torquing bolts, nuts

[tee-boy]

How necessary is it to torque to specification? Some engineers tell me that torque specifications are a bunch of #$$%. I know the traditional reasonings for proper torqueing (bolt holds, doesn't break, etc.) So what if I'm at 34 lb-ft or 26lb-ft on a 30 lb-ft specification? This may be a dumb question as I can already imagine the reponse I will get. What do you think/know?

 

[jim dozier]

Re: torquing bolts, nuts

Get new engineer friends. Tightening to set ranges of torque does 2 things. It sets the proper amount of clamping force for the part. Probably equally important is that for larger parts with multiple fasteners it prevents warpage such as in a cylinder head or other plate-shaped part. Perhaps a third but related to the first reason is to prevent stripping threads when either the hole or the bolt is made of a softer material than the other (i.e. aluminum block and steel bolt).

 

[BoatBuoy]

Re: torquing bolts, nuts

Old auto mechanic bragged to me years ago that he never torqued head bolts. When I asked how would I know how tight to tighten them, he answered "Just go ahead and break one off. By the time you get the broken end dug out, you'll know".<br /><br />Back in the 60's, I started rebuiding a '57 Johnson without manual or torque wrench. After about 3 trips to the machine shop for heli-coils, I at least got the torque wrench. Do as jimd said.

 

[Paul Moir]

Re: torquing bolts, nuts

The clamping force a bolt creates is related to it's turning torque by the pitch of the threads (threads per inch) and the coefficient of friction between the bolt and what it's threading into. There's friction between the threaded part of the bolt and what it's threading into, and between the bolt head and the thing you're bolting on.<br />Predicting the coefficient of friction is notoriously difficult since there's so many factors that can affect it. Like how clean the threads are, how the threads are lubricated, condition of the threads, that sort of thing. Maybe this is the reason those engineers you've been talking to consider the specifications to be ridiculous. <br />Actually, in some critical fastening situations rather than measure the torque applied to the bolt, the bolt is measured for how much it stretches. Since bolt stretch is related to its clamping force by factors that are more controllable, like bolt diameter and material, it is a more reliable way to measure clamping force. Unfortunately, you need more complicated equipment and access to both ends of the bolt to measure this. So it's application is limited to things like rod bolts.<br />I expect the reason for keeping the specifications tight is to limit the error. You don't need to make a bad thing worse.

 

[seahorse5]

Re: torquing bolts, nuts

Another thing to keep in mind is that aluminum "grows" as it is heated at a different rate as a steel or stainless steel bolt. The torque setting, which may seem low to car mechanics, is so the clamping action (force) between parts is correct when the motor is at operating temperature.

 

[18rabbit]

Re: torquing bolts, nuts

There are tables for bolt material/size/thread/dia/torque range for clamping. For applications where even surface pressure is not an issue I can see a statement that any given spec is a bunch of #$$% as long as you remain within the engineered spec/torque range for the bolt size/material.<br /><br />Ditto Paul. Bolts and their torque specs are engineered.<br /><br />I was reading an esoteric mil spec (forget the application) where bolts were to be installed, tightened, marked, removed counting the turns, measured, reinstalled with the same number of turns. Don’t remember why.<br /><br />As far as heating metal goes, yes, it expands when heated. However, a hole in metal shrinks…the heated material around it is expanding into the space that is the hole.

 

[Dunaruna]

Re: torquing bolts, nuts

I can't speak with any certainty about marine engines but in the auto industry head bolt torque is critical. All of the above is very important (clean threads, lubed threads & bolt washer etc).<br /><br /> Head bolts stretch, its called 'plastic range', the bolt goes through a designed stretch range until the ideal torque is applied (ever seen this,eg: torque to 45ftlb & then 90 deg). Once torqued it does not require retorquing in fact once loosened it must be replaced with a new bolt. The bolts do not return to their original length and therfore the torque specifications don't apply.<br /><br />In the example, going past 90deg puts the bolt into a different plastic range, it passes the engineers specifications. The desired clamping force becomes null and void, it can actually lessen the clamping force. Releasing it and torquing again becomes a useless exercise.<br /><br />Torquing to manufacturers specs is critical.<br /><br />Aldo

 

[robbyusa]

Re: torquing bolts, nuts

I am not as big on torque wrenches as most just because of the fact that in practice there always tends to be one or two of every set that you cant reach with the bulky torque wrench. <br /> In practice a good feel and even pressure on all seems better than torquing 9 out of 10 bolts perfectly with one too loose or tighte.<br /> I used torqe wrenches for years to find my feel for a properly tighted bolt diminishing. I enjoy trying to feel the bolt as it stretches transitioning from elastic to the brink of plastic deformation. I also enjoy feeling the little hitches and glitches that occer as I tighten past them smiling as I think about all the dunmb torque wrenches that cant tell the difference and stop prematurely. <br /> I only use torque wrenches that tell you real time torque as you are tightening, never the click when you get there sort that fool so many into false security. <br /> Remember that when working in critical areas like heads and rods undertightened fasteners will cause more broken bolts than overtightened because of the increased magnitude of cyclic fatigue as the fastener is stretched and released.<br /> The human hand can tell the diference between a bolt that just hit a bad threaad and stopped as opposed to a properly tightened bolt but a cheap torque wrench just clicks!

 

[jim dozier]

Re: torquing bolts, nuts

I was just trying to put myself in the position of an engineer who has designed and built a mechanical device (like an outboard engine) and was trying to write the repair manual for the consumer and mechanics. Would I specify torque numbers that would hopefully be used with clean threads and good torque wrenches or would I rely on the occasional psychic mechanic to feel the proper bolt tension?

 

[Mark42]

Re: torquing bolts, nuts

I like torque wrenches. Just remember that most torque specs are for clean dry threads. Lubing them up changes the amount of torque that is actually applied.

 

[dajohnson53]

Re: torquing bolts, nuts

Interesting discussion, thanks.<br /><br />What comes to mind to this very unskilled *NON* mechanic - who uses a torque wrench for installing spark plugs?<br /><br />The reason I ask this is that plugs are one thing that almost everyone does for him/herself. Lawn mowers, weed whackers, chain saws, snowmobiles, etc. etc. There is indeed a torque spec (for the larger engines, don't know about the lawn mowers). I've never owned a torque wrench, nor have I actually with my own eyes seen anyone use one for spark plugs. I was taught, probably like everyone else: "tighten that plug nice and tight, but not too tight or you'll strip it or it will break off". <br /><br />Comments?

 

[tee-boy]

Re: torquing bolts, nuts

I'm thinking the best solution is the one referred to in the manuals I have seen--Tap the hole with a clean tap, clean the bolt, run a die over the bolt, and torque to specs with a torque wrench. this eliminates much of the problems discussed earlier; except for the one about not being able to reach that 10th bolt in the turn sequence. I don't know what to do about that one.

 

[jim dozier]

Re: torquing bolts, nuts

For those hard to get to spots simply weld 6 pieces of drop-forged steel around all the obstructions so that the horizontal wrench arm is exactly 1 foot. Then hang the desired number of pounds from the end to equal the proper foot-lb torque. (Actually that is how (minus the welding) I get the proper torque on my Honda front-wheel drive axle nuts. I stand my 185 lbs the proper number of feet out to achieve the desired torque).

 

[Paul Moir]

Re: torquing bolts, nuts

I always torque my spark plugs. Hey, why not if the wrench and torque specs are sitting right there? It's just as easy afterall.<br /><br />jimd -

Then hang the desired number of pounds from the end to equal the proper foot-lb torque.

Don't laugh, but that's how I 'ball park' my torque wrenches to make sure they're close to right. Since I use mine only ocassionally, and not professionally I never end up sending them in to be recalibrated. A quick check once or twice a year helps keep the worry down. I clamp the drive square in a bench vise and hang known weights from it.

 

[18rabbit]

Re: torquing bolts, nuts

Originally posted by tee-boy:<br /> ... except for the one about not being able to reach that 10th bolt in the turn sequence. I don't know what to do about that one.

Don't worry about it. 90% is good enough.

 

[swist]

Re: torquing bolts, nuts

I never used a torqur wrench on plugs - they have metal gaskets - it's pretty easy to feel when you've hit the top of the gasket, then another 3/4 turn to compress it. Been doing this with cars and boats for 35 years with no problems.

 

[robbyusa]

Re: torquing bolts, nuts

I hired a mechanic to do some work on a set of catipillar D399' V16's I was in charge of in Gibralter where good help is hard to find. <br /> The Marina supplied a professional mechanic who took pride in the fact that he torqued all bolts to spec and as I was in charge he constantly bothered me to find the torque specs.<br /> All I could think was that I could have done the job myself easier than holding his hands through the project. On our first sea trial we developed exhaust leaks That were so severe we had to return to port. <br /> Turns out that he had tightend the intake and exhaust to each others specs and because he just waited for the click never caught his mistake. <br /> Also the torque wrench was huge reducing his feel. <br /> On a D399 Vcat the exhaust reside between the heads and the intake is bolted to the outside were the exhaust is usually located on a v motor.<br /> So if you use a torque wrench make sure you use your common sense an a proportional torque wrench to the job

 

[jim dozier]

Re: torquing bolts, nuts

So the moral of your story is to use the correct torque specs and an appropriate torque wrench.

 

[robbyusa]

Re: torquing bolts, nuts

I was actually hoping that readers of my post got a little more out of it than just that.<br /> Lets also add that one of the most scarry things you can do is hand an inexperienced mechanic a torque wrench! <br /> It lulls them into a false sense of security and slows down or ends there learning how to properly tighten a bolt without a torque wrench. <br /> You would be better spending your money on a book called "Zen and the art of Motorcycle maintenance". The psychic mechanic

 

[massimofinance]

Re: torquing bolts, nuts

Just wanted to say I really find this discussion helpful. I am rebuilding my 115HP Evinrude, and have to tighten the bolts for the Cylinder Head. The book says 18-20 Ft/LBS, but when I set my torque wrench (automotive) to some place below 20 (it starts at 20) It is tight and I am careful to not blow the bolt out. (Do I need to buy a smaller torque wrench?) Is automtive and Marine torque wrenches the same? The book says I need to use special torque socket part No.: 331638, what exactly is that?<br /><br />Thanks.<br /><br />MAX