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Wet Torque vs Dry Torque
05-09-2008, 01:56 PM
The issue of wet torque (lubricated threads) vs. dry torque on common fasteners such as lug nuts has come up in another thread and I will bring it to everyone's attention here.
What is everyone's experience with this issue? It is common knowledge that the torque value expressed in Service Manuals are dry torques. There are many exceptions to this generalization. For example, all bolts tightened into aluminum threads should be coated with anti-seize compound (at least I hope this is a good generalization). Also, critical fasteners such as rod cap nuts and main bearing cap bolts are always lubricated with engine oil.
Lubricating fastener threads is common under critical use conditions as described above. Why would automakers create manuals using dry torque as the standard when no lubrication is specified? It requires a lot of effort to remove thread contamination to the point that the threads are in new condition. Even so, this cleaning does not help with lubricant that is in the bolt hole. No one ever cleans out bolt holes under normal service conditions. Solvent treatment of bolts and blowing the residual solvent off with air is barely adequate. I doubt in general that mechanics go to the trouble of using new bolts or cleaning the used bolts well.
The use of dry torque makes no sense to me except under conditions of original manufacture of the engine. Except for then, wet torque gives more even tightening pressure since it reduces the variability of the surface condition of used fasteners and holes.
I was taught to always lube bolts with either antiseize compound for aluminum holes, or engine oil for steel holes or nuts. This would give some over-tightening if the torque values are specified as dry torques for steel holes/nuts.
Does this extra torque really matter? Isn't an evenly applied torque more important to sealing and preventing warpage than absolute torque? I have never had a part fail from over-torquing using lubricated threads.
05-09-2008, 02:57 PM
Brian, my theory here, is after 40 years of building competition engines, that between torque wrench inaccuracies and engineering tolerances that the minor change in torque applications in almost all cases is too minor to be an issue. On the other hand, todays yield to failure bolt tightening techniques using a torque angle gage, it becomes more critical for everything to be exact. For example high performance rod bolts, today, require measuring bolt stretch after an initial basic torqueing. Common sense, care and high quality tools probably are more important in the overall equation.
05-09-2008, 04:23 PM
I agree with maxwedge. When talking about non-essential components, such as lug nuts, where the clamping force just needs to be enough to hold it on, you can be off by 10 or 20% and its unlikely to be an issue
Engine assembly can be a big difference though, since it can cause mechanical failure if things aren't done right, but even there you have some room to error.
Too often when dealing with used components, the threads won't be flawlessly clean, let alone dry, and there's no simple way to clean them. You have little choice but to work with what you have. I can't say I've seen many failures I would blame on such either.
05-09-2008, 04:33 PM
Like the others have already said, your average torque wrench can only provide a bolt tension accurate to +/-25%.
So you'd expect the makers with their torque settings to have shot for the middle of the window between "enough tension" and "bolt failure".
I do have formulae which relate bolt torque to tension somewhere, they include different factors for different thread finish (dry, galv, oiled etc). Let me know if you're interested and I'll try to find them.
05-09-2008, 07:18 PM
anytime old bolts are reused they should be cleaned and inspected....by using a chart you can determine if the bolts are torqued to there maximum stretch ...if so they should be replaced....like some engine head hardware....
on the use of antiseize this can be replaced in some app's with thread sealant....in any case using the manufactures recommendation is always the safe way....
when steel bolts are used in aluminum i would replace bolts with new.....but thats my 35 years experience...in the transit business....
05-10-2008, 05:16 AM
Yup... the critical goal is to get the proper stretch. Fasteners are at their greatest strength when they are at a particular tension/stretch point. They also use that stretch to prevent the fastener from loosening.
Torque specs are somewhere in the middle of acceptable tensions. That way if you're off by a few left or right it should still be within a range of acceptable installation. Highly critical fasteners these days use either a stretch gauge or a torque-to-yield spec where you take the fastener to a lower torque spec and then using a special wrench take it another X degrees.
Whenever possible, I like to use the stretch method, but this is only possible when both ends of the fastener are exposed like with rod bolts. Then you're not guessing with torque specs which can be affected by friction, you're measuring the actual stretch. That is the most accurate way to set fastener load.
But... billions of engines have been assembled with old-school flex torque wrenches (the kind with the pointer that indicates how much the wrench flexes) and have gone on to live perfectly normal lives.
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