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Old 04-12-2002, 02:22 PM
JimSmith JimSmith is offline
Join Date: Jul 2001
Location: Old Lyme, Connecticut
Posts: 3,596
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
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

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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