sjsingle1

iTrader: (1)

Now that I know the bolts holding the caliper bracket are TQ to yeild......I decided that something a little stronger might be in order....the bolts are M12 x 1.75 x 40...ARP p/n 665-1004

sjsingle1

iTrader: (1)

wssix99

iTrader: (5)

I haven't come across any information proving that correct. There is popular mythology that has spread on this and other sites about this. The stock bolts are high strength and don't get close to their yield. My math for this is a little rusty, but I calculate that the torqued stock bolts have around 50-60 ksi of strain in them. (They are rated for 100 ksi.)

I think the reason this idea has spread has been because the GM Factory Service Manual instructs us to replace the bolts after every use. I haven't seen a good reason why. Maybe this is a copy/paste error from an earlier version of the manual from an earlier F-Body? Maybe it is because some of the black oxide coating rubs off with each use? I don't know, but I'm re-using the bolts with confidence and without issue.


These bolts match in size, but there could be unintended consequences. With the stronger bolt, the torque applied to the bolt won't stretch it as much in the threads. This reduces the friction on those threads and the bolt would be more likely to vibrate out.

You could increase the torque to the bolt to a higher, proper value, but that leads to a question as to if the spindle or mounting plate threads can take the higher strain and match up to the bolt. (They could strip out.)

If you know the exact type of steel the bolts go into, the proper torque can be calculated, but that takes a little bit of engineering to make sure it's going to be safe and make sure the bolts won't vibrate out.

Chris Paveglio

iTrader: (2)

Always use red Loctite with any bolts. I've had a bracket bolt back out, and boy it was ugly (a huge pain but not terminal).

sjsingle1

iTrader: (1)

it does not say in the manual that the bolts are TTY so that may just be a internet rumor....it just says that the bolts should not be reused ...curious why GM would state that too

now that the question is raised i am curious as to what TQ to use with the ARPs....use the manual at 74 ft lbs or ARPs recommended the for bolt size at 98 ft lbs

I did chase the threads on the bracket with a balaax tap ( best tap made IMO ) used blue locktite on the threads of the bolt and ARP lube under the bolt head and did use the supplied ARP washer on the bolt and TQd per GM spec

sjsingle1

iTrader: (1)

so i called ARP tech support today with my TQ spec question.....he said since it is not an application tested by ARP he suggests sticking with the GM spec

wssix99

iTrader: (5)

Thread locker in this application is redundant and really doesn't do anything. It takes more force to break the torque of a properly fastened bolt, so if a bolt can come loose under the proper torque, the same factors will tear through thread locker. (This is why we don't see GM recommending to use it here.)


He probably shouldn't have suggested anything at all, then. The answer to the problem (either way) requires some design work, which he would not be required to do.

If you want to stay with the ARP bolts, I would calculate a better torque. If the stock GM torque is used, the bolts will not elongate which means: http://www.zerofast.com/proper-bolt-torque
- the parts will not be held together as tightly as GM's engineers originally intended
- the bolts may not develop enough friction to be held in place as tightly as the original bolts

* DISCLAIMER * - The ARP Bolts are stronger. The metal they are going into has not been upgraded. Therefore, if my method below is not correct or if the spindle has any flaws, the threads in the spindle could become stripped.

You could go to this site and reverse calculate the characteristics/type of steel the spindle is made of, using the stock specs: http://www.futek.com/boltcalc.aspx?mode=metric
Bolt Size - 12mm
Bolt Pitch - 1.75
Proof Strength - 830 MPa
Mating Material Length - <This needs to be measured by the Spindle thickness.>
Mating Material - Test the various steel values until the calculator gives back the stock 74 ft. lb torque

Once a similar to stock mating material has been discovered, the new proper torque can be calculated by changing the Bolt Proof Strength to the new value.

wssix99

iTrader: (5)

... and I've been looking for that "Proof Strength" value for the 8740 steel that the ARP bolts are made out of and I can't find it - but with what I have found; it looks like they are far weaker than stock.

I was able to find Yield strength. The stock 10.9 bolts yield at 940 MPa and 8740 steel yields at 550 MPa. So it doesn't look like the ARP bolts are stronger and it also looks like they would yield much sooner.

In terms of torque, the stock torque should be fine since the ARP bolts should stretch more and APR says they can take that force. I would not question if they are really going to be more durable than the stock bolts. ¯\_(ツ)_/¯

Darth_V8r

iTrader: (4)

That 550 MPA value (about 80,000 psi) assumes not heat treating was done. You can heat treat 8740 to pretty high numbers.

Now, that said ... you think you are actually getting 80,000 psi on those bolts with ~50 lbs of torque?

Hint: you are not

TYY is a misnomer. You are torquing to strain, not to yield. If you truly torque to yield, the bolt is permanently longer after torquing it.

A true TTY is typically done with a reduced segment of the bolt to give it a place to stretch.

wssix99

iTrader: (5)

Great point. Any idea where one could find the proof strength of APR bolts? They list delivering "tensile strengths between 180,000 and 210,000 psi." but I assume that is an ultimate strength.


Right. I only care so we can find the metallurgical property of these bolts and calculate what their torque should be.

Darth_V8r

iTrader: (4)

Yes tensile = ultimate. In s heat treated scenario, a good number is 80 to 85% of tensile strength for the yield strength. So approx 140-150 ksi (1000psi is 1 ksi) on a tensile of 180 ksi.

How that translates to a proper torque value, idk.

sjsingle1

iTrader: (1)

I spoke to ARP again and referenced this thread and he agreed I needed more TQ to reach the bolt pre load....I think the bracket can take the additional TQ

wssix99

iTrader: (5)

Here's a good diagram of the relations between the key values. We need the Proof Strength to confirm the proper torque. (I would expect that ARP can provide this. It looks like they are publishing Ultimate Strength on their website.)

wssix99

iTrader: (5)

For sure. If you can confirm the Proof Strength of the bolts with ARP, you can take a shot at a new torque. If you want to take the risk of stripping out the knuckle/bracket, you can always use heli-coils as a fall back since they should be stronger than the original threads.

sjsingle1

iTrader: (1)

the bracket is 3/4 inch thick so I thought it would easily take 98 ft lbs of TQ.....so I lubed up the bolts with ARP lube ( all their TQ specs on their website is with bolt threads lubed and under the bolt head lubed ) and TQ it to 40...60...80....98 ! since my spindle is loose the hard part was holding that sucker while i TQd it

wssix99

iTrader: (5)

At 3/4" and considering the calculations above, you'd be in the neighborhood of 98 ft lb.
I expect you are in a much better place with your bolts, now.

ZEN357

As long as the bolt thread correctly, you torque them to spec and use some blue loctite, I would think you'd be good to go.

wssix99

iTrader: (5)

Please read through the entire thread and the links provided and you'll learn why torquing a non-spec bolt to the OEM spec is dangerous and why loctite is pointless in this application.

biketopia

iTrader: (5)

Yea, lots of good, concrete, scientific and engineering data in this thread, not just hearsay. I learned a thing or two reading through it. Thanks for taking the time on it!!

sjsingle1

iTrader: (1)

while speaking to ARP I asked if my use of threadlocker was a positive thing.....he said an under TQd bolt will loosen regardless of use of threadlocker .....a false feel good band aid in this case