Torque decrease for lubricated bolt

Any definitive answer for how much to decrease torque for a lubricated bolt or nut?

Pocket Ref by Thomas J. Glover
ISBN 1-885071-00-0

There are plenty of bolt torque tables about for various types of lubes, if you hunt around.

This one suggests about a 25% reduction if using oil on the threads - over the dry spec torque.
http://raskcycle.com/techtip/webdoc14.html
There are other charts that deal with using grease or anti-seize, etc.
Mostly with a similar 25% to 33% reduction.

Unless you have a good reason to oil or anti-seize the threads though, its better to keep to dry threads, and torque them to spec ?
Its really only stainless bolts into stainless fasteners left for long times that are likely to show any problems ?

Absolutely NOT. Without detailed measurements on scope of range of fastener dimensions and female alloy threads conditions and type of lube and temperature and how many times its been run in and out to clear or create tread bind, you are depending on by gosh and by golly ballpark torque listings to get correct bolt tension and stretch. Best wishes if you don't already have a calibrated muscle tension sense on the size and length and grade of fastener that you have to refer to a chart. I even got Nortoneering God JIm Comstock to admit - often enough he just re torques by feel or till the leaks stop w/o bothering to put a T-wrench on it, like he did so very professionally disciplined on initial assembly but was mis led on what it actually required. So this shade tree tends to look way down on the skill level of any one bothering with a T-wrench. Only way to really know is to measure the stretch and only a few places that is possible, rod bolts and crank bolts are about it but thank goodness the only critical one to get about right. Best wishes on by gosh and by golly then a good bit more till fasteners stay clamped down stable w/o leakage and not twisted off.

Thx for responses. I have to admit that if you put something together by the numbers
and then put a hand wrench on it they seem loose. Avoiding too tight important esp.
with alloy. Think center front stud on cylinder base.
Basic rule of thumb for torque: " Tighten until it breaks, then add a quarter turn".

No Sir Ree Bob, by this point in life you should already have a sense of fastener tension sufficient before the sudden decrease of your sick joke. All's I can add is that both my own and my decades long experienced buddy Wes's fasteners end up rather tighter more torqued long term than the wimpy wishful levels listed in manuals but not too extreme beyond them. Part of being a real mechanic with trial and error guiding them. If you pull out the threads well they would of any way sooner of later so might as well find and fix it now for the next generations. I was rather impressed when I tired to unloosen Wes's head bolts so took his example to heart and crank down till teeth gritting tension.

As a one time airframe fitter I can only think that "going by the numbers" is a good thing. However it does help to have a feel for such things. Right or wrong I have always oiled or greased pretty much everything on assembly as a matter of course. Also I'm convinced that most (NOT ALL!) fasteners are overtightened anyway. I have always liked mechanical locking methods as well (locking wire, lock washers, etc etc), but that's probably a hang over from when I were a lad.
cheers
wakeup

Dear aircraft assembler, your remark is offensive BS for our Commandos , unless of course we were dealing in new parts examined and certified as aviation parts requires then fine to use ball park numbers to go by. Bringing in advanced new age assembly line protocols is very misleading for DIY guidance. Going by Norton manual torque values is more a saftey practice to prevent initial install damage but not what may actually be requred in practice or by actual measure of stretch as I do on rod and crank bolts to find the manual values are about 5+ lb ft under valued. I lube my critical fasteners and so should you and run em in and out a bit to make sure no hang ups to stop ya short of necessary clamping.

Thanks for that, I was about to let hobot wind me up a bit. Im sure he has a lot of time
in on Nortons and he has a pretty good shade tree handle on things. However, once you
play with aircraft you tend to go by the book and so I simply asked what the reduction
was generally considered to be. It is then up to my relatively skilled hands to see how
much more or less I would feel comfortable with. But the book carries a fair amount
of weight for me.
Agree too with the locking things up.
Thanks again to all.

OH my reply was to wakeup, not to hobot.
I have no fight with hobot, we just look at things
differently.

Although it seems simple, tightening a bolt is a complicated engineering problem. Torque is not a great measure of clamping force because of friction variable. It is cheap and better than nothing though.

Go to www.boltscience.com and read all you want.

This will stir up a hornets nest. Even though it seems logical, oil is not typically a bolt lubricant. Only stuff designed to be a bolt lube like moly or graphite based is a bolt lubricant. This being said, I have scientifically tested equal clamping at 150% dry to lubricated.

Use the values in the Norton manual for dry or oily. It is critical to torque incrementally and in sequence for best clamping results.

Seems I have unleased another oil thread.
Agree with the sequential and incremental approach
fully. Even load on each bolt important.
In general Id say better to err on the loose side
and would take a bit off of an oiled bolt compared
to a completely dry on.
Scientifically Im very likely to be full of it!
Have a lovely evening, Im off to buy some towels
for the inevitable weep....the bikes and mine

Onder, regarding the nuts on the cylinder base flange, I have found that when reassembled with a new gasket, they often loosen off within a very short time indeed and need to be retorqued a couple of times thereafter within the first 100 miles or so.

The only fasteners Norton factory used a t-wrench on were the rod cap bolts. These should be stretched to .0065". My measuring showed this takes closer to 30 ft than 25 listed. 28 lb ft needed on lubed cleaned up threads. Admittedly Norton didn't fly their piston engines in those days. Creep up on twist as your bravery-experience allows but realize too loose a fastener in an engine is more famous for damage than 'too' tight, short of just breaking the fastener of course.
I find if I don't stay ahead of the re-torques and get leaks, often as not its too late to seal back up just by re tightening. If following the manual blindly be aware it has a few damaging torque levels listed for some important fasteners.

If hobot was accusing me (wakeup) of being guilty of "offensive BS" I'm not sure why. Maybe whats offensive BS to one may be helpful advice to another. The point I was trying to make is that generally the information in manuals is written by people who understand and know the situation, and in the absence of any better information (note "better", in this context more qualified) can be reasonably safely relied on. Assembling used parts, as obviously Commando bits will be, requires more than just reassembly. The parts should all be inspected for faults, including threaded parts. As far as I'm concerned threaded parts with rounded hexs, damaged threads etc are fit only for the bin. Obviously some of the alloy parts with female threads take more care, because they are harder and more expensive to replace and can be subject to damage more easily than steel bits. However even damaged threads in alloy parts can be repaired. So at the reassembly stage there is no reason to treat the thing as anything other than new parts, so that the manual will (again generally) provide an answer which at least will cause no damage.

Personally there are things on this board that I find deeply offensive, but in the interest of maintaining a degree of civility I am quite happy to keep shtum, because there is a lot that is very interesting and sometimes very funny.
cheers
wakeup
ps An aircraft assembler is not the same as an airframe fitter, at least where I come from. Meaning no disrespect to anyone of course.

One can only wonder how in the world, all over the world, the vast majority of DIY assemblers in all sorts of things can get away as well as they have and do w/o t-wrenches. Its is offensive to imply the vast majority, like me and my mentors are stupid and or careless or even damaging mechanics w/o measuring torque. Its BS to imply that Norton actually did the study required to list a torque value per fastener application. Mostly the manual is just repeating the well known ballparks of torque per the size of the fastener. After a few re-torques how many of ya really bother with a T-wrench? If ya even retighten once w/o a t-wrench then why bother in the first place?

Sorry if I've offended you hobot. I certainly wasn't implying that you or any other torquewrenchless Commando fiddler/assembler/mechanic was stupid. I'm not sure how you could read that into what I wrote, but then I don't have a particularly thin skin. Personally the only fasteners I would be really careful about, and use a torque wrench on would be the big end bolts. But then I've been assembling things (motorcycles, cars, machine tools, engines etc etc) for most of my adult life and I believe that I have a feel for such things. That "feel" only comes with experience. My concern was, that in a moment of enthusiasm someone could cause some damage to their pride and joy, if that damage could be prevented by reference to the manual then that's a win in my book.
Its been 50 odd years since I bought my torque wrench. At the time it was a fair amount of money, but if I was starting out as a motorcycle assembler I would think fairly seriously about getting one. After all they are very cheap compared to the cost of a Commando, even if only for four fasteners (big end bolts) per motorcycle.
How did the manual writers arrive at the torque figures in their manuals, by exactly what you are talking about...... experience, gained from assembling hundreds if not thousands of motorcycles. As one of my old bosses used to say, "Manuals are provided for the man who doesn't know everything"
If you or your mentors and mentees don't want to use a torque wrench then that's fine by me, just be careful is pretty much what I'm saying.

cheers
wakeup

wakeup said:

"Manuals are provided for the man who doesn't know everything"

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Some folks might even suggest that manuals are for the folks who can't REMEMBER everything. !
It only takes one slipped neuron in the ole memory banks to remember a torque number wrongly, and do some serious damage by overtorqueing something.

There are enough errors in Manuals that cause this anyway ?
Some Matchless manuals apparently overquote the head stud torques - so that sales of headstuds have boomed over the years.

Perhaps a good shade tree mechanic would figure the number quoted was too high ?
Apparently not....

P.S. Isn't the correct quote to tighten it until it breaks, and then BACK OFF a 1/4 turn...

Rohan said:

P.S. Isn't the correct quote to tighten it until it breaks, and then BACK OFF a 1/4 turn...

Click to expand...

Ah!! Common sense, assuming that its got some lube??
cheers
wakeup

wakeup said:

Ah!! Common sense, assuming that its got some lube??

Click to expand...

If its broken, its the operator that needs the lube ? (beer !)