Question on Torque Importance - PeachParts Mercedes-Benz Forum

#1 05-07-2002, 02:39 PM

I have done quite a few things now on my car and don't own a torque wrench. I pay attention to how hard the nut/bolt is to remove and use same approximate effort to redo (for example the nuts on the transmission pan are not stressed down very hard).....but I know that is far from precise.

Note that I am not working inside the engine.

Any comments on how important it is to torque exactly right? Even the wheel nuts have a torque requirement "by the book".

I have seen some posts where apparently experienced types talk of a one "ug" vs. two "ugs" torque concept and I have watched qualified mechanics work on my suspension without ever touching a torque wrench. So, just how important is it?

Nic
'85 300CD

mbdoc · #2 05-07-2002, 05:54 PM

I guess that depends on whether or not you care that the bolts are being damaged OR left too loose.
Have seen many damaged wheels, lug bolts, & even wheels falling off from this type of practice. ie; GOOD LUCK!!

73MB280SEL · #3 05-07-2002, 06:12 PM

NIC,

I'd strongly recommend getting one. As M.B.Doc said, you really need to torque alloy wheel lugnuts. You should probably torque steel wheels because the tendencay is to get them too loose and they WILL come off that way.

Mechanics who have use a torque wrench often get used to the torque that needs to be applied. Weekend hackers like me don't use it often enough and really can't just guess.

Just get you a craftsman 3/8-inch wrench which goes to 80 ft-lbs. This will do most of your general purpose stuff. I have that, a used snap-on 1/2 inch wrench, and a snap-on 1/4 inch wrench for the small stuff. I still use my craftsman alot because the ratchet mechanism is better than the snap-on IMHO.

The craftsman wrench is really not expensive at all just a bit less well built than the snap-on unit. Torque and ratchet wise, the craftsman unit is just fine.

Sholin

BWatson · #4 05-07-2002, 07:09 PM

I rarely ever use a torque wrench. I have always relied on the 1 or 2 ug method and have never had a problem.

gsxr · #5 05-07-2002, 11:56 PM

Oddly enough, I just posted on this subject in a different topic. It applies here too, so here ya go:

==============

One last tip - which may get me flamed - don't use a torque wrench for "little" bolts like this. I have gotten myself into more trouble than I care to discuss by using the proper tool. (No that's not a typo.) The extra leverage of the wrench can end up masking the true force you are applying to such a small bolt, and most of the time, it just doesn't need to be that tight, or at a very specific torque setting! Use a 1/4 or 3/8 hand ratchet, snug them all down evenly, and then a little past "snug". Use anti-sieze if possible, especially on aluminum threads. If using LocTite please ONLY use the blue stuff (the red stuff should be banned.)

If you insist on a torque wrench, make sure the threads are all clean and DRY- use NO anti-sieze or other stuff - and maybe set the wrench a little lower than spec. Also, the wrenches are most accurate at the top of their rated range, so be VERY VERY careful if using a wrench rated (for example) 20-100 lb-ft on a bolt set to 25 lb-ft!! Much better to use one rated (for example) 5-30 for a 25 lb setting. Many are off as much as 20 percent in the lower rating range.

IMO, the torque wrench should be saved for "big" bolts like head bolts, suspension stuff, etc that are about M8 or bigger. Oh, and DEFINITELY use one EVERY TIME when torquing your alloy wheel lug bolts! That is actually the #1 use of my 1/2-inch torque wrench.

Let the flames begin!

(Nomex suit on...)

Regards,

rickjordan · #6 05-08-2002, 12:10 AM

The way I look at it, I love the way the cars are engineered, and it was the engineers who determined the various torque specs. I like using the torque wrench because it assures that everything is tightened down as it should be. I use mine on the oil drain plug to avoid cracking the pan. The torque spec. for this bolt on the 617 engines is 30 ft lbs. I definetly use it on the wheel bolts, which have a spec. of 81 ft lbs.

Jim Anderson · #7 05-08-2002, 01:04 AM

At my work they torque everything. They have lots of little ones, like 1/4" drives and screwdrivers, they go down to the inch/pound.

BobK · #8 05-08-2002, 07:56 AM

I've used moving-beam type torque wrenches for many years. Not all the time. Many bolts still get the snug and a bit more treatment. I had two wrenches. One rated to 150 ft/lbs the other rated in inch /lbs to 50 ft/lbs. These covered most jobs pretty well. As I have been buying and servicing MB cars lately, I have decided to upgade my work practices. Bought a new click type wrench (150lb) and am ordering a smaller click type for smaller jobs. Should have done this years ago. The click type make job so much easier, you are more willing to use the wrench. The one thing most of us overlook is that the torque specs call for (and most of us disregard) NEW fasteners. Some are more prone to stretch/fatigue than others, but all have probably changed after being used. This being said, I'll still probably use the wrench when possible, but I'm real big on having clean threads on both the bolt and the hole it goes to. my $0.02.

leathermang · #9 05-08-2002, 09:21 AM

Using some method of determining the amount of friction which the bolt is requiring to turn it is very important to gasket and machine performance....... The amount of area rubbing together when you figure the sides of the threads gets large fast...as you screw it in.... Not only does it need to be measured but you can not measure anything relavant unless the THREADS ARE CLEAN AND 'LIGHTLY' LUBRICATED.... this is the standard way engineers measure this....it is important because you are not actually measureing the HOLD DOWN AMOUNT of the article being bolted on.... rather a hoperfully ' translates to' amount based on trials of similar tries in the past.... You can be using a million dollar torque wrench and one bit of maching tailing or broken thread off the last bolt cause you to get NO pull down at that location... even though the torque wrench WILL READ PERFECTLY.... as if you are set to go.... \
Therefore, it is important that one use a torque wrench AND have a clean and lightly lubed thread surface...... DON'T ARGUE WITH ME ON THIS,, TO SAVE FUTURE EMBARRASSMENT.... LOL....
The only way to do this properly... and I always do it on any head gasket situation .... is to invest in the proper tap and die for the hole you are working on... and learn to use it properly.... so as not to have to learn how to drill out a broken tap.... NO LOL.. If you are interested in this sort of stuff I suggest the three ' Machinist Bedside Readers" by Guy Latuard....... Greg

#10 05-08-2002, 09:59 AM

Thanks for info folks. As I expected, some difference in opinion but to be safe, and in respect for my very nice mercedes machine, I'll buy the sears wrench and use it carefully and appropriately. Will pay special attention to the small nuts and wheel lugs.

Nic

gsxr · #11 05-08-2002, 10:52 AM

Sorry, leathermang, I'll have to argue on this one. Every source I've seen says to have the threads clean and DRY - no lubrication of any type put on the threads. If any lube is added (oil, anti-seize, etc) the torque value MUST be reduced. The exception is torque specs that specifically state lubricated threads, such as Mercedes head bolts. :p

Regards,

Jim Anderson · #12 05-08-2002, 10:59 AM

Machinist Bedside Readers, now there's a book for me. Now all I need is one for the bathroom

leathermang · #13 05-09-2002, 12:55 AM

I thought I might get away with not having to find the backup material on this... Everyone argued with me on the Sube forum ,,, then came back and agreed with me... LOL.. will find it sometime... but the rest of you .. trust me on this one.... maybe we should have a poll on who trusts me and who does not.... LOL....If you have any of your sources handy how about posting their names.. ? Maybe I will be so busy I will just cave in.... LOL... NOT likely if you know me.... Greg

It might be best if we agree that it should come with ASME indorsement ... would that be logical ?

NIC, I don't know which T-Wrench you are talking about... but the small nuts and bolts require an inch/pound... not a ft/pound to be safe.. even more important when dealing with aluminum heads and stuff....

rickjordan · #14 05-09-2002, 01:46 AM

Just a little FYI, to convert inch pounds to foot pounds, divide by 12. For foot lbs to inch pounds, multiply.

leathermang · #15 05-09-2002, 09:03 AM

Jim, it is very hard to find any of the " Machinist Throne Readers " but they are available, you will recognize the originals because the pages were laminated in plastic. Also, they included some plumbing tips just in case....but people shy away from buying the used ones..... an image problem I suspect....Greg