Insane crank pulley bolt torque???
05-21-2010, 09:45 AM
#1
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Insane crank pulley bolt torque??? I've asked before, but nobody seemed to know or my question was unclear. Why is the bolt torqued to such crazy specs? Or why does the bolt need to be "seated" at that high rating? Gen 1's never had a balancer bolt that needed to be that tight, why the Ls1? Maybe an engineer on here would know why. Just curious that's all. I know the process and specs on installing a pulley and bolt so, that's not the question. Thanks.
05-21-2010, 11:48 AM
#2
Crazy? Not really...
1. It is a big bolt (16mm if I'm not mistaken)
2. You need to achieve a certain amount of preload on the bolt to keep it from backing off (usually causing failure), but not so much that you introduce too much stress on the bolt (which can also cause failure)
3. The amount of preload required depends on the material used, but in general goes up with the size of the bolt. Big bolt = more torque to properly preload it.
You can reach the correct level of preload required a few different ways - GM uses a torque to yield bolt design at the factory, so the bolt has to be turned a specified number of degrees after reaching a set torque value. The bolt ends up yielding (hence why they shouldn't be reused), but the method usually provides a consistent amount of preload, especially in a production environment (they have machines that can tell when a bolt has yielded - for head bolts, it tightens all bolts at the same time). It is apparently not as affected by friction as well, which is hard to control consistently, especially in a production environment.
You can also do the specified torque method, which is what you are supposed to do with an ARP bolt. The specified torque value given by ARP is around 75% of the fastener's yield strength, but they usually provide two different numbers - one using their moly lubricant, and one not. The one using the lubricant will be a lower number since the friction is more controlled with that versus 30wt oil or just putting it in dry, at least according to them.
The other method of tightening a bolt properly is by using a stretch gauge - connecting rod bolts are a good example of this. With the crank being a blind hole, you won't be measuring stretch any time soon like you can with a connecting rod. From everything I've ever read, the stretch method is by far the most accurate for measuring preload.
There might be other reasons, but the torque required isn't just some random number designed to frustrate the shadetree mechanic trying to do a pulley swap on an LS1.
1. It is a big bolt (16mm if I'm not mistaken)
2. You need to achieve a certain amount of preload on the bolt to keep it from backing off (usually causing failure), but not so much that you introduce too much stress on the bolt (which can also cause failure)
3. The amount of preload required depends on the material used, but in general goes up with the size of the bolt. Big bolt = more torque to properly preload it.
You can reach the correct level of preload required a few different ways - GM uses a torque to yield bolt design at the factory, so the bolt has to be turned a specified number of degrees after reaching a set torque value. The bolt ends up yielding (hence why they shouldn't be reused), but the method usually provides a consistent amount of preload, especially in a production environment (they have machines that can tell when a bolt has yielded - for head bolts, it tightens all bolts at the same time). It is apparently not as affected by friction as well, which is hard to control consistently, especially in a production environment.
You can also do the specified torque method, which is what you are supposed to do with an ARP bolt. The specified torque value given by ARP is around 75% of the fastener's yield strength, but they usually provide two different numbers - one using their moly lubricant, and one not. The one using the lubricant will be a lower number since the friction is more controlled with that versus 30wt oil or just putting it in dry, at least according to them.
The other method of tightening a bolt properly is by using a stretch gauge - connecting rod bolts are a good example of this. With the crank being a blind hole, you won't be measuring stretch any time soon like you can with a connecting rod. From everything I've ever read, the stretch method is by far the most accurate for measuring preload.
There might be other reasons, but the torque required isn't just some random number designed to frustrate the shadetree mechanic trying to do a pulley swap on an LS1.
05-21-2010, 08:20 PM
#3
I've asked before, but nobody seemed to know or my question was unclear. Why is the bolt torqued to such crazy specs? Or why does the bolt need to be "seated" at that high rating? Gen 1's never had a balancer bolt that needed to be that tight, why the Ls1? Maybe an engineer on here would know why. Just curious that's all. I know the process and specs on installing a pulley and bolt so, that's not the question. Thanks.
05-22-2010, 05:29 AM
#4
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As was stated, torque value is a function of bolt diameter and material. That said, your Gen I engines had keyed balancers, the key being used as a locator. The LS* motors do not use a key and depend on the "crazy" torque spec to keep the balancer properly located so it doesn't spin on the crank snout.
05-22-2010, 08:08 AM
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GM must have seen something real "ugly" during R&D to warrant that size bolt and specs. Everything above clears it up for me, should've thought a little deeper into it. I know it takes time out of your day to respond to off-the-questions like mine so I really appreciate your time, thanks again!!!!
