jhg41977

I have always replaced them because the manual says to. Why are we replacing them?

JasonAndreas

Dacromet or whatever they're using now?

TMc993

EDIT: Deleted due to apparent misinformation.

jhg41977

Where did you get that info Terry?

Leander

How common are failed caliper bolts? Because the new bolts were the wrong length, I didn't replace mine but instead re-used the existing bolts when I did my pads and rotors six months ago.

TMc993

EDIT: Deleted due to apparent misinformation.

Mike J

I have never heard on one failing for street or even light track duty, but I have a limited view. I was told by a mechanic (who is a race mechanic, racing since the 964 days) that the reason was not due to stretch, but the wearing of the coating which is essentially loctite - his recommendation was to reuse the bolts with some Blue loctite ....

OverBoosted28

And is it just for the fronts?

pp000830

When torqued to the specified value a fastener is stretched within its elastic range to maximize clamping force. No one designs a cap screw application where a threaded fastener is torqued to the point where it is stretched beyond their elastic limit resulting in reduced clamping force or worse yet setting the stage for fastener failure. Recommending the replacement of the fastener probably has more to do with corrosion on the existing bolt that may significantly weaken the fastener. Even a small corrosion particle that causes an abrasion on the fillet under the threaded fastner head dramatically weakens the fastener. When disturbing the existing screw to remove it there may be the potential for corrosion to damage the bolt. Using a fresh bolt likely reduces the chance of having a compromised bolt in a safety related application where a failure could have catastrophic results.
Andy

Kein_Ersatz

And then there is this ...

Replace Caliper Bolts with Studs

nine9six

From the Workshop Manual...

Brake caliper to wheel carrier* (Note the asterisk)
Size: M 12 x 1.5
Torque: 85* (63) (Note the asterisk)

* Replace bolts (only on front axle) whenever the screw connection has been undone

As for the reason why; I suspect Andy has the engineering rationale.

jhg41977

I am aware of that, just curious why. As mentioned above, these bolts should be well within their plastic zone so replacement after a single use due to stretching is unlikely. If it were a fatigue issue, I would expect them to be replaced at a regular service interval. So it seems the best reasoning so far is corrosion or locktite.

nine9six

When you paraphrase, things get missed.

G-MASSA

Andy- are you sure that's accurate? I got into the weeds on this subject a few years ago and found some caliper bolts are in fact TTY (torque to yield). Some head bolts also. This means permanent fastener elongation with specified torque applied. This was application specific. I replaced mine on the boxster as the repair manual instructs.

pp000830

Interesting. I worked for many years in the fastener industry at a company called Barnes Distribution. I was exposed to the engineering issues of fasteners while creating instructional literature and how to videos with the engineers. I never ran across an application where a cap screw is torqued to the point beyond the fasteners elastic limit. Some of the video demos were interesting. In one the presenter makes a tiny scratch on the head fillet radius of a cap screw with a pin. The result is the head of the cap screw just popped right off long before it was torques to its recommended value. In another a Skidmore clamping force meter was used to show the characteristic cure of the clamping force as it passes through its elastic state and on to where the bolt begins to stretch and ultimately fail. These tests were to demonstrate why a fastener should not be torqued past its elastic limit. The tests also seemed to suggest that once a a fastener passes beyond its elastic limit the point at which it will continue on to complete failure can not be reasonably predicted.

Having said the above, I found the below that seems to negate all the rules of fasteners as I have known. One being that is a fastener that is torqued into its elastic range and during clamp load cycling it never exits the range it will never exhibit a fatigue related failure. go figure?

http://en.wikipedia.org/wiki/Torque-to-yield_fastener