Go BottomRe-Torquing Head Bolts Valve Adjustment
 | Thomas M. Meza
Join Date: Posts: 1 
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2000-07-06 17786
I'm about to undertake the first maintenance required valve adjustmenton a 1725 Ford. What is the consensus on the practicality or necessity to loosen thentorque to specifications each of the head bolts? Or should new head bolts be installed and torqued to specifications? Your comments on this issue would be greatly appreciated.
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-07-06 17796
What an interesting thought! I just finished reading a technical paper on
bolts - mostly on arguments for ways of measuring bolt strain. Hopefully
this is what you are doing when you torque a bolt of course....making it
into a stretchy spring...and then using that spring to clamp the cylinder
head down to the block - compressing the gasket at the same time. BTW, the
tech paper (about 100 pages) is from Bowman....maker of Bowmalloy bolts. If
the subject interests you, I think that they sell copies for $20 or just
give them away depending on how they are approached.
I don't know your level of expertise on bolts, but basically torque is a
pretty crude way of measuring what you want - which is to stretch the bolt a
certain amount. The only time that you really get the stretch that you are expecting out of torque values is when the two mating threads are clean and dry, and the head of the bolt is clean and dry plus it is bearing on a hardened washer.
The slightest bit of oil, or water, or even dust on the threads or
under the bolt head can allow you to stretch the bolt more than 30% more
than you had intended for the same reading on a torque wrench. On a soft
grade two bolt this might just deform it (losing the spring property); on a
hardened head bolt which are usually grade 8 or 10 it could easily snap it.
It depends on who built the bolt, the value that the design engineer put
into the torque specs (not all engineers are equal), and of course what the
original guy down on the assembly line put onto the bolt when he installed it. Did they do it dry or with lube? What % of lube is left or added by the working
of the engine? How about the washer? And the washer to head contact? A whole lot of what a torque wrench measures isn't bolt stretch at all, it is head and washer friction...
Er....cut to the chase. This is all my opinion, and I will call on a
friend who does a lot of warranty service today and see if he agrees. I
wouldn't loosen the bolts to re-torque them. You run a risk of changing the
forces on the head gasket, and there is no guarantee that the re-torque will
come anywhere near to stretching the bolt as it was originally stretched. For example, I would think that there is a good chance of having at least a little roughness and corrosion on the threads after a break-in period. ..And if true, this will cause the bolt to require a substantially higher torque to achieve the same spring tension. So if you merely loosen and re-torque to the same value, you are may have REDUCED the forces holding the head to the block...Something to think about, isn't it!
I'll break this into two parts and post the rest of it below.....R.L. ....
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| Roger L.
Join Date: Jun 1999 Posts: 0
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2000-07-06 17799
Well, continuing right along..... My argument for not re-torquing is that the tension of the bolt should not have changed unless it came unscrewed (unlikely), and what you really want to be doing is to insure that the compression of the Head Gasket has not progressed to such a point that the bolt is no longer clamping it properly. Compaction of this gasket was a problem years ago, and with gasoline engines. It isn't a problem with modern diesel gaskets, which are required to be much stronger than gasoline engine gaskets in the first place.
What I would do is to put my carefully calibrated torque wrench on each
head bolt and twist them up to say 10% UNDER the recommended torque. They
should not move! This insures that the bolt has not loosened in service, and
that the gasket/head did not do something odd at that clamping position.
Then leave them there.
I think I've talked enough! But you did ask a question dear to my
heart...We could also talk about calibrating torque wrenches, I prefer the
beam and pointer type, although the break-away type is decent as long as you
calibrate it at least once in its life....
Enjoy! Roger ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley 
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2000-07-07 17804
I'd add that tightening is usually done in a particular sequence of head bolts. The pattern is usually circular starting from the centre of the head, across, and progressing outward. It's probably a good idea to use such a sequence when checking the torque.
For the reasons Roger mentions, I don't think I'd loosen the bolts and re-torque them either. An assumption in the torque specs probably is that the torquing is done in shop conditions and using new nuts and bolts.
I think the case for checking the torque is that the factors Roger mentions generally would to produce high rather than low readings. So, if the nut moves when it's torqued to value, the bolt was almost certainly below specs. However, if the bolts are loosened and then re-torqued, there is no certainty that the readings are accurate.
I think that some loss of torque can be expected in high hour engines from stretching of the head bolts themselves. I know that replacing the head bolts as part of an engine rebuild is often recommended. However, I imagine that loss of torque from such sources is built into engineering design, and failure due to these factors shouldn't be expected before a rebuild is required. ....
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| Rob Munach
Join Date: Posts: 1
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2000-07-07 17806
I would also like to add that torque values are usually based on the bolt being in motion when the torque is checked. This is because the coefficient of static friction is higher than the coeff. of kinetic friction. If you stop tightening the bolt and then try to start tightening it again, the torque reading will be very high until the bolt starts "moving" That is why people first loosen bolts to re-torque them. They have to be loosened to the point that the static friction is less than the kinetic friction at the proper torque. ....
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| TomG
Join Date: Feb 2002 Posts: 5406 Upper Ottawa Valley
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2000-07-08 17815
Rob: Thanks for the note. What you say makes sense, and I now remember that readings should be taken after the nut cracks. I guess that leaves the idea unresolved. If both ideas are true, then whether to loosen or not? Maybe the resolution is answering the question whether checking the head torque for purposes of valve adjustment requires re-torquing the head or not. One approach might be reasoning that the purpose of checking the torque as part of a valve adjustment is to correct any gross looseness that might be progressive. Head tightness certainly affects valve clearance. I imagine it's possible to loose all valve clearance in areas around loose head bolts.
As always, a lot to consider. I'm coming up on a 600-hour valve adjustment myself. Suppose I should figure this one out. However, my instincts are to disturb something that is working as little as possible.
....
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