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Old 10-04-2017, 06:52 AM   #1
Spurshot
 
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Torque to Yield Bolts?

I guess I’m not familiar with this type of bolt for the brake calipers.


I read up on these which seem to be common for head bolts lately. Which bolts in our SS brakes utilize these? It would seem they’d be the long thin cross bolts holding the caliper halves together?
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Old 10-04-2017, 07:05 AM   #2
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The two bolts holding each of the calipers in place are in fact "torque to yield" bolts. Meaning, Chevrolet "highly" recommends one time usage and replace.

I recently purchased a complete Brembo setup from another Forum member who sent me a close up picture of a one time used bolt and a new OEM bolt side by side. You could actually see the one time used bolt had stretched. You just wait and see how much pressure you put on those bolts when installing, the torque on them is 30 foot pounds then 90 degrees! The 30 foot pounds doesn't seem like much but the extra 90 degree turn will have you grunting (hence the stretching)

On September 21, 2017 I purchased all new caliper bolts from my local GM dealership. He sells me parts at cost plus 40% (which isn't really to to bad compared to most).

Front bolt (4) part number 11570788, suggested retail $4.48 each, I paid $3.36 each.
Rear bolt (4) part number 11515781, suggested retail $3.48 each, I paid $2.61 each.

For just shy of $25.00 why take the risk?

Larry
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Old 10-04-2017, 08:35 AM   #3
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2010 SS list, prob same for most applications
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Old 10-04-2017, 08:57 AM   #4
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TTY bolts on the motor of course need replacin..different stress...as for the drive train? Once maybe twice re-use with lock-tite and "mechanic" tight...if you don't understand this, then replace..never had a prob...many will agree and sum will not...and this is per my Master Chebby Tech....good luck!
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Old 10-04-2017, 11:11 AM   #5
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Quote:
Originally Posted by frankwjr View Post
TTY bolts on the motor of course need replacin..different stress...as for the drive train? Once maybe twice re-use with lock-tite and "mechanic" tight...if you don't understand this, then replace..never had a prob...many will agree and sum will not...and this is per my Master Chebby Tech....good luck!
After reading up on these bolts, I’d say if you want to re-use them, don’t torque to the full stretch value in the manual. If you do, you permanently stretch it twice. That means you’re that much closer to failing the bolt.

In the stress-strain chart below, the vertical axis represents force or tension on our bolt. The horizontal axis represents stretching of our bolt.

A regular bolt would be tightened to point A or maybe point B. Point C is the Yield Point. A and B are stretch below the Yield Point and therefore, remain elastic and will return to original length.

From Point C an onward to the right, is the “plastic region”. The bolt is permanently stretched in this region.

Point D is where the metal becomes stronger from work hardening. At the crown of the curve, between Point E and F the metal begins to fail.

TTY bolts are intended to be permanently stretched somewhere between Point D and E. If you permanently stretch it twice, it could put your bolt beyond Point E or F.
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Old 10-05-2017, 07:43 PM   #6
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IOW, it's not about how much you tighten a TTY fastener in terms of torque, but how far you stretch it.


A concept that's probably familiar to those who have at least looked into tightening connecting rod bolts even if those weren't TTY fasteners.


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Old 01-09-2018, 09:24 PM   #7
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are they 30ft pds +90 on both the front and back? I have seen conflicting numbers. Some are saying it is closer to 140ft pds for the rear.
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Old 01-12-2018, 05:14 AM   #8
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Make a mark on one of the sides of the hex head through to the caliper. Torque bolt to mark when installing new one. Done. Did mine this way and have been fine for over 10k miles.
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Old 01-12-2018, 05:54 AM   #9
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Torque to yield bolts should never be re-used. After one use if tried to re-use the portion weakest of the bolt begins to "neck" and is no longer in the same stress/strain curve. And further use degrades the bolt. Always safer to replace with new hardware. Heat expansion also effects said bolt differently in situations. Again always safer with new hardware.
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Old 01-12-2018, 06:02 PM   #10
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Those that say never reuse a TTY bolt are absolutely correct. GM brake caliper bolts have been TTY for decades and some reused for that long. The list of TTY bolts in the 5th Gen is long. Who replaces clevis bolts after every alignment?

The best practice is to replace them after a single use. When they are not available use red LocTite and don't lose any sleep.
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Old 01-13-2018, 08:14 AM   #11
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All TTY fasteners are "one-time use", but not all "one time use" fasteners are TTY.

The fasteners that you can get away with re-using with Locktite (or a surface layer of Nature's own threadlock if present) aren't TTY fasteners. They'd be using some other means of ensuring that they stay tight, such as interference thread profiles or some other sort of threadlock compound supplied originally on those fasteners, which is not as reliable on successive loosening/retorquing cycles. These fasteners won't have any angle spec ("turn of the nut", in some engineering disciplines) for tightening them.


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