Engineers: torque wrench & fastener accuracy?

This is a question for you metallurgists, engineers, and ironworker types I’m sure can chime in. I was involved in a dispute between a vendor and the project over TC bolts failing the Skidmore test. Nucor makes them and coats them with lubricant, what with all of the inspections we do, the lubricant dried up after the kegs were cracked because every inspector has to put his pecker tracks on them. That screwed up the Skidmore test, can’t lube them with WD40 because they tighten too much then.

Anyway, it got me thinking about the need to keep threads clean and properly lubed when assembling anything. How many of us have reused fasteners that are OE? I wonder about stuff that gets torqued to specs and how you can tell if the fasteners you are using are doing their job, or if they can. Seems to me that new fasteners are the way to go. What kind of thread lube will provide the best results? Best way for application?

 

hey bro, we reuse fasteners/ bolts in the airline industry. To see if they are still good you can do a run on torque test to see if the predetermined torque was applied.. If the fastener comes out and the threads look good and there does not seem to be any stretching I am sure you will be fine.. You may have trouble determining the torque applied since the lube is gone and it may be frozen some which would show an increase in torque to remove. Remove one and then retorque as intended. I am sure there are others out there that will provide more detail or info

 

I re-use all OEM fasteners EXCEPT for headbolts on the newer engines. They are a "torque-to-yield" fastener and are normally scrap when you remove them. Everything else,I clean, re-lube with a light oil and re-use. $.02

 

To be actually anal about torques.. you can drive yourself nuts if you want. There are so many variables when torquing that its impossible to keep all of them under control. The biggest culprit I see other than reusing old bolts, where the threads have been stretched, along with cracks and imperfections caused by overtorqueing and the engine heat factor. Is the problems with the imported fasteners being shipped and distributed into this country. These fasteners are inferior in design and quality, From hardness problems to incorrect or inaccurate plating such as zinc or chromium.Many times I find threads which are not form correctly due to non control of manufacturing processes. The distributors who supply these fasteners will use whatever manufacturer is the cheapest for the month. There is actually no way to monitor there change of suppliers and we who use them are stuck with there junk.
Another problem when using the same threads in engines or the reusing even a good bolt is the dirt left in the threads.
That's why it essential to get yourself a tool that will run the threads, Thread runners as the tool is called... These clean the threads without cutting or distorting the surfaces as a thread die or tap would. My advise would be to get yourself a good clicking torque wrench, run the threads, use a new bolt or nut and torque to the value three times as the engine is heated and cooled each time.

 

as long as the plating is still on, that provides the friction coeffient...lubrication. most fasteners that we use are standard hex head cap screws which are through hardened, the only way they will stretch is if they are overtorqued, and at that point you are exceeding the tensile strength, and BAM, now you have a broken bolt to remove... imported vs. US bolts...I hate to say it but almost everything is overseas product or canada.

 

am i the only one who read this that doesn't know what the "Skidmore test" is?

 

It is a test that measures how much torque a bolt can take.

 

You guys might like this. I have 2 cat 3608s at work(these engines are about 3250BHP @900 ERPM). The torque sequence for the rod caps on them is about 17 steps long. Any way the 2nd to last step involves a inch# torque wrench. Then the last step is 300 degree turn. I have strecthed fasteners with hyd. pressure then wrist tighten the nut and release the pressure. tim

 

Another problem arises when the bolt goes into water as with a flathead. In that case it is coated with Aviation Permatex and any lube is a moot point.

When working with aluminum heads I always use new bolts. With iron heads I have reused if they look clean and no corrosion damage. Thread chasing is mandatory.

With studs, they always get replaced with new. PITA, Im not a stud fan.

 

[ QUOTE ]
...Another problem when using the same threads in engines or the reusing even a good bolt is the dirt left in the threads.
That's why it essential to get yourself a tool that will run the threads, Thread runners as the tool is called... These clean the threads without cutting or distorting the surfaces as a thread die or tap would. My advise would be to get yourself a good clicking torque wrench, run the threads, use a new bolt or nut and torque to the value three times as the engine is heated and cooled each time.

[/ QUOTE ]

This raises a question I have about a "thread runner" vs a conventional "tap" or "die."

What makes them different?

As far as "streching" fasteners; don't some high performance engine builders use a dial indicator to determine when the (e.g. head studs) are at their optimum strength?

BTW - When us Canadians want "good" fasteners (like ARP "rolled thread" bolts) we buy them from US sources; we don't manufacture "knock offs."

 

Any thoughts on El Caballo's other question:
"What kind of thread lube will provide the best results?"

 

Differences between thread cutters and chasers (runners is an incorrect term actually called chasers) A chaser gently reshapes damaged threads without removing as much metal as a die would.
As far as lubricant, I don't have any technical data on that only personnal experience. I use a lightweight oil such as WD-40.

 

I would prefer to use the stretch method for tightening a bolt in all applications. unfortunately the equipment to do this for blind holes is very expensive. The threads should be cleaned and chased witht a thread chaser to ensure cleanlyness. I like moly lube(like what ARP uses) because it burnishes in the threads as you tighten. ARP recommends that if the streatch method is not used that the bolt be cycled by tightening to spec and then loosened and retightened three times. This is to burnish in the threads which in turn reduces the friction that the threads cause. Each time you tighten the bolt the friction is less and and the bolt will stretch more and create more clamping load. As long as a bolt or studs threads are not damaged in any way and the bolt has not permanantely stretched it is suitable for reuse. I prefer the moly lube but oil can be used as well but the torque required to stretch the bolt will be more with oil than with moly(check out the ARP catalog, lots of info there)
Another important point comes up as well when using the stretch method to tighten a bolt you should bring the bolt to its full stretch in one motion, in other word not tighten 5 ft. lbs. and check stretch and tighten 5 more and check stretch. What does this mean, it means you must be checking the stretch while you are tightening the bolt.

A couple great books on fasteners etc. are
High Performance hardware by Forbes Aird HPBooks 1304
and
Nuts, bolts, fasteners and plumbing handbook by Carroll Smith