torquing!!

[jan lauwers]

Hello gentlemen,

I have ruined a few bolts by trying to tighten them to the torque settings as specified in the MB or Haynes books. E.g. I have ruined a new stabilizer bar bolt and also a couple of the subframe or caliper bolts.
First I thought this is because I am not always using new bolts..., I only replace those of which a worn threat is obvious; but the stabilizer bolt WAS new!
I am now starting to be bit afraid of torquing to the specs.

Is there any wisdom here that I am missing?

Thanks guys,
Jan

[Rolf-Dieter ✝︎]

I would question the accuracy of your touque wrench ... Have you checked it? Is it a high end wrench? What torque values are you using for what bolt size? For example size, number of threads per inch must match the torque value you are using.

I know you are aware of all of the above, I am just thinking out loud here since you asked ....

[UJJ]

Like R-D says your torque wrench maybe off. You ca check your torque wrench by setting it to 9.8 Nm then clamp it in a vise, add a lever/pipe to lengthen the handle of the wrech, mark it at 1m (1000mm or 39.4") and use a fishing scale or a very light container with 0.98 liter of water on it. Your wrench should "click" at this setting or your fishing scale should read 9.8 N (0.74 lbs.) Do not forget to deduct the weight of the lever/pipe you are using.

However, "number of threads per inch" does not apply to metric screws/bolts. Regular metric screws/bolts are defined by the letter M and the nominal diameter (M8 for example) Metric fine thread is defined by the letter M, the nominal diameter and the pitch (M8 x 1 for example)
If you look at the attached sheet you'll see the tightening torques for regular (not fine thread) metric screws/bolts.
Note: Most screws/bolts on our cars are 8.8 quality. Cylinder head screws for example are of higher tensile strength 10.9 . You can indentify the tensile strenght by looking at the head surface. It should have the 8.8 or otherwise on it.
Hope this helps.
Urban

[jan lauwers]

Thanks Urban,

I knew some of this but not everything. Always nice to learn something new. I will check the wrenches, they are pretty new, but it is always nice to verify.

Now I have one more question about what this table is telling us. If I take the example of an M8 bolt in Nm, we can tighten it to 23, 30 and 40 Nm. Yes so what... I guess I am not sure what this table is telling me.

Thanks!

Jan

[Paul & Dolly]

Jan,

The figures  8.8   10.9    12.9  refer to the different material specs for the Bolts, so effectively there are 3 vertical columns , one for each spec.

The metric system designates strength capabilities via property classes rather than grades.
The number that appears before the decimal, when multiplied by 100, will provide the approximate minimum tensile strength of the bolt.
The 10 in 10.9 multiplied by 100 tells the user that this bolt has an approximate minimum tensile strength of 1,000 MPa (Mega Pascals).
The number which appears after the decimal, when multiplied by 10, will provide the approximate yield strength percentage in relation to the minimum tensile strength.
For the 10.9 bolt, the 9 tells the user that the yield strength of the bolt is approximately 90% of the first number: 1,000 MPa. Thus, the 10.9 bolt has an approximate yield strength of 900 MPa (940 MPa by specification)
Another table ...

https://www.swtc.edu/ag_power/diesel_engines/lecture/metric%20torque%20values.pdf

Generally the bigger the numbers, the stronger the bolt,
8.8 is often considered the default, but if you torque up a say 4.8  or lower bolt, to the value for a 12.9 it could break.....
 
Hope the above does not muddy the waters

Keep well
Paul

[Shvegel]

When I was working as a line mechanic I sent out my very expensive torque wrench out for calibration.  When I got it back with a clean bill of health I welded 2 old 1/2" drive sockets face to face which left me with a way to connect 2 torque wrenches to each other to test one against mine.  Most were fairly accurate except for the most meticulous guy I had working for me who's was about 20 foot pounds low at 100 foot pounds. His jaw dropped and I thought he was going to faint then he just stared straight ahead and said he had just assembled 4 race engines for some friends.

[ja17]

Yes, the "click type" torque wrenches can become inaccurate if not stored with the setting at zero. Even my good expensive Snap-on "click-type" torque wrench became inaccurate when stored for a long time without being set at zero. The old less-expensive "beam-type" torque wrenches are a bit harder to use, but never become inaccurate from storage, since they automatically return to zero. These days I own a few different styles of torque wrenches; click type, dial type, and beam type. I use my old inexpensive Craftsman beam type to check my various click types. I hate to say it but I have a Harbor Freight click type, which I have been using for years. I often check it for accuracy with my Craftsman "beam type", and it is doing fine. My Snap-on "click type" needs to go back to Snap-On for an expensive re-calibration from being stored wrong. The less popular "dial type" as used in the BBB seems to be a good choice, but harder to find. They automatically return to zero when not in use. I have a couple of these also.

Use the torque specs as supplied by Mercedes in the Technical Data Manuals or BBB. If you still are having problems make sure you are using good tools. Sometimes 12 point sockets will not due the job. 6 point sockets may be needed to keep from damaging bolt heads. Allen type bolts heads and tools must be made of good steel and must be undamaged in order work properly. Unfastening torques (from corrosion etc.)  can be much higher than tightening torques so damage to bolt heads is not always avoidable. Abrupt impact with a hammer or heat can encourage stubborn bolts to loosen. Stripped-out allen head sockets can be hammered-tight again, then loosened. Replace these after removal. Clean bolt threads and threaded holes for good accurate torqueing. Early BBBs specified "graphite oil" for good accurate initial installation of head bolts. I still use graphite oil on my head bolts and main bearing and rod bearing caps for good accurate torqueing.

[wwheeler]

I was curious about Joe's reference to storing the torque wrench at 0. I had never heard that before and probably because I never read the instructions. Of course he is right (#9) and I am headed outside to set mine to 0. I left a couple of other interesting points from Snap-On below and deleted the obvious stuff. I have a question about #12. By handle, are they talking about the grip at the end of the wrench or are they referring to the entire length of the wrench? I always have one hand at the END of the wrench when torqueing. Any thoughts on that?

TECHNICAL REFERENCE
Released by Snap-on Tools
December 22, 2008
Proper Torque Wrench Use and Maintenance
3. Do not exceed the recommended working range of the torque wrench. Reliable measurements are
based on a percentage of the working range. In general, most mechanical wrenches have a useable
range from 20% to 100% of full scale. Most electronic wrenches have a useable range from 10% to
100% of full scale.
6. Avoid dropping or sliding a torque wrench. Dropping a torque wrench on a hard surface can cause
the instrument to lose reliable calibration. If you suspect that a wrench has been dropped, have the
tool inspected by the manufacturer or reputable calibration service.
9. If using a click-type torque wrench, always store it at the lowest level on the scale.
11. Use a torque wrench to apply a specific torque value during the final assembly process. Do not use
a torque wrench as the primary means of tightening or loosening fasteners.
12. As most torque wrenches are length specific, always grasp the torque wrench in the center of the
handle. If two hands need to be used, place one hand on top of the other.
15. After 5000 cycles or up to one year of use, whichever comes first, have your torque wrench
inspected and recalibrated by the manufacturer or reputable calibration service.