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Need torque values Rear wheel bearing bolts

To say I am pissed is an understatement. Just got the rear wheel bearing in and had begun to reinstall everything when at 25 lbs the bolt snaps on the rear wheel bearing cover.

Prior to this I had looked for a torque value in a number of places but couldn't find it just so this wouldn't happen. The bolt number i believe is 900075079.02

Does anybody have the torque value for these bolts? Also is there a torque value for the bolt on the lower end of the rear shock? Reasonable hand tight my asXXXX!!!!

This is the second bolt that has snapped on this project. I am now gun shy

Just asking before I blow my brains out

Will post complete rear wheel bearing fiasco when this is done





Old 06-03-2017, 02:01 PM
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Would you call that a bearing cover to body? M10? I see in my SC spec book 47 Nm. 35 fl lbs.
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Old 06-03-2017, 02:46 PM
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I looked up the bolt its a M 8 not an M10
This is on a 1988 911 Carrera. Do you want think the bolt fatigued ?
Old 06-03-2017, 04:40 PM
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Are those common, grade 8.8 bolts (can't quite read print on bolt heads, but looks like it)? If so, the max torque on those m8 bolts would be 17 ft. lbs.
Old 06-03-2017, 04:45 PM
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I have in my notes that I used 18 ft lbs, but I see in Bentley figure for the rear suspension they call for 34 ft lbs.
I can't account for this (yet)

PET shows them as M8 and the OP's PN matches the one in PET
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Old 06-03-2017, 04:54 PM
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The Bently has to be wrong, as the max torque on super heavy duty m8 bolts (grade 12.9) is typically 29 ft. lbs. and there is no way those are 12.9 (which usually are black oxide steel - not zinc plated).
Old 06-03-2017, 04:59 PM
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Agreed - I think they copied the torsion bar cover plate torque.

Rekstein1 - the shock should be 92 ft lb

Here is the questionable figure I mentioned (for item 16).
It must be correct elsewhere as I lucked out.
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Old 06-03-2017, 05:07 PM
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Just for the sake of discussion and future consideration, torque specs that are published in manuals can always be double checked by measuring the diameter of a fastener (the threaded part) with calipers (m6, 8, 10, etc.), reading the grade (if marked on them or if you can tell based upon the material they are made with), and going by a standard torque chart.

Of course there are many exceptions to this - such as the pressure plate bolts and other precision fasteners like rod bolts.

Last edited by Rawknees'Turbo; 06-03-2017 at 05:17 PM..
Old 06-03-2017, 05:14 PM
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Here are the Factory Manual axle torques.






Bentley admits that they made a mistake on this torque value and has issued a revision to the page online.

Wheel bearing cover plate tightening torque correction

Harold
Old 06-03-2017, 06:11 PM
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A good strategy is to compare whatever Bentley or other books say to a standard metric bolt torque chart. If there is much of a difference then investigate deeper, there are not a lot (my feeling after checking a few) of fasteners on the Porsche that don't follow the standards. Having said that I should compare to the list above.
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Old 06-03-2017, 06:44 PM
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I checked the castle nut M20 x 1.5 Grade 10.9

Porsche Chart (above) 300-320 Nm
Chart I found 530 lubed and 675 dry Nm

I expect this is a "special" case because of the application and/or castle nut.

Checking the M6 8.8 bolt on the chart.
Porsche Chart (above) 5 Nm
Chart I found 8.8 grade 9 lubed Nm and 11 dry Nm
Using the 4.8 grade 4 lubed Nm and 6 dry Nm

So maybe the M6 grade bolt is not 8.8 but is 4.8 on the Porsche or the factory thinks torquing the bolt to the maximum clamping force is not needed or the bolt is going into AL.

I could be bringing up more questions than answers - maybe I'm not providing much help.
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Old 06-03-2017, 07:14 PM
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Assuming the axle and double row angular bearing need a certain clamping force it is curious that the nuts (M20 vs M22) are torqued differently 300-320Nm vs 460Nm. Both sizes have the same course and fine pitches 2.5 and 2mm.
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Old 06-03-2017, 09:22 PM
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HC - I believe Porsche shows a torque less than what the 22mm nut could do is because what you are doing is compressing the bearings with this nut, and most likely the bearings call for less compression than the ultimate strength of the nut.

For the bad luck guy, I think you had a bad bolt, as 25 lbs/ft is low for an 8mm 8.8. The charts typically are quite conservative as to max recommended torque. It may be because of the way fasteners are checked - I believe there is a sort of percentage pass used - if X percent of a sample survive a test, the batch is good. By testing to a higher number, you pretty much guarantee that even the worst of the lot will hold the specified torque?

I tighten my 12mm flywheel bolts to 150 lbs/ft. Porsche's spec is something like 110, and a chart I consulted was maybe 130. Never broke one, and I reuse them as well. You only need this for race motors running consistently above 7,000 RPM, to be sure.

Torquing many of these fasteners is overrated. If you use a socket wrench of an appropriate length, and get things good and tight on the suspension, things aren't going to get loose. The loading is often in shear, which doesn't persuade fasteners to loosen the way tension cycles like rod bolts and head studs and case through bolts see does. The shock to banana is a good example of this - as long as you don't use a breaker bar, you'd be hard pressed to tighten that bolt so much that it would fail.
Old 06-04-2017, 09:24 PM
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I know this is a silly questions but:

What are you using for a torque wrench? If you use a 150 ftlb "clicker" type torque wrench it may not even"click" for a light torque.

Using a torque wrench is pretty crude way to achieve bolt stretch so I wouldn't be too concerned about better than 5% accuracy [1] but I *would* be concerned it was way off, broken or if you were using a wrench for the wrong range i.e. using a 150 ftlb wrench for 15 ftlb fasteners for example. You want to avoid the lowest and highest 10% of the torque wrench's range, something like that.

[1] I've heard that up to 80% of the torque can be attributed to friction. A pound or 2 one way compared to the error contributed but improper fastener conditions. Rusty bolts, lube (or lack thereof) can cause a lot more error than that.
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Old 06-05-2017, 10:36 AM
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This had me concerned because I have 18 ft-lbs in my notes and the Bentley and Porsche specs show 34.
Either Rekstein1 over torqued (34), or I under torqued (18) .

So I dug out my Bentley and found another torque reference in the Rear Bearing section and it shows 18 (Bentley is inconsistent with itself and the Porsche specs apparently).

I believe we are talking about an M8, so the lower torque seems correct.
This would also answer why Rekstein1 had his result.

Here is the table from the rear bearing section 420-20.
(Wheel bearing cover plate)
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Old 06-05-2017, 02:53 PM
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I wouldn't go past 20 ft-lbs. Ask me how I know (as you do): My (almost) no good, terrible, horrible day
Old 06-05-2017, 04:14 PM
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I snapped one of those bolts as well. I didnt even get to torque them with the torque wrench. It snapped when I "pre tightened" it with my ring spanner. It was easy to remove the rest though, I just cut a slit in it with the dremmel and used a scew driver to remove it.

Lesson learned: 25 Nm is not much...

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Old 06-06-2017, 01:16 AM
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Wish I had read this thread before last night. Grrr! I torqued all my bolts to 34 ft lbs because ALL the books say to (except for that one spot in the Bentley--which I didn't know about). All my bolts held 34 ft lbs except one. I felt the bolt get soft and yield before the head came off. I was able to back it out with the socket. Ran to FLAPS and got eight new bolts and installed them at the proper 18 ft lbs.

This could have been a nightmare, but I learned a very important lesson. Trust the manuals, but verify and use your brain. I'm going to work up a large-print chart of general torque values for different size bolts so next time I have a general idea of what a certain size bolt can take regardless of what the book says.



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Old 02-09-2018, 09:51 AM
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Quote:
Originally Posted by guli View Post
I just cut a slit in it with the dremmel and used a scew driver to remove it.
Nice trick!
Old 02-09-2018, 10:43 AM
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Learn to tighten things like that by hand, by feel.
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Old 02-09-2018, 11:31 AM
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