Torque wrench spec strips threads

[Seymour Snerd]

...cast iron head is still going to be a fair bit cheesier (technical term....) than a grade 8 equivalent bolt? In other words, still the weakest link?.

I think it would depend entirely on the depth of thread engagement. But what's interesting to me isn't "which breaks first" but rather "why did it break at all?" And (again) to get to the bottom of this particlar case we need to know the thread conditions (oily?).

 

[Seymour Snerd]

The source I use is 'How to build small block Ford engines' by Tom Monroe. Now using this procedure has been ok with one major exception -The inlet manifold, Monroe describes a 2 step procedure and sequence for torquing these bolts 1/ 15 to 17 ftlb 2/ 23 to 25 ftlb

I finally got home and opened up my copy of the book. On page 158 he says "Finish manifold installation by torquing bolts in the recommended sequence." Above that he has this picture:



So now I'm really confused. The picture clearly says 5-7 followed by 9-15.

Are we talking about two different books?

And, BTW, on page 154 "When using bolts, lubricate the threads" although this is buried in the context of head bolts.

 

[Dave Bilyk]

I'm with Cliff on the point that standard bolt torques are to do with the bolt, rather than the kit it is inserted into. They are usually associated with attaining a % of yield stress in the bolt, but when it comes to a weaker female thread material the torque has to be de-rated to suit, or choose a weaker grade of bolt that will fail before the female thread. (rather have a broken bolt than a pulled thread right?).
In an engineering application where pressure containment is involved, such as cylinder heads, or for main bearings and rods, that have to contain a force, the selected bolt diameter, and grade, is all to do with the clamping pressure pre-load required to contain that internal pressure or force without lifting or separation. This is the situation where torque is critically important and torques need to be carefully established and adhered to. It is bolt tension load, not torque that is the desired end result and unfortunately there is the variability between the two that gives uncertainty in the result. Incidentally for larger studs 2" diameter or more on end covers for pumps, we are containing loads of several hundred tons, so use a hydraulic tensioner to pull the studs to the required pre-load, nip up the nut, then release the tensioner. The result is a known tensile load without a torque in sight.

For applications like manifolds, we don't generally need to worry about internal forces, only effective sealing and maybe vibration. In these cases I always use intuitive torque.
As far as aluminium goes, I am always on the side of extreme caution and on any case use high temp copper or moly anti-galling compound whether needed or not.

FWIW, I bought OEM bolts for a Mini flywheel, tried to tighten to recommended torque and felt a couple yield. I phoned Minisport who did racing builds, they said 'yeah that always happens, we now use 5 ft.lb less and don't have problems' the moral is never trust anything anyway.

Dave

 

[Dave Bilyk]

Alan, Tom, regarding torquing sequence, intuitively I always feel I should be working from the centre outwards, like smoothing a piece of cloth. It moves any 'rucks' out, rather than trapping them where they are. Maybe with a soft gasket in place that doesn't figure so much, but it does go through my head every time I see a typical tightening sequence. Any comments?

Dave

 

[Seymour Snerd]

FWIW, I bought OEM bolts for a Mini flywheel, tried to tighten to recommended torque and felt a couple yield. I phoned Minisport who did racing builds, they said 'yeah that always happens, we now use 5 ft.lb less and don't have problems' the moral is never trust anything anyway.

I grant that these occasional cases occur where the provide torque spec is just plain wrong, but in my experience the vast majority of "trouble with torque" is because the "mechanic" didn't follow the directions. So I'm going to stand by my usual position that following torque specs gets a bad rap for all the wrong reasons. The vast majority of torque specs were arrived at competently and with reasonable margins, and if the threads are kept clean, new fasteners used with an accurate wrench, and lubricated (or not) according to instructions, will do exactly what they are supposed to do.

The case we have here, I believe, supports my position.

Using these "corner case" failures like the mini flywheel bolts as an excuse to "torque by ear" is in my opinion irrational and dangerous.

 

[Dave Bilyk]

Alan,

Using these "corner case" failures like the mini flywheel bolts as an excuse to "torque by ear" is in my opinion irrational and dangerous.

it was not my intention to suggest that, only that torque values are a minefield, and suspicion, care and attention are the order of the day.

In the rest of my post I only suggested intuitive torquing for non critical items like manifolds, and for items subjected to pressure and force I said 'torque is critically important and torques need to be carefully established and adhered to'.

So no disagreement here.

Dave

 

[Seymour Snerd]

it was not my intention to suggest that

Oh, I know. You clearly get all this. I was reacting to smart people here and elsewhere who deny the value of routinely measuring fastener torque. It's a debate that never seems to go away. Sorry for the misunderstanding.

BTW I have one of those corner cases: Porsche 928s have steel coolant drain plugs in their aluminum blocks, and a torque spec to go with them that, although probably reasonable with a new block on the assembly line, easily strips the threads in the real world. And that day I thought all I was going to have to do was drain and replace the coolant.....

 

[Seymour Snerd]

..I always feel I should be working from the centre outwards

I hadn't noticed that about this one. You're right the heads I've worked with usually spiraled out from the center. OTOH, unlike a head, a 4bbl intake manifold is pretty non-uniform in stiffness across it's length. Perhaps that explains this pattern. ???

I looked at some other books (eg FE) and the do a third pattern: inside first, outer corners next, in between last.

 

[Seymour Snerd]

I finally got home and opened up my copy of the book. On page 158 he says "Finish manifold installation by torquing bolts in the recommended sequence." Above that he has this picture:

View attachment 56134

So now I'm really confused. The picture clearly says 5-7 followed by 9-15.

Are we talking about two different books?.

Ok I'm the one with the wrong book, but it's not very comforting. It's the later Monroe book "How to Build Small Block Ford Racing Engines" that contains my diagram.

Ross is using Monroe's earlier book "How to Rebuild Small Block Ford Engines" (mine is dated 1978!) that contains on p.119 a similar picture calling for 25 (or in the case of Boss 302, 32) as the final torque figure. Not only that, but the text calls for 22, not 25.

This really sucks. Tom Monroe has some explaining to do. Although I can imagine that after 30-40 years it might not be realistic to apply the original mfr. torque to "real world" blocks and heads, although the fact that the latter book is about racing engines and discusses aluminum heads is a better explanation for the different figures.

Dave's right, you can't trust anything. What a mess.

PS: Edelbrock instructions say "Torque all of the manifold bolts in two steps by the sequence shown in Figure 2 to 18-20 ft/lbs."

PPS: This is starting to bug me. Why such high torque for fastening down an intake manifold? (32 for boss 302??). Is this about stiffening the engine?

 

[Dave Bilyk]

Alan, a lot of the differences might stem from differing viewpoints and sources of information, and what the engineer who calculated the torques wants to take into account (rightly or wrongly). When working with stresses, he might choose the minor diameter of the thread, or the bolt shank diameter and so arrive at a different torque. Then there will be a choice of different dry and lubed friction coefficients. He might use common coefficients, or he might use coefficients appropriate to the materials used.
Here's another thing, he may want to take into account the real world situation where the surfaces are not quite flat, and ask the question 'what pre-load do I need to account for this and make sure that I will get a seal even when there is a small warp?'
If a thicker softer gasket is used, this wouldn't matter and a small torque would guarantee a seal, while for a thinner stiffer gasket, a higher pre-load / torque might be required to deal with imperfections and warp.
What about a backfire, how much load will that produce, and we don't want the manifold to lift so what pre-load / torque do we need to deal with that?
What about differential thermal expansion, and so it goes on.
Some engineers will take things into account, some not, and there are differing standard methods and simplifications to choose from, hence the variance of the data.

As for the tightening sequence, it probably doesn't matter if you use say 4 steps or more, you can use any sequence you want.

Dave

 

[zollis]

Allan you are correct in spotting the 22 in the text . Page 119 left hand coulomb 13 lines from the bottom .

Do you really want to call Tom to do explaining ?

Keep in mind that book has 160 pages . With thousands ( if not millions ) of words letters and digits. One digit is "off " . Could have been a typo.

Be reasonable.


I have used and enjoyed that book for many years.





Z.C.

 

[Seymour Snerd]

Allan you are correct in spotting the 22 in the text . Page 119 left hand coulomb 13 lines from the bottom .

Do you really want to call Tom to do explaining ?

Keep in mind that book has 160 pages . With thousands ( if not millions ) of words letters and digits. One digit is "off " . Could have been a typo.

Be reasonable.

I have used and enjoyed that book for many years.

Z.C.

You're picking on by far the least important of the inconsistencies I found. And I don't need a lecture on the complexity of technical writing. I did that for a living for awhile myself, and it involved much more dangerous systems than car engines.

I know what's involved and it's called proof-reading. HPBooks says the book is worth $18. They can afford an editor to check for this stuff. The two figures are inches apart on the same page. I don't think it's unreasonable to expect better proof-reading in a mass-market $18 paperback technical book. But that particular figure is not the one needing explanation. It's the one that changes from 25 to 15 between two books by the same author.

Yeah the book's great. So are a lot of other engine building books. I'm not picking on Monroe; it just happens that it's his book that Ross had the problem with.

My real point is that the general automotive tech-writing world's casual behavior with respect to fastener torque has led a lot of amateurs to dismiss the issue as unimportant or impossible to figure out. This is just one example, but the general problem is serious.

 

[Roy Richards]

I finally got home and opened up my copy of the book. On page 158 he says "Finish manifold installation by torquing bolts in the recommended sequence." Above that he has this picture:

View attachment 56134

So now I'm really confused. The picture clearly says 5-7 followed by 9-15.

Are we talking about two different books?

And, BTW, on page 154 "When using bolts, lubricate the threads" although this is buried in the context of head bolts.

The pic you posted is for a Boss 302 intake and I don't remember the specs for it but that seems a little low.

Roy

 

[Pat Buckley]

I use ARP studs in this application.

Makes life a bit easier.

 

[T. Stinchfield]

I recently had the same problem with stripping out intake manifold bolt threads. When it happened I thought I had lost a cylinder head....Fortunately the bolts I had were simple too short. As an exercise in overkill I got some grade 8 5/16 course all-thread and cut it into studs. Now I am absolutely sure that all of the threads in the head are used. It also makes aligning the intake easier as it will just drop on over the studs. From what I remember I have it torqued to between 15 and 18 ft/lbs.

 

[Cliff Beer]

I noticed the last time I had the inlet manifold off my little lowly Ford Racing 302 that the manifold didn't line up that well with the heads. I actually loosened (then later re-torqued in the proper way.....) the heads, gradually tightened it all up together (heads and manifold, and also heads/block) then did the heat cycle/re-torque drill. Perfect now, no drips or leaks or issues.

Have to admit, I rarely use a torque wrench. Maybe I'm wrong but I think I have a pretty good feel for all the variables here. I'm no dummy, my old company made the most precise thread gauges in the world, and was responsible to checking the structural integrity (inc. bolting of sections) of much of the worlds oil lines. There were a lot of propeller heads involved who devoted their entire professional existence to the minutia of bolting one thing to another.

I know how a brake caliper bolt should clamp, same for a head bolt or a fender bolt, what specs each should be, and what's going to stay in place, and what's not. There are a lot more variables here than a single torque chart can fully reflect. Frankly, to use charts you need a different chart for different metals, dry or lubed, surface degradation due to environmental factors, even a different chart for different weather conditions. And that's just a few of the static variables. Dynamic variables such as the thermal loads, incremental pressures, and contamination of the bore and threads by chemicals in proximity also affect things dramatically.

But if you don't have a good sense for it all then a torque chart is a fine basic starting position...may result in a few stripped threads or loose bolts here and there though. Just my little $.02.

 

[Seymour Snerd]

....I rarely use a torque wrench. Maybe I'm wrong but I think I have a pretty good feel for all the variables here.

Hey Cliff -- you started a thread recently that begins

"and your car runs like crap because.....<!-- google_ad_section_end -->..some HACK has been there before you!....Don't be a HACK!<!-- google_ad_section_end --> "

OK then use a torque wrench. :laugh:

 

[Cliff Beer]

Hey Cliff -- you started a thread recently that begins

"and your car runs like crap because.....<!-- google_ad_section_end -->..some HACK has been there before you!....Don't be a HACK!<!-- google_ad_section_end --> "

OK then use a torque wrench. :laugh:

Ha! I'm busted.......