Will reusing torque-to-yield bolts cost you an engine?
Engine building has progressed so much in even recent history that it is borderline insane how complicated it can get. Of all the new technology and progress put into building ever-higher horsepower and reliability, one of the interesting pieces that has now become standard is torque-to-yield bolts. They make assembly a game of degrees and always provide lively discussion about potential re-use. Luckily, Toque Test Channel on YouTube just released a video that aims to separate fact from fiction when it comes to these special bits of hardware.
The concept of torque-to-yield is less scary than the name implies. The clamping force of any bolt is basically what material the bolt is made from and how much the bolt stretches when tightened. Using torque setting in foot-pounds does set a tension, but it can be inconsistent based on how precise the threads were cut or if there is unexpected lube or friction under the head of the bolt. Remember, tightening something too much can be just as bad as too loose when it come to things like critical engine components.
And that is why math entered the process. Threads are nothing more than a spiral incline plane, one of the six simple machines most everything is based on. That’s right, we are going back to Renaissance times where the foundations for mechanics were set. The thread pitch of a bolt determines exactly how far it will stretch when rotated a given number of degrees. This is why torque-to-yield bolts use a light torque spec plus an angle for a precise stretch of the bolt. This allows for calculating exact clamping force by knowing exactly how far that bolt is stretched.
Now the part that sparks great debate: Can that stretched bolt be re-used? Will it have the same clamping force once stretched? Less? More? Some builders even believe torque-to-yield bolts are better after they have been used, despite R&D departments funded by millions of dollars stating that TTY bolts should not be reused. Could that just be a cash grab?
The Torque Test Channel designed a rig to scientifically test the clamping force, and then it tested all the internet myths, including whether or not you can re-use that already stretched bolt. In the hydraulic rig, the bolt was tensioned and then cycled to see just how much it stretched under 6500psi of force. Then it was taken apart and reinstalled. The same test showed that the bolt stretched significantly more than a fresh bolt, all the way to the point of failure.
It’s a tough thing to order new bolts for everything when a visual inspection tells you the ones you pulled are good. It’s even more difficult when there may be a backorder or other delay in getting those parts. This test tells us that wait is worth it if you want to do it right. Could your engine survive if you reuse head bolts? Absolutely yes. That doesn’t make it the right thing to do.
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