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View Full Version : Initial Torque vs. Final Torque?



gulfcoastbeemer
07-22-2007, 08:06 PM
What is the difference between "initial torque" and "final torque".

Can someone give me an example of how/when you use these two values. For instance, the engine oil drain plug on my R1200RT has an "initial torque" of 23 NM, and a "final torque" of 32 NM.

Obviously, a nut or bolt can have only one (final) torque value. So, what are you suppose to do -- tighten it and then tighten it again? What's the point?

Thanks

mika
07-22-2007, 09:28 PM
I am not certain of the point of two torque settings in your example beyond taking care not to over torque the bolt.

In cases where you are torqueing with multiple bolts hold something, a head for example, the idea is to bring the part evenly into the final torque spec and at the same time not unevenly crush a gasket causing a leak of fluids or compression.

gulfcoastbeemer
07-22-2007, 11:54 PM
I understand what you mean about multiple bolts; however, the example I gave of the single engine oil drain plug having an "initial" and "final" torque value is straight out of the BMW repair CD-ROM manual.

Does anybody know what "initial" verses "final" torque value is supposed to mean???

mika
07-23-2007, 01:09 AM
I kept wonder about this myself and did a bit more research. There seem to be combinations of reasons in this case. I read a number of articles and this seemed to present the point in the clearest manner.

From: The Nuts and Bolts of Torque ( http://www.americanrider.com/output.cfm?id=1029349)

Torque - the nuts-and-bolts kind - is important, but many of us don't understand exactly why. We know things like fenders and spark plugs tend not to fall off or unscrew if we tighten them without the benefit of a torque wrench.

Since this is the case, why are torque specifications and torque wrenches so important? I can best answer this by examining how a bolt works and what happens when you tighten one. Bolts are like springs, and they hold our motorcycles together. The tighter they are (within limits) the greater the spring pressure they apply to the parts they're clamping.

If the bolts are too loose, oil falls out or the head gaskets blow. If they're too tight, parts become distorted or even break, and the oil falls out or the gaskets blow. A torque wrench allows you to apply a reasonably precise amount of spring tension to the bolts. Bolts and the parts they hold together resist distortion. For example, if you flex a fender to the side a bit, it resists the force and springs back to its original shape when you remove the pressure.

When you tighten a bolt, you flex it and make it slightly longer. The bolt resists this flexing and becomes a spring, just like the fender did. If you severely over-tighten the bolt or push too hard on the fender, you could bend them. A bolt that's been bent (permanently stretched) by over-tightening doesn't apply the correct amount of clamping pressure and is likely to eventually fail, either by loosening or breaking.

The trick is to apply a correct amount of tension to the bolt and have that tension remain when the bolt is in use.

In several other articles it was made clear the toque applied to a bolt will release after time because of this distortion. Taking it to the initial will cause the maximum distortion of the bolt. The final few pounds will minimally distort it. Any release of torque after the final torque has been applied will be within the acceptable design range.

The practical reason is the simple. The bolt and case are different materials. By asking the mechanic to apply torque in incremental stages you are less likely to strip the case.

RandyB
07-23-2007, 01:20 AM
The other application of initial and final torques are to seat parts with a high value and then retain with a lesser one. Swingarm pivots on airheads come to mind. 15 ft/lbs to set, then 7.5 to retain, IIRC.

Not strictly applicable to this post, but it comes up from time to time.

That's good stuff, M1ka.

knary
07-23-2007, 04:20 AM
I kept wonder about this myself and did a bit more research.

:ha
Understatement of the week.

84674
08-06-2007, 01:42 AM
Do you have a list of torque values for most of the nuts and bolts on the R1200RT? Or do you know where I can find them.

theverns
08-06-2007, 03:17 AM
Respondent above is correct about the spring quality of bolts. When the thread is torqued to the proper value its threads act as a spring to hold the bolt in place.

For initial torque: tightening a bolt in a single bolt application only requires one torque value.

If tightening multiple bolts that cover a large area such as a cylinder head or oil pan the bollts should be tightened at incremental steps until the specified torque is reached. This will ensure proper pull down and/or gasket crush. Also the order in which bolting is tightened should be from the center outward in a criss-cross pattern on each tightening pass.

bmurphywa
08-06-2007, 05:29 PM
Do you have a list of torque values for most of the nuts and bolts on the R1200RT? Or do you know where I can find them.

Haynes just published a BMW R1200 Twins manual that provides all of the torque values for this bike and much more. ISBN 978 1 84425 598 6. I ordered directly from their website - took about 2 weeks to get here. Do not know if it's available from a domestic source yet.

larrydk
08-07-2007, 09:14 AM
The initial and final torque requirements are not that unusual.

Tighten to the first torque. Wait. Tighten to the second torque.

If you went right to the final torque, you would find when you went back it was low. By "stepping" up to it, it tends to be closer to the final

PAGoldsby
08-07-2007, 01:52 PM
This is EXACTLY why we structural engineers don't use "torque" as a measure of the tightness of a bolted connection. It is extremely difficult to establish a direct relationship between the applied torque to a fastener and the resultant tension in the fastener, at least for structures. Mechanical engineers feel better about doing this, as they think they can control the condition of the bolted joint better. The type and amount (or absence of, for that matter) lubrication on the threads, galling and faying surfaces, not to mention the finish of the fastener itself (bare steel, stainless steel, cad-plated, etc.) all serve to greatly influence the relationship between torque and tension. If the threads are dirty, it is possible to reach the specified torque before the fastener has been properly tensioned; if the threads are overly-lubricated, it is possible to exceed the maximum tension before the specified torque is reached.

I refer to the torque values that are provided in repair manuals when working on machines, but I don't obsess over them. The values, for the most part, are ideal target values. If you can hit +/-10%, you're done. Sure, if I'm tightening head bolts on a big-block Chevy race motor, I'm going to quadruple-check them, and then have somebody else check them again. Tightening the bolts that connect the rear subframe to the main chassis on my /7? Snug tight, plus 1/4 turn.

Oh, and by the way, when was the last time your torque wrench was calibrated? :stick