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Old 02-03-2004, 02:23 AM
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How do thread lubes affect torque values?

I have been looking through the site and other places and really haven't found a solid answer to this question, so here goes.

I am planning on reusing my bolts throughout my new sbc 355, the original build was advertised at 140 HP and I know the entire history of the engine back to '73. So I know this is the first time the bolts have been touched. My machine shop and performance parts store (Sure some of you have heard of Dick Harrels on the drag strip...) recommend using the ARP moly grease. But because it is a higher performance lube than the 30 wt that the GM engineers set the torque values to, I am told to knock the torque values down 10 ft-lbs across the board. This does not entirely make sense to me, because there are vast ranges of torques for the various components...

So I log into the ARP web site and see what they have to say. I look up what seems to be an appropriate strength spec and compare the 30 wt to ARP moly grease torque values. They difer by a relatively consistent 33%. So I look up the fatory torque spec and set the wrench to the new value. 54 ft-lbs compared to the spec 70 ft-lbs. Put all my plastigage down just to check for good measure, but 54 pounds seems awful loose... And my journal clearances all appear to be at the upper limits.

So after all this, what can you all tell me about adjusting torque values based on the lubricant used?

(And yes I will consider buying the new bolts, but for now the old ones stay...)

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Old 02-03-2004, 02:45 AM
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if it sounds to good to be true it probably is
if it were my engine i'd doit by the book settings
the factory spend lots of dollars on R/D
and some jazzy grease anit going to make those bolts
screw in much further be sides if the grease is as good as they tell you it is whats to say it wont make your bolts come lose or id them to wind back out do them up properly
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Old 02-03-2004, 05:26 AM
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bolts

What holds a bolt in place is stretch.... Think of the bolt as a spring, as you tighten it you stretch the spring.
The kind of lubricant that you put on any bolt will effect how much torque it takes to get the proper stretch. As your turning that bolt in you have friction areas in the threads and where the bolt head contacts what ever you bolting together. So if you have dirty threads,dry threads, moly,oil ect this will all change the amount of torque needed to pull a bolt up to a set amount of stretch.

I just spent about 10 hours doing some test's on rod bolts in my shop. You can take the same bolt and get totally different stretch depending on what kind of lube you put on the threads and under the head of the nut.When it come to rod bolts the only way to get the proper stretch is with a stretch gage. If you are building a stock or close to stock motor you will be fine just using the factory spec's on torque on factor bolts (i believe all the manuals recommend oil on the threads)

Try this simple test. Take a block of steel drill a hole through it and put a bolt and nut together thru it dry. Take a set of mic's and check the length of the bolt. Now start torquing the bolt in like 10 ft/lb increments. Check the length after every pull. Take it apart and put some molly lube on it and try again. Record the lengths per ft/lb dry and with molly you'll be surprised!!!! If you really want to get wild try it with different grade bolts...

keith
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Old 02-03-2004, 06:06 AM
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Arrow

I use ARP hardware and do agree with the lube comments mentioned, there is a difference. I would, however, use stock torques if using stock hardware.

Last edited by woodz428; 02-03-2004 at 05:10 PM.
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Old 02-03-2004, 08:15 AM
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There is no advantage at all in using a moly (or any other "slippery stuff") on fasteners. Best procedure is to use 30# motor oil and make sure that the threads are absolutely clean. For cleaning the threads, use only a thread chasing set. Do not use a tap and die set to clean the threads as these will actually remove metal from the threads. The amount of metal removed by tap & die set is very minute, but it will affect the clamping force of the fastener.
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Old 02-03-2004, 08:21 AM
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Also, you mentioned that you are using bearing clearance as an indicator of bolt tension. DON'T! Clearance has NOTHING to do with bolt tension, rather it is solely a function of the machining process. Joints are a necessary evil to allow assembly of parts in machinery but engineers strive to create bolted joints that come as close to a solid structure as possible. Bolt size, spacing, and pre-stress (stretch or torque) are all designed so no single component is overstressed by normal operating loads, plus a healthy safety factor. The engineer determines the tensile, compressive, and shear loads on a joint then sizes the bolt material, diameter, and tension so the bolt takes all of the loads, protecting the joint yet the bolt itself is not overloaded.
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Old 02-03-2004, 09:38 AM
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I am not using the journal clearance as a guage of bolt stretch. However, if a 33% reduction in torques if not appropriate, meaning significantly off, will make a differences in structure deformation. The static load that I am putting on the journals for a gage test will not be affected very much from a few lbs difference in pressure exerted on them. Unless it is too low to bring the caps into full design deformation.

Maybe I just need to revert back to using 30 wt with the oem hardware, but it would be nice to know if there are any definitive rules for changing lubricants.

Rod bolt stretch can be measured, but cap bolts are another story. The higher performance lubes have definite advantages, but only if they are matched with the corresponding torque values.

At this point the engineer in me is probably just rearing its ugly head. It seems that there has to be a definitive way to translate torque values based of the kinetic friction coeeficients of the various metals and lubricating agents.

Thanks to all who have thrown in their two cents so far!
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Old 02-03-2004, 10:15 AM
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There is no deformation anticipated in cap design, especially since it depends on very imprecise torque specification which would result in indeterminate bearing clearances which is unacceptable. Torque is used as the bolt tensioning standard simply because it is easiest for the average technician to measure. As you are finding out it is a pretty bad way to do precision assemble. It is close enough for government work but for very high demand applications, there is no substitute for measuring bolt stretch which is the base design parameter. Torque is simply an easily measured secondary indicator. For highly stressed industrial applications technicians use instrumentation to measure bolt stretch only and aren't concerned with the torque level. Chilton and other assembly manuals specify torque levels that assume a certain thread condition and lubrication that yield a known range of bolt stretch. Change the lube or thread condition and the torque needs to be re-calibrated to the desired stretch. You are getting vague answers because there is no 100% way to tell what the effect on bolt stretch without measuring it directly. Best bet is to defer to the bolt or lube suppliers who have hopefully done the lab work to calibrate torque spec to bolt stretch.
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Old 05-11-2014, 11:06 AM
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I some good answers here and some not so good.
if you are using stock fasteners, absolutely use the factory recommended lubricant and torque values.

Good design practice will tension bolts in the 30% up to 60% of yield. Bolt preload must be higher than any loads acting upon the connection with safety factor.

The torque wrench method is not very accurate for tensioning a bolt. But, with the design attempting to land in between the minimum preload and the point in which we go from elastic deformation of the fastener to plastic deformation (permanently deformed) factory torque specs work pretty darned good.

Consider that when torqueing a bolt, about 40% of the force applied is used to overcome the friction in the threads. About 50% of the torque applied goes to overcoming face friction. The remaining 10% is applied to preload.

Now if we look at the coefficient of friction here, often called the nut factor, we can see that this can range from a high .17 all the way down to a .05 as in the case with some moly type lubes like JL-M paste.

It would be very easy to have a connecting rod bolt over loaded and into the plastic zone with such a huge reduction in nut factor. This would be a bolt likely to fail.

ARP has some very good information on all of this you can read on line.
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Old 05-11-2014, 09:41 PM
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I would not apply "ARP" suggested torque values to other bolts based just on using their moly-lube. There is more to the suggested values that the lube as the fastener material itself is different too.

Most of the time, suggested lubrication with moly (or 30w oil) is for head bolts and not every fastener on the car. If you are using ARP fasteners then go by their suggested lubrication and torque values. If using OEM then use OEM suggestions. If you want to use moly on your head bolts for whatever reason and the OEM calls for engine oil then yes you would reduce the torque value by a %. Not as high as 33 IMO but by some. Maybe 10-15%. There are way too many variables to have a 100% factual and concise answer to that question.

On the rest of your fasteners they are best installed clean and dry. The suggested torque specs usually provided are for that unless specifically calling for lubrication.

ARP has good info and resources as mentioned by other users. Another good source for you would be Bolt Science Web Site which has many good write-ups of the technical aspect of fasteners and in laymens terms.
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Old 05-12-2014, 10:04 AM
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Quote:
Originally Posted by jonathanks View Post
I have been looking through the site and other places and really haven't found a solid answer to this question, so here goes.

I am planning on reusing my bolts throughout my new sbc 355, the original build was advertised at 140 HP and I know the entire history of the engine back to '73. So I know this is the first time the bolts have been touched. My machine shop and performance parts store (Sure some of you have heard of Dick Harrels on the drag strip...) recommend using the ARP moly grease. But because it is a higher performance lube than the 30 wt that the GM engineers set the torque values to, I am told to knock the torque values down 10 ft-lbs across the board. This does not entirely make sense to me, because there are vast ranges of torques for the various components...

So I log into the ARP web site and see what they have to say. I look up what seems to be an appropriate strength spec and compare the 30 wt to ARP moly grease torque values. They difer by a relatively consistent 33%. So I look up the fatory torque spec and set the wrench to the new value. 54 ft-lbs compared to the spec 70 ft-lbs. Put all my plastigage down just to check for good measure, but 54 pounds seems awful loose... And my journal clearances all appear to be at the upper limits.

So after all this, what can you all tell me about adjusting torque values based on the lubricant used?

(And yes I will consider buying the new bolts, but for now the old ones stay...)
I don't recommend the use of ARP lubricants on anything but ARP fasteners. The factory fastenres use 30wt engine oil on the threads and under the heads and this works on clean threads without having to fudge anything.

However, many Chevrolet bolts penetrate in to coolant or lubrication passages and zones which will leak past what ever liquid is present past the threads without a sealer. At the same time you need a sealer that comes extremely close to the friction values of 30 wt oil when using the factory torque values. It is widely recommended that hardware store Teflon impregnated plumbers pipe sealant be used. This both keeps coolant or oil from getting into and past the threads and it produces a coefficient of friction between clean threads that is for all intents and purposes the same/similar to the factory's use of 30 wt oil, therefore, the factory torque values to obtain the needed stretch on the factory fasteners holds true. No guessing or hopeing required!

Bogie
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Old 05-12-2014, 07:00 PM
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The reality of all this conjecture over lubricants etc is as long as the torque on the fastener does not exceed the limit of proportionality and is within the designed clamping stresses to be held on the interface it does not matter what the actual value is.

Torque values given for all fasteners are rated for this clamping stress, the limit of proportionality for any given material is a range of stresses and all the torque value does is ensure you are at least within that range.

The proportionality range is quite large on heat treated steels, adding more stress is only that...more stress. As long as it doesnt exceed into the plastic range of permanent deformation it will be fine.

It has been shown lubricity of a fastener interface does little more than place you higher in the range of stress allowable on the fastener.

In other words, unless the fastener has been designed on the ragged edge of allowable stress for its size...its not that important. What is important is reaching the minimum stress designed into the interface, and there is always a large margin of safety in the system.

Of course if your building spacecraft or high performance aircraft where each fastener has only a minimum safety factor to save weight, in those cases the fastening system would be engineered and carefully spec'd to ensure the stresses are controlled and duplicated from lab to real world.


Your typical small block chevy is not in this realm, torque it to factory spec and stop worrying about it. Even top fuel engines cylinder heads are simply torqued in the pit with only oil on the threads, the torque ensure minimum clamping stresses in the fastener and the high quality studs used have a steep proportionality ramp which means they resist stretch with high stress (i.e. Higher clamping forces).

Thats what your buying when you purchase a high quality fastener such as an ARP stud or bolt...a steep proportionality stress ramp which is typical of high quality fine alloy modern fastener steels.

Got a sealing issue with your head gasket on your 14:1 compression race engine? Maybe a higher torque with a better quality fastener would help...just remember those cast iron threads might need to be helicoiled to handle the added stress.

Want those same fancy ARP studs and bolts on your mildly built 500HP hotrod using a stock block, torque to stock values unless your prepared to reengineer the gasketed interface which includes the tapped block threads. Use whatever lubricant you want, the induced stresses will be within the range of acceptable stresses for the fastener unless specified by the manufacturer.

Lets put this 2004 thread to bed, it really is a non issue unless your pushing the envelope in top fuel or launching your self to Mars in your backyard built rocket.
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