Threads, Lubricants, and Torques: Home Mechanics Alert

[motoman]

If you are concerned about corrosion and removal of a bolt or nut, you may have cleaned,lubricated and then torqued the fastener as usual , maybe by the torque settting in the owner's or workshop manual. Welcome to the world of limited information.

Almost all torque callouts are for dry , clean threads. When you lubricate and tighten to your favorite torque you are probably over -tightening by up to 45% (yes forty-five percent). This stresses the fastener toward its elastic limit where it looses clamping ability, necks down, work hardens and distorts threads. Sometimes you can see this with the naked eye. Do not reuse over- torqued fasteners

In this forum the most obvious fasteners subject to concern must be blade bolts and nuts. They are usually corroded or rusty on the outside and hard to get off , especially if not frequently removed.

Look at the charts below carefully . If you dip that clean dry 7/16 " x 20 bolt into motor oil the dry torque of 78 ft lbs should be reduced to about 55 ft lbs. If you use graphite/oil the figure should be reduced to 42 ft. lbs. Note this chart is for Grade 8 fasteners. People are usually concerned about spark plugs into aluminum heads. The plugs are often plated. A dab of motor oil and 15 ft lbs should be reduced to 10-11 ft lbs. If you dont' have a starting point on your fastener read the head marking in the diameter you have and put the these tables on your shop wall (Hot Rod Magazine 1970's). PS 12-27, The authors mentioned that lock tite has very little effect on torque.

 

[SeniorCitizen]

QUOTE If you don't have a starting point on your fastener read the head marking .

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If both components aren't of the same grade I would think that point would be moot .

Mine will get lubricant every time and the lesser value. I just don't care for thread galling .:biggrin:

Some interesting facts in this link even though they are for stainless.

http://www.estainlesssteel.com/gallingofstainless.html

 

[motoman]

QUOTE If you don't have a starting point on your fastener read the head marking .

**************************************************************
If both components aren't of the same grade I would think that point would be moot .

Mine will get lubricant every time and the lesser value. I just don't care for thread galling .:biggrin:

Some interesting facts in this link even though they are for stainless.

Stainless Steel Galling / Locking Up / Freezing Up

What was meant: If the grade (hardness ) is unknow then the dry torque starting point is unknown. I would start with grade 5 or 6. If one assumes the female or nut is an 8 and applies high torque that would probably not be good for a grade 6 thread , no? I realize earlier talk on the forum has stated grade 8 used on blades. My craftsman seems more like a 5 or 6 based on manual callout of 45-55 ft lbs (dry).

Anyway ,identify (or assume) the grade , look up the torque on a chart. If you must lubricate, the charts above should help.

Craftsman blade bolts have held up well with removals about 2-3 times per summer for sharpenng, wiped clean and dry. Although the heads and washers look a little rusty they clean up easily . Once loosened, finger removal. The threads are not rusty so the washer seat in the blade and the bolt head on the washer hole is effective in keeping water out of thread area. Curiosity , my bolt heads are stamped with alpha-numeric gibberish I cannot decode (not metric). Probably proprietary factory marking.

 

[Elias40]

Thanks Motoman for sharing this very important information with us. We have always cleaned: and oiled, depending on the fastener type, and it's duty, but never paid much attention to what different lubricants would do with torque specifications.

 

[Elias40]

What was meant: If the grade (hardness ) is unknow then the dry torque starting point is unknown. I would start with grade 5 or 6. If one assumes the female or nut is an 8 and applies high torque that would probably not be good for a grade 6 thread , no? I realize earlier talk on the forum has stated grade 8 used on blades. My craftsman seems more like a 5 or 6 based on manual callout of 45-55 ft lbs (dry).

Anyway ,identify (or assume) the grade , look up the torque on a chart. If you must lubricate, the charts above should help.

Craftsman blade bolts have held up well with removals about 2-3 times per summer for sharpenng, wiped clean and dry. Although the heads and washers look a little rusty they clean up easily . Once loosened, finger removal. The threads are not rusty so the washer seat in the blade and the bolt head on the washer hole is effective in keeping water out of thread area. Curiosity , my bolt heads are stamped with alpha-numeric gibberish I cannot decode (not metric). Probably proprietary factory marking.

I realize earlier talk on the forum has stated grade 8 used on blades. My craftsman seems more like a 5 or 6 based on manual callout of 45-55 ft lbs (dry).

Grade 8 used on blades- there is a brittleness factor to be concerned with as well. If you do hit something, there would absolutely be no flex in the bolt material, and it would snap.

 

[motoman]

Grade 8 used on blades- there is a brittleness factor to be concerned with as well. If you do hit something, there would absolutely be no flex in the bolt material, and it would snap.

Not so sure of that Mr Elias. Brittleness is tempered out. Grade 8 is widely used in race applications and one gentleman on this forum was surprised I once mentioned grade 5/6 as he had not seen them in south. Usually grade 6 is bagged and sold in auto stores (three slashers). Also I think in open stock in H Depot and in the specialty drawers at Lowes. We all know to avoid plain, unmarked commercial grade bolts for blade retention, I hope.

Even though many are now saying "motoman, STFU!" I now think about a lot of archived material on metric, but you will really have to convince me you need it.

 

[Elias40]

Not so sure of that Mr Elias. Brittleness is tempered out. Grade 8 is widely used in race applications and one gentleman on this forum was surprised I once mentioned grade 5/6 as he had not seen them in south. Usually grade 6 is bagged and sold in auto stores (three slashers). Also I think in open stock in H Depot and in the specialty drawers at Lowes. We all know to avoid plain, unmarked commercial grade bolts for blade retention, I hope.

Even though many are now saying "motoman, STFU!" I now think about a lot of archived material on metric, but you will really have to convince me you need it.

The '70s F subframe used by General Motors; Used 4 grade 8 bolts attaching the subframe to the body. They had a problem with the bolts snapping off from a 30mph front end collision, causing the front of the car to fold in, crushing the driver. Inspectors stated, that because of the bolts used, it created a weak assembly, that would otherwise be an easy to survive fender bender.
Just an example

 

[motoman]

The '70s F subframe used by General Motors; Used 4 grade 8 bolts attaching the subframe to the body. They had a problem with the bolts snapping off from a 30mph front end collision, causing the front of the car to fold in, crushing the driver. Inspectors stated, that because of the bolts used, it created a weak assembly, that would otherwise be an easy to survive fender bender.
Just an example

I cannot comment further. Beyond my knowledge, but IMO its hard to understand how many on this forum have stated they have grade 8 bolts holding their blades on, without stories of catastrophe since the buggers are exposed to rocks, pipes, boulders, concrete abutments and the like. Rivets or Il Engine or Chance or others chime in please . :anyone

 

[Rivets]

Interesting thread. You've made me go back to my machinist training. First two websites will give you a basic understanding of bolt grading for the average engineer and consumer. These grades are standardized by the SAE (Society of Automotive Engineers). They are the most commonly used in the automotive and small engine industry. The last site shows a much more comprehensive chart and breaks the relatively SAE standards into a more precise and tighter ISO ( International Organization for Standards) standards.

You will see that there is no grade 6 bolt in the SAE standards and I assume that the bolts you are talking about (3marks) are grade 5. Don't really know about the grade of blade nuts, but in my opinion anything higher than grade 8, for this application, would overkill and possibly to brittle.



Bolt Depot - Bolt Grade Markings and Strength Chart

SAE Grades Steel Bolts

American Fastener - ASTM, SAE, and ISO Grade Markings

 

[soulman]

Having spent 32 years in quality assurance for a major tractor mfg. I have never heard of a grade 6 bolt.