SOME THREAD LOCKERS CAN AFFECT BOLT TORQUE - BE CAREFUL
Can Thread Lockers SNAP your bolts in Half?
During the 6 Month final assembly of our brand new $300,000 Off Road Race Truck, I noticed a few of my very expensive, 12 point, Military Spec Bolts were broken (snapped in half), after being torque up using our new “Digital” Torque Wrench, before we had EVEN DRIVEN THE TRUCK.
The search for the cause of our broken bolts was immediately underway!
Based in Western Australia, Proformance Motorsport design, manufacture and build Off Road Race Trucks and components. We CAD design and CNC machine our own Differentials, Gearboxes, Wheel Hubs, Axles and even manufacture the entire chassis and fibreglass bodies - all in house.
Running a business that charges money to take customers for rides in our race trucks, we lose money every time a component fails or we have a truck that is disabled. We pride ourselves on only using the best materials and components available to ensure our vehicles are the most reliable they possibly can be. We all try to use the best bolts we can and 12 point bolts when we can get hold of them. As many of us know, a bolt is held in place by the preload caused when torque is applied to the head.
The best bolt manufacturers in the world will provide race car engineers with the recommended amount of “stretch” recommended for their bolts to achieve the correct amount of preload. Professional engine builders will always use a micrometer to measure the physical stretch of a con rod bolt when it is tightened.
In 99% of circumstances, it is simply not possible to use a micrometer to measure the length of the bolt when it is being tightened and therefore bolt manufacturers will supply us with Bolt Torque Charts to guide us how much to tighten the bolt. Any one that has ever rebuilt an engine will be familiar with the need to use a torque wrench to tighten the Head Bolts.
Most of us are aware that amount of resulted bolt stretch (preload) for a particular applied torque varies greatly due to the following:
Dirty/Rusty Threads – The Torque Wrench clicks (Beeps, or similar) at the preset torque setting, but the bolt is not stretched to the desired preload and the parts are not clamped sufficiently and the bolt can easily come loose.
Oily Threads – The oil lubricates the threads and the bolt can be over-tightened when the recommended torque is applied and the bolt may fail prematurely.
Most High End bolt manufacturers will provide torque settings, showing the required torque for a Dry Bolt, and a lubricated bolt. See the example below.
The type of lubricant used can significantly alter the resulting preload created by the torque applied when tightening the bolt. See the data below, again from ARP:
Plated Bolts vs. Black Bolts:
Something many of us are not aware of is that plated bolts (Silver or Gold appearance) require different tightening torques than plain, Black Bolts. If this is news to you, then take a look at the fine print in the Unbrako table below (look for the asterisk).
Looking the fine print above (after the asterisk) you will clearly see:
Zinc plated bolts require 40% more torque than Black (Unplated) bolts
Cad Plated bolts require 25% less torque than a Black (Unplated) bolt.
The Silver or Gold “Zinc Plating” causes the threads to bind just like a dirty or rusty bolt and the bolt stretch (preload) is often not achieved after the required torque is applied and the Cad Plating acts a thread lubricant, increasing bolt stretch per applied torque. Now you know why High End ARP bolts for race engines are always BLACK!!
Proformance Motorsport only use Unbrako Cap Screws (Allen Bolts) or Military Spec 12 Point Bolts when (we can get hold of them) and due to the mud and dirt associated with off road racing, Proformance Motorsport always use Pacified Zinc Plated bolts to prevent corrosion. Our crew uses high end “digital” torque wrenches on every bolt we tighten and we always use thread locker!!
The question was “Why are we starting to see broken bolts even before we have driven the new race truck?”
We had some of the bolts removed and tested for hardness and tensile strength and found no problems. We checked and double-checked the manufacturers recommended torque settings and found nothing that would cause the breakages that were occurring.
Like many race shops, we had lots of different brands of and many different grades thread-lockers and all different colours and viscosities. They all seemed to hold the bolts in place ok and we never really problems with bolts that came loose, but we found that not all brands provided the same preload to the bolt. We found that some brands of thread- lockers were lubricating the bolt so well that the bolt would simply snap in half if the manufacturer’s recommended torque was applied.
After many hours of testing, and chewing through few sets of batteries in my very cool digital torque wrench I put together the results of my testing in a nice spreadsheet of my findings PROVING that various thread-lockers acted as lubricants and the lubricating properties were very unpredictable. After my bench and vice were completely covered in patches of red, green, purple and blue thread locker, I contacted Loctite Australia for some advice to make some sense of all my testing and results.
Chris Manger of Loctite Western Australia visited our premises and after a few minutes explaining our problems to him, Chris presented a test case where Loctite had previously performed similar tests in a controlled laboratory. The results of the Loctite testing clearly confirmed my findings that different brands and types of thread-lockers provide very very different amounts of preload/stretch when the bolt is tightened to the recommended torque. Some (Many) thread-lockers simply act as an unreliable and non-predictable lubricant and most provide unpredictable, non-constant preloads.
It seems that some thread-locker manufacturers have simply “forgotten” to advise the user of their products on the product label that the lubricating action completely can and will result in excessive preload resulting in broken bolts if you actually tighten them to the required torque using your bright and shiny torque wrench.
After more discussions with Chris from Loctite, we were advised that the Loctite range of Thread-Lockers are designed to simulate as close as possible to a dry bolt torque condition. After seeing the data we are now able to torque our bolts to the recommended torque settings without fear of snapping the bolt in half or having the bolt snap in half when we drive our vehicles.
In summary:
Check with the manufacturer of your bolts to obtain the required torque setting
Apply the torque “correction factor” if your bolt is Cad or Zinc plated
If using a thread-locking compound, use a product from a manufacturer that can provide you with data sheet
showing how the thread locking product will affect your bolt tightening torque and the resulting bolt preload.
If in doubt – Only Use Loctite Products like all the major race teams (designed to replicate dry assembly
torques/preloads)
I hope someone out there in race car land learns from our findings.
Happy engineering
Scott Bryce - Owner Proformance Motorsport
