 

They, like bmwgirl's cap headed bolts (aka capscrews) are not necked down, so after some research that I detailed in my original thread, I reused them.

 

My research also suggests that stretch bolts tend to be class 10.9 bolts. My bolts were class 8.8.

 

Capsrews seldom have any markings on them so its oftem difficult to tell what class they are, though the presupmtion is that theu are higher class  rather than your common or garden class 5.9 or 8.8 varieties.

 

Lars I am not doubting what you say, but "Dehnschrauben"   translates as expansion screw. Is that really the same thing?

 

One final point about stretch bolts is that according to the engineer who specified the k-series head bolts, they are re-usable so long as they have not exceeded there stretch point. Various manuals give the exact bolt length so that this can be determined.

 

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