buckshotglass;1114449 said:
Would you still do it if the studs are used, and have been heat cycled? I'm wondering that myself.
LOL...the million dollar question...I was wondering if someone would ask
mazzer;1114483 said:
it has more to do with a steel headgasket,and an alum head sitting on it
And...ummm no that's not it. The greater expansion of an aluminum head on a steel block has a lot to do with it though
Read this first from ARP's website concerning using a torque wrench to install their hardware:
ARP said:
If the stretch method cannot be used in a particular installation, and the fasteners must be installed by torque alone, there are certain factors that should be taken into account. ARP research has verified the following “rules” pertaining to use of a torque wrench:
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, 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.
3. Surface finish is also important. For example, black oxide behaves differently than a polished fastener. It is therefore important to observe the torque recommendations supplied with each fastener.
NOTE: It is possible for even the most expensive of torque wrenches to lose accuracy. We have seen fluctuations of as much as ten (10) foot pounds of torque from wrench to wrench. Please have your torque wrench checked periodically for accuracy.
"Seasoned" hardware has the advantage of heat temper, it also has the disadvantage of "work hardening" from thermal cycles. The biggest impact is proper preload on the fastener...if done properly, it will last almost indefinitely. If done improperly where the thermal expansion (dynamic load) force exceeds the preload and results in cyclic tensile stress...it's "work hardened" and becomes brittle. To answer the question...it depends if the hardware was installed properly in the first installation. A previous over torque also has a significant impact. Fasteners are designed to stretch within a specific range...if over torqued, it can stretch and not rebound. If the fastener is longer than manufactured (only .001") it is in a partially failed condition.
To install properly, there is a step most people miss...from above:
"...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."
This does two things...1) reduces friction to the point where readings on a torque wrench is as accurate as possible and 2) insures a proper preload by torquing to 90% of the spec on each of the 5 cycles stretching the bolt for proper clamp, then to final torque on the last. If folks would do this, it begs the question if a re-torque would ever be necessary. It takes quite a bit of time...most people don't take that time to preload head stud/bolts properly.
Realizing 95% of the people here are not going to use this technique, I usually recommend a re-torque. If you think about it, the thermal expansion of the head for a minimum of 5 heat-up/cool-down cycles kinda does the same thing...stretches the fastener to produce the clamping force it's capable of, but reduces preload. A re-torque resets preload to where it's greater than the dynamic load caused by thermal expansion. You are now at exactly the same place
Clear as mud?
BTW - I have never seen a fastener loosen up (including OEM) when I used the proper technique. I have seen them loosen up (including studs and seasoned hardware) when only once torque cycle was used.
