greiswig wrote:
So being inherently skeptical as I am, and wanting to get more of the straight scoop, I called a friendly guy at ARP and talked to him quite awhile. My dad was a metallurgist, so I grew up getting edumacated about things like yield, malleability, elasticity, etc.
Anyway, the guy made pretty persuasive arguments. His bottom line was that he would tighten every fastener the same (inner row and outer row), in the following stages: 40, then 85, then 125 lb/ft. And no, he says you're NOT supposed to loosen each one between stages.
Why three stages? - their recommended number of stages, because doing many more than that and you risk entering a situation where each stage is actually not adding as much as you think, because it is getting closer and closer to the margin of error and sensitivity of the wrench.
Why no more than 125 lb/ft? - because that is the maximum torque that should be put on those studs, no matter what the application. It puts the fastener at the right point in its elasticity range, so the clamping force and dimension will be unaffected by yield (which might happen if you go farther than that) and will still be plenty to make a good seal as long as your gasket is good and both surfaces are good. In other words, if you start to make the stud yield, you will end up with net less clamping force for more torque value...the opposite of what you want.
As for why the outer rows have a different torque spec than the inner rows in the manual, he doesn't have any idea. He pointed out that they make the same recommendation for Cummins, etc. that use the same diameter studs and MLS gaskets.
I also asked him about the difference in clamping force for a given torque value given a different thread pitch. He reluctantly agreed, but said that it was little enough difference that I shouldn't worry about it, and in this case it has been taken into account in the 125 lb/ft. spec they recommend.
So that's what ARP says about it. He also admitted that they have not bought one of these engines to test, but indicated that doing so would principally tell them whether the threads in the block were at risk before that 125lb/ft. limit was reached.
If you read the ARP literature or do the math, 125 FPT is 75% of the yield torque for that stud, using the ARP lube on the threads and the nut/washer assembly. Max torque is 165 FPT torque, but you will cause general yielding of the head before you get to that point. I did not, do not and would not recommend loosening the nuts between torque steps, and if you re-read the thread, you'll see that I recommended multiple torque steps, too, but I have a very good torque wrench.
As regards the slightly lower torque for the outside studs, the reason is that the outer rows of bolts/studs are at the outside edge of the head and block, and the head is prone to crushing if you exceed that torque value. I know this because I did it. As regards the engagement depth in the block, I checked that and there was plenty for typical CI material, although I did not do a finite element stress analysis of the block, lacking the time, interest or software to do so; my assumption was that as the studs slightly exceed the factory bolt engagement, and since the studs replicate what a real TTY bolt ought to do, then it was a reasonable assumption that this install would not cause the block to yield. Especially since the studs cause less stress on the block when installed and they distribute the force across the block threads more evenly.
Read the thread, make your own calculations and do what seems best to you. I can say that I have run this now for about 14k miles with no problem at all, and so have a number of others. My considered judgement is that the use of studs significantly reduces the chances of a warped head, loose "TTY" bolts, or a HG leak, but there are lots of CRDs that don't have them. Yet. My guess is that in 5 years, most CRDs still running will have studs installed. The factory installed bolts are not conducive to engine longevity.
Login
Register
FAQ
Search
