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Discussion Starter #1
Hi all, I searched but no answer, so here are my questions....

Putting on my summer tires a few months ago I noticed that the lug bolts THREADS for the front wheels only have some rust. Yes, the nut/caps themselves have some minimal cosmetic rust -- which seems to be what every other thread here that came up during my search was discussing -- but my issue and question is with rust on the bolt threads. Weirdly, my rear wheel bolt threads do not have any rust. Only the front ones do. The bolts came off easy (i.e., were not seized) and torqued back up to 96lbft just fine. I change my wheels/tires with the seasons.

Thoughts? Does anyone else have rust on the front wheel bolt threads? Should I be concerned? Should I clean the bolts threads (I plan too..) and the bolt holes, or replace the bolts and clean the bolt holes in the front hubs? If cleaning, would a soft conical metal wire brush work to "clean" the bolt hole threads?

thanks! :cheers::cheers:
 

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I would expect that the rust is superficial and nothing to be concerned about. I've seen rust on wheel studs on older cars, but I do not recall ever seeing a wheel stud broken off. Moreover, if the stud did crack, I'd expect that it was because of some careless guy at the tire shop rather than because of some rust. If you clean the threads, you would probably be the only guy who ever did, but if it makes you feel better, than clean them.
 

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I have not read my manual in this area lately, but can tell you that for many years, Porsche always recommended lubing the lug bolt threads, but not the load bearing surface under the head of the lug bolt.
 

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From my manual:

Apply a thin coat of Optimoly TA (aluminum paste) on the thread and between the bolt head and movable spherical cap ring (arrows). The bearing surface of the spherical cap facing the wheel must not be greased.
 

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From my manual:

Apply a thin coat of Optimoly TA (aluminum paste) on the thread and between the bolt head and movable spherical cap ring (arrows). The bearing surface of the spherical cap facing the wheel must not be greased.
To be clear, note it does not say to put the paste on the bolt threads. The purpose here is to make sure the conical cap can rotate freely and does not get stuck to the bolt - it has nothing to do with the threads . This is always a point of contention, but since the correct torque of the bolt is determined without using grease or paste on the threads, if you do apply grease or paste on the threads and then tighten to the specified torque you could potentially over-tighten the bolts. Leave the threads dry. FWIW, the threads on my lug bolts all have a bit of rust, some worse than others, but with no ill effect.
 

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So I wonder what "on the thread and..." means in the manual? Apply a thin coat of Optimoly TA (aluminum paste) on the thread and between the bolt head and movable spherical cap ring.
 
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From my manual:

Apply a thin coat of Optimoly TA (aluminum paste) on the thread and between the bolt head and movable spherical cap ring (arrows). The bearing surface of the spherical cap facing the wheel must not be greased.
Here is the graphic that goes with the instructions:

pg_206_img.jpg
 

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While I think this may inhibit much of the rusting; I believe it is intended more to keep the lug bolt from corroding and seizing.
All steel lug bolts will rust to some extent over time with exposure to moisture.

I personally don't think you will be the "only guy that ever cleaned the rust off the threads". There are enough posts here on P9 to prove, beyond the shadow of a doubt, that many of us Porsche owenrs suffer from OCD in varying degrees.
However, if the rust really, really bothers you; you can always go with titanium.
 

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Thanks for the graphic. Didn't know how to get that on my post.
 

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I cannot stress this enough. Follow the manufacturer's instructions.
My 981's owner's manual says in no uncertain terms, "Wheel bolts must not be greased." Also "Always clean the wheel bolts before fitting." Significantly, the 981 torque spec is 160 Nm (118 lb-ft) as opposed to the greased spec of earlier cars of 130 Nm (96 lb-ft.)
According to NASA and documented elsewhere, only 10% of the torque of an un-lubricated fastener creates tension in the bolt. Thread friction accounts for 40% and 50% goes to the bearing friction under the nut or head of the bolt. So greasing the threads and bolt heads will cause a huge increase in bolt preload, as shown in the link above.
My company experienced a major "Ah Sh!t" moment when an 800 lb radar antenna fell out of the nose of a B1 bomber. The autopsy revealed that the bolts used to secure the antenna had been greased. Since the bolts were torqued to 80% of yield based on un-lubed bolts, the addition load cause bolt failure.
Do not let this happen to you.
 

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I cannot stress this enough. Follow the manufacturer's instructions.
My 981's owner's manual says in no uncertain terms, "Wheel bolts must not be greased." Also "Always clean the wheel bolts before fitting." Significantly, the 981 torque spec is 160 Nm (118 lb-ft) as opposed to the greased spec of earlier cars of 130 Nm (96 lb-ft.)
According to NASA and documented elsewhere, only 10% of the torque of an un-lubricated fastener creates tension in the bolt. Thread friction accounts for 40% and 50% goes to the bearing friction under the nut or head of the bolt. So greasing the threads and bolt heads will cause a huge increase in bolt preload, as shown in the link above.
My company experienced a major "Ah Sh!t" moment when an 800 lb radar antenna fell out of the nose of a B1 bomber. The autopsy revealed that the bolts used to secure the antenna had been greased. Since the bolts were torqued to 80% of yield based on un-lubed bolts, the addition load cause bolt failure.
Do not let this happen to you.
Yep, when in doubt RTFM. Mine says lube the threads so lubed they will be. My father was also an aerospace engineer (Titan launch vehicle) and his hobby was rebuilding general aviation aircraft in our garage. "Thread no fastener fowled or dry was his motto". Gotta choose the best method from our combined knowledge, documentation, and mechanical experiences.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900009424.pdf
 

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Discussion Starter #14
Thanks you everyone for the responses. I apply aluminum paste on the thread portion indicated in the manual. But the rust in on the portion of the threads that don't have paste (ie, area towards the end of bolt away from bolt head). The curious thing is that only the front bolts have rust. The rear ones are good. I inspected and it look like the front rotor/hubs are open on the other side to the elements, whereas the rear rotor/hubs are closed. Meaning, the bolt holes are open ended in the front and close ended in the rear. Unless the dealer screwed something up during a service, this is how my car is. So I guess the open ended nature of the front hub bolt holes is what allows water etc to get in there and rust the bolts. Does his makes any sense to anyone?(the open ended vs closed ended bolt holes).

Investigation continues. Thanks all!! :cheers:
 

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Chicago Ill. Yes the fronts are noticeably more exposed to the elements and you have a lot of "elements" in Chicago as noted in your avatar..
 

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Well I certainly learned something here! I was so sure that bolts should not be greased that I misread the quote that ottobon100 posted. I looked at my manual and sure enough:

Greased_Bolts.JPG

So indeed I stand corrected. Does this mean that if you don't apply grease and torque the bolts to 96 ft-lbs (which has been my practice) that more of the torque goes into overcoming friction of the threads and less into stretching the bolt? It's interesting that the 981 manual specs a higher torque for ungreased bolts than the 987 with greased bolts - that seems backwards to me IF the bolts are identical.
 

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Does this mean that if you don't apply grease and torque the bolts to 96 ft-lbs (which has been my practice) that more of the torque goes into overcoming friction of the threads and less into stretching the bolt? It's interesting that the 981 manual specs a higher torque for ungreased bolts than the 987 with greased bolts - that seems backwards to me IF the bolts are identical.
That is correct. And the 981s indeed need more torque to attain the same preload for exactly the same reason, more torque is necessary to overcome the higher friction.
When it comes right down to it, torqueing a fastener is a very poor way to set preload. There are just too many variables. Other methods include measuring bolt stretch or a combination of torque and turn angle.
 
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