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  #1  
Old 04-12-2002, 01:30 PM
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Question How many 90 degree stretches for true torque?

I understand angle of torque by 90 degree, but the service CD only says "torque to xx nm and and then gives the angle (90 - 100). This means one ninety degree turn, right? If it needed two stages it would say two wouldn't it?

Last edited by Cap'n Carageous; 04-12-2002 at 08:43 PM.
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  #2  
Old 04-12-2002, 03:22 PM
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Cap'n Carrageous,

The real point of applying torque to a fastener is to preload the fastener, by stretching it, so the loads it sees in operation never exceed the preload value. If you did not do this the fastener would be stretched some incremental value, and the mechanical joint would open slightly, with every cyclic application of load. In relatively short order the fastener would fail from fatigue.

So, when you get to the details the most uncertain way to apply a preload to a fastener is by measuring torque. The condition of the threads and the details of how they were cleaned and lubricated control the coefficient of friction between the male and female threads. This factor can range wildly within the limits of what is considered good workmanship with regard to cleaning and lubricating and deburring. In uncontrolled circumstances it can vary so much torque loses all meaningful relationship to preload.

To address this, the stretch measurement method by controlling angular rotation once the fastener is "seated" is often used. Seating involves some prelimiary torque value and is presumed to stop before any real stretching takes place. The axial stretch is a consequence then of the thread pitch and the angle you turn the fastener through.

If you seat the fastener and record the torque value you see when you subsequently turn it the specified number of degrees, it should be near the torque value in the manual. Once you check all the fasteners after they have been preloaded, if there is a gasket of significant thickness in the joint, you will see some relaxation of stretch in the fasteners as the gasket compresses. Consequently you may have to go back and check torque again, possibly bringing the value up to the specified number more than once.

In your situation, I would not tighten the fasteners in a single 90 - 100 degree increment. I would break it up into a few segments and follow the manual's bolt tightening pattern/sequence. My interpretation of your manual instructions is that when you get to around 100 degrees of rotation the fastener torque should be at the specified value. If you have something with spigot fitted O-ring, you will go metal to metal on the joint, detected by a sudden feeling of tightness on the fastener, and then I would use the manual procedure for preloading the fasteners. If it is a head gasket or some other relatively highly engineered closure, I would repeat the process until all the fasteners had been stretched the full 100 degrees or so in increments, and they stayed at the specified torque. It is not a good practice to fully tighten a fastener in a complex closure in one increment. It can misalign other parts of the closure and add unintended stresses. I would not assume the manual means do it in one step. Hope this helps, Jim
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1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #3  
Old 04-12-2002, 03:50 PM
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As I said, I understand the concept and theory. I just think that the manual is unclear about how many times to cycle the process. In this case I am referring to rod and flywheel bolts.
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Old 04-12-2002, 04:01 PM
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The only item on most engines that requires more than 1 (one) 90deg rotation, is the cylinder head. Rod & flywheel bolts require only one stretch! They also have a min thickness(when re-using old bolts).
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  #5  
Old 04-12-2002, 04:10 PM
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Thanks again Doc.! I have a digital caliper and will measure all before reusing.
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Old 04-12-2002, 04:21 PM
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I might have been a little unclear in my wordy dissertation. I do not think you should be repeating the 100 degree or so rotations after you seat the fasteners and complete the crushing of the gasket, if there is one, unless the manual specifically says to give the closure fasteners two 100 or so degree increments of stretch.

I think you need to draw the closure together evenly and reach a point where the fasteners are through crushing any gasket material that may be part of the joint design. The specified torque value will not be achieved if the gasket is still being compressed, and neither will the designed fastener stretch. I also do not think it is a good practice for a joint to be assembled by fully tightening the fasteners in one step. I would break the 100 degree/specified torque value into several increments that add to the specified value, and apply the torque in the fastener assembly sequence or pattern prescribed for the joint.

If there is no gasket the crushing of the gasket step does not apply. It is seat the fasteners, then apply the stretch. I would still not do it in one step though.

Jim
__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)
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  #7  
Old 04-12-2002, 04:35 PM
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I looked up the spec for the head (finally found it) and MB recommends inital torgue then two stages of 90 degree angle. There is an interesting footnote also. It reads " The cylinder head gasket is not watertight until the engine has reached it's normal operating temperature and the gasket has swollen . For this reason, do not pressure-test the cooling sytem until the engine has reached operating temperature". I would tend to think not following MB's instructions would be foolish, although I see your point, Jim, about the increments. Your method will probably not disregard the "crush" aspect of the gasket.
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Old 04-12-2002, 09:13 PM
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I am confused what are you trying to torque (a head or the flywheel) . ?
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Old 04-12-2002, 09:22 PM
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He's talking about the cylinder head bolts on a 300E. Cap'n tighten them to the torque specified, wait 10 minutes and turn them 1/4 turn, wait another 10 and tighten them another 1/4 turn and you are finished. Oh yea, make sure you go in the order as specified in the book.
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Old 04-12-2002, 09:28 PM
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Thats what I thought , we covered this like 8 posts ago on this one post. Go by the book dont second guess it.
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  #11  
Old 04-12-2002, 10:09 PM
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I was originally asking about conrods and flywheel. The head came into play on Docs' reply but it was also a concern since I'm torquing that too. I did the search before posting, but I found nothing about the number of times you perform the cycle, which was question of the thread. I thought the book was vague on the subject. However I have to remember that these manuals are designed for trained techs with experience already. Plus, I don't think I truly understand American English translated from German!:p :

Last edited by Cap'n Carageous; 04-12-2002 at 11:35 PM.
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