89supturbo said:
its not the lube used that affects the tourque spec its the metal that the bolt is made out of, bolts are tourqued to 70% of the elastic limit, the bolt is actually stretched to increase the clamping force, with a stonger metal the elastic limit is much higher and therefore to get the same clamping force you need more tourque
Actually we are kinda talking about the same thing...but, you're mixing apples with oranges
He's talking about the stretch method of determining preload on the fastener...it requires a stretch gauge to be done properly. Stretch is measured in thousands of an inch...I couldn't find the spec, but for most bolts/studs it is somewhere around 0.005".
I was talking about the torque wrench method of installation...perhaps I wasn't clear in my statement above...the lube used determines the friction coefficient that must be overcome for proper torque. From the ARP site:
"1.
The friction factor changes from one application to the next. That is, the friction is at its highest value when the fastener is first tightened. Each additional time the fastener is torqued and loosened, this value gets smaller. Eventually the friction levels out and becomes constant for all following repetitions. Therefore, new fasteners should be tightened and loosened through several cycles before applying final torque. The number of times depends on the lubricant. For all situations where ARP lubricants are used, five cycles are required before final torquing.
2.
The lubricant used is the main factor in determining friction, and therefore, the torque for a particular installation. Motor oil is a commonly used lubricant because of it’s ready availability. If less friction is desired in order to install the fasteners with less torque, special low friction lubricants are available. With special lubes, the required torque can be reduced as much as 20 to 30 percent. It is important to keep in mind that the reverse is also true. If the torque value has been specified for a particular fastener on the basis of low friction lube, installing the fastener with motor oil will result in insufficient preload; the torque has to be increased to compensate for the extra friction caused by the motor oil."
The material used in the bolts does determine the amount of stretch required to reach the proper clamping force. However, since very few of us own a stretch gauge, the torque wrench method is commonly used. When using this method and using the ARP spec for the lube used, the bolts are torqued to 75% of the rated yield strength of the bolt. Since moly is a low friction lube, you can reduce the stud torque to 81 ft/lbs instead of the 105 ft/lbs for motor oil.
Note: To do this correctly, you should torque through several loosening/tightening cycles. ARP (per above) recommends 5 cycles.
