Brake rotor threads

[dxhall]

I just replaced the wheel bearings on my '03 V11 Sport.  To get the old bearings out, I had to remove the rotors and heat the hub with a heat gun.  When I bolted the rotors back on to the hub tonight, several bolts won't come up to spec - they go to about 15 ft/lbs, but no more.  Anyone had this problem before?  Torque spec is 18 - 22 ft/lbs, bolts are 8mm 1.25 pitch.

[fotoguzzi]

Use some lock tight and call it good at 15#.. you really don't want to shear off a bolt head do you?

[rodekyll]

Be sure everything is completely bottomed in their seats and your 15# isn't because something is moving.

Those are long bolts.  If they're stretching at 15# you need to choose if you want to make them all 15 and use thread sealer or replace them on account of they're used up.

[lucky phil]

Use some lock tight and call it good at 15#.. you really don't want to shear off a bolt head do you?

Whaaaat.....the bolts wont torque to spec? I read that as they are stripped. If thats the case using thread locker and calling it good would be......... well not a smart move.
Ciao

[fotoguzzi]

find out why . you really don't want to shear off a bolt head do you?

Whaaaat.....the bolts wont torque to spec? I read that as they are stripped. If thats the case using thread locker and calling it good would be......... well not a smart move.
Ciao

your right of course if stripped but if they tighten to 15# it doesn't sound like threads stripped.

[dxhall]

The bolts aren't stripped.  The threads in the aluminum hub are letting go.  I guess my choices are locktite them at 15 ft/lbs, or install helicoils.  I'm just wondering if this is a common problem.

[fotoguzzi]

  The threads in the aluminum hub are letting go. 

now that sounds frightening. option 3 new rim?

[Triple Jim]

As long as there's enough meat in the area, I wouldn't hesitate to Helicoil them, but I sure wouldn't use them as-is if I thought the aluminum threads were failing.

[mtiberio]

remember the tip from the old british manual, stop turning 1/4 turn before the fastener strips.

[Lannis]

remember the tip from the old british manual, stop turning 1/4 turn before the fastener strips.

Before?  BEFORE??   

NOW you tell me!!!  And all those years ....

Lannis

[Chuck in Indiana]

Tighten it until it loosens, then back it off a quarter turn.

[Lannis]

Tighten it until it loosens, then back it off a quarter turn.

People think "Forlorn Hope" is a term originally applied to the first squad to storm the breach in the fortress.   It's not.

It's what happens on the side of the trail when you've turned the spark plug for the 50th turn ... HOPING that it will tighten up .....

Lannis

[Wayne Orwig]

The threads in the aluminum hub are letting go.  I guess my choices are locktite them at 15 ft/lbs, or install helicoils. 

 ???

Helicoil would be the only option IMHO.

[Triple Jim]

Question:  Are all the bolts original?  18 lb-ft is a lot of torque for a bolt that size, and if some were replaced with low strength bolts like grade 2 or stainless, they could be twisting/stretching, but not look damaged.

[dxhall]

The bolts appear to be original, are "flange" type bolts, no washers.  The spec (from memory) is 18 -22 ft/lbs, so 15 ft/lbs doesn't exactly give me confidence.

[Triple Jim]

OK, you ruled out wrong bolts.  I would have zero confidence if I couldn't get to the specified torque, and would not consider just using it as-is.  Helicoils and similar things like Time-Serts are easy to install and will create threads that are better than the originals.

[canuguzzi]

You aren't using the same bolts you took off are you? That would be a bad move.

Those are the wrong bolts. The originals have stretched when torqued and they aren't the same going back in.

New bolts, clean the threads real good and then assemble. You'll get the proper torque then.

Of course, if the threads are already letting go, then it is repair time, not loctite time. Helicoils aren't a good way to go either. What you're doing then is putting a steel insert into aluminum and then guessing about the torque. These are your brakes.

If there is enough material available for a rethread, that would work and then you could use standard torque figures and go that route. Use new bolts.

[canuguzzi]

Question: how high was the heat gun set? A heat gun can easily go past the temperature that will remove any heat treating temper on the aluminum. When the heat is too high, the "T" rating goes to zero and then you can easily strip out threads or they begin to let go as you've described.

At about 775 degrees, you've removed the temper from T6 and gone to nothing.

If you got there, rethreading isn't going to work and neither is a helicoil in any safe way.

[Triple Jim]

Question: how high was the heat gun set? A heat gun can easily go past the temperature that will remove any heat treating temper on the aluminum.

Yes, but only on small parts.  It's very unlikely he got a whole hub anywhere near that hot with a hot air gun, and with the conductivity of aluminum, he wouldn't have been able to do much localized heating.

[canuguzzi]

It is easily possible to remove the temper from a localized area. I routinely use a heat gun to bring T6061 bar stock up to temp and bend it. It happens PDQ and with thinner stock like 1/4 x4" takes less than a minute.

Don't forget, he was applying the heat to the hub itself (according to his description) and those bolts are steel and retain the heat longer than the aluminum. The threads are very thin and could loose their temper in very short order.

Many heat guns easily go to 1200 degrees or more, plenty enough to do damage to the temper on aluminum.


Correction: read where he removed the rotor before heating the bug so no bolts in when the heat was applied. Comes down to how long long the heat was applied and at what temp. If it's one of those low/med/high guns then the temp was over the threshold to remove the temper. Then it comes to how long.

[dxhall]

While I always replace rod bolts, I don't usually replace bolts like these, which are loaded in shear, not extension (as rod bolts are).  So long as shear bolts are stretched to a point below the limit of plastic deformation, I've always thought that they were reusable until the threads become worn.  I suppose I could have taken the temper out of the hub.  I guess I'll try helicoils, and see what happens.  Maybe check ebay for another wheel.

[Idontwantapickle]

I would strongly suggest finding a good shop to install Timeserts. They kick sand on the whimpy helicoils and take their girl.
Brakes are not to be left in any doubt.
Hunter

[redrider]

Timesert is the only choice for me. I gave up on helicoil decades ago. YMMV

[Triple Jim]

Either product would fix the problem for good.

[blackcat]

Timesert is the only choice for me. I gave up on helicoil decades ago. YMMV

Same here, especially for this application.

[dxhall]

I'm wondering if the torque figures in the manual are intended for dry threads.  The threadlocker I put on the rotor bolts when I installed them could have acted like a lubricant, and caused the 18 ft/lbs shown on the wrench to be much higher.  I've done experiments with rod bolts where I've compared torque figures shown on the torque wrench to stretch figures shown on the stretch gauge.  There's a big difference between the torque required to reach a particular stretch number with dry threads and the torque required to reach that stretch number with lubed threads.

I usually use timeserts over helicoils.  For some reason, this looked like an application for helicoils.