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donmlewis 10-27-2005 01:56 PM

flex head torque wrench worth the $?
I am going to replace my old Sears torque wrench, am considering SK and was wondering of a flex head model is worth the premium on price, e.g. an SK 74251 1/2" 25-250 ft-lbs retails for about $83 and an SK 74256 flex head 1/2" 25-250 ft-lbs retails for about $137. It may be that other factors add to the price but I am unaware of them. How often is a flex head really handy and wouldn't using an extension work in most cases?

crazy larry 10-27-2005 04:46 PM

just thinking outloud..... it's my understanding that using an extension will change the actual torque applied, so wouldn't a flex head do the same? I dunno.... :confused:

oldred 10-27-2005 05:16 PM

It is a myth that using an extension changes torque readings, that is as long as the extension is held straight and not allowed to lean over from the fastener. A crows foot type extension wrench will however change torque values and must be compensated for.

onesweetpink63 10-27-2005 05:25 PM

sounds cool. i give 220$ for my mac wrench :( as for a flex head, ive built many types of engines, and hadnt the need for one

oldred 10-27-2005 05:44 PM

Mac torque wrench :thumbup:

As far as the flex head goes I personally can not remember a time when I could not finish a job for lack of one and in fact have never owned one but that is not to say they are not useful and maybe I have missed something.

that SK is a darn good wrench for the price.

onesweetpink63 10-27-2005 06:36 PM

ya, i think mac tools is a sk distributor also

Mr.NutCase 10-27-2005 07:51 PM

i would buy it

Bad Rat 414 10-31-2005 12:32 PM

My Snap-On torgue wrench has a flex head and sometimes it does come in handy.

Canadian Charlie 10-31-2005 01:09 PM

I just bought a new torque wrench today after work. I was considering ordering a Snap On Tech wrench on line but at over $300 I decided to go for something a little cheaper. I bought a Beta torque wrench, they are Italian made and comes with an interchangeable head which will come in handy for torqueing down the centre bolts on an intake. It has a range of 30-150 ft/lbs which is great for just about everything I need to torque down. I got it at a great price, something like $180 US Dollars. I know I could have gotten a regular Snap On wrench for that price but after shipping costs to Europe and taxes the Beta was worth it.

farna 11-03-2005 01:22 PM

You need to talk to an engineer on the extension not affecting torque thing. That's not exactly true! While a short, stout extension won't affect torque a noticeable amount, the longer the extension, the more likely it is to affect torque. For small numbers, I'm guessing 50 lb/ft or so, it probably won't make enough difference to be concerned about if the extension is kept at 6" or less, 1/2" drive. With 3/8" better keep that to no more than 4". The longer and thinner an extension is, the more there will be some torsional flex. With a 6" 3/8" drive extension you might "lose" 2-3 lb/ft through (unnoticeable) twisting of the extnesion itself. Most fasteners won't mind the 2-3 lb/ft "loss", depends on how critical the torque is. Don't take any of these lengths and loss figures as gospel -- I'm guessing and using the "rules of thumb" I've picked up over the years. But extensions DO affect torque. Some machinists and/or engineering books will have a couple pages of formulas showing how to compensate.

I found this while searching for a good source, it's an engineer trying to be a bit humorous, but works!


That which gives you an offset position for the wrench from the
socket . .

For that we would have a torque loss equation . . .

You apply 50 ft-lbs of torque to a six inch extension to the
socket . . the extension is steel and steel is still the most
"elastic" material we have . . . .

Elasticity is defined as the amount of load that can be applied
to a material under certain constraints and the material will
return to its original shape . . .
Full text:

This link: has a chart with the formula used for an extension on the drive (between wrench and socket) and for change in length of wrench.

Make sure you click on this link too...

oldred 11-03-2005 06:58 PM

Farna, this old wives tale has been argued for years and no an extension will not change the torque value. It does not matter if the extension flexes you would simply pull the wrench farther. It is simple laws of physics, whatever force you input into one end of that extension is what is going to be exerted on the other end period. If you were getting less energy out of it than you put into it where on earth does that energy go? If the extension twists then as I said the wrench would require that much more input as long as it is moving but when the torque setting is reached and movement is stopped then at that point the input force and the resistance is equal and that is what it will read. Now if you allow the extension to lean over from the center of the fastener so that the wrench turning center is no longer centered over the fastener then you would get an erroneous reading because of the leverage factor, but this is something else entirely. For this same reason a crows foot extension must be compensated for and some wrenches include a chart for doing this but again this is for leverage because the wrench is not centered over the fastener. If an extension required compensation then they would include that also.

Incidentally that link is refering to a crows foot wrench type extension not a long socket type extension.

Bad Rat 414 11-03-2005 08:03 PM

I used to believe an extension would change the torque but over the years I've become wiser and I've changed my mind about that idea. Now a crow's foot extension WILL change the value.

redlightning 11-03-2005 08:07 PM

I work on imports and the flex head is extremely nice to have.

NXS 11-04-2005 03:39 AM

An extension DOES affect Tq applied to a bolt.

I used to work in a tranny shop and just as the previous post said if you have 4 foot worth of extension your 400 ft/lb wrench won't Tq 75 ft/lbs.

I now work for a transport refrigeration company and the compressor heads are torqued to 23 ft/lbs on some models and 26 on others. We like to put a standard ratchet on the head bolts and see who can get closest with-out going over. Using a "click-it" style 3/8 Snap-on Tq wrench we then step up the Tq rating one ft/lb at a time until the bolt head turns. If a 6 inch extension is used we need one extra ft/lb at 28 ft/lbs {for the discharge manifold} to cause the bolt to turn again.

As a matter of fact they make special extensions just for impacts that will torque the rims on your car to a preset rating.

But this was about flex head tq wrenches. It seems to me that a flex head has the possibility to change the tq rating. If the wrench has been calibrated with a 24 inch handle and you need it at a 45* angle you have effectively reduced your leverage by 50% otherwords if you are torquing a 100 ft/lb bolt with a 24" wrench you will need to pull a full 100 lbs of force on the handle as opposed to a 50 lb pull.{in addition to the fact that you have now increased the lateral strain on the bolt}.
Since the ratchet reads the tq from the handle side of the joint it will now read that 100 ft/lbs are being applied to the bolt head when in reality 50% of that, or 50 ft/lbs is being applied.

oldred 11-04-2005 09:07 AM

:rolleyes: Ok As I said this has been argued for years and disproved in many ways although some just will not accept common sense. Let's do a little physics.

One cannot argue with accepted laws of physics governing force such as "For every action there is an equal but opposite reaction" Thus if you apply a rotary force to one end of a shaft(in this case an extension) it will take an equal but opposite force (resistance in this case) to prevent the shaft from turning, no more no less. If the force is less on the other end the shaft would continue to spin but if it is more then it would turn in the opposite direction. If you use a shaft(extension) that is made of a material that will twist and then if you apply, say, 35 ft/lb of torque with a torque wrench then the wrench will read 35 ft/lb when it encounters that much resistance at the point of connection to the shaft(extension) then if the shaft(extension) "twists" under this much force it will simply have wound-up until it reached 35 ft/lb of resistance on the fastener end. Fellows like I said it is simple laws of physics and there is no way around it, Whatever force you put into one end of that extension is how much resistance it is going to take to stop it from turning and if you apply a turning force to one end of a shaft then it will turn until the resistance on the other end is EXACTLY equal, no more no less.
NXS If you allow an extension to tilt even slightly then you are dealing with the lever effect and that would affect the reading so it is important to keep this extension centered over the fastener and the impact extensions are dealing with impulse force and and is an entirely different matter. If what you guys are saying were true then a long bolt would require a different torque value than a short one but this is not true. And as far as saying that 400 ft/bs of torque applied to one end of a 4 ft extension will only amount to 75 lbs on the other end this is so ridiculous that it is almost comical and I would like to invite you down here to the mines where this is done all the time on certain mining equipment and you can explain to all these guys that they have been doing it wrong all these years and Caterpillar Tractor company does not know what it is talking about and needs to change it's service manuals! :)
Now take a look at that first link farna provided and check out the illustration and it shows the type of extension(wrench extension) that does affect torque values but if you had really checked it you would see that this(which really has nothing to do with what we are talking about) actually INCREASES the applied torque value from the torque wrench reading which is just the opposite of what you are saying. Also notice that although this is a list of factors that DO affect torque wrench readings there is no mention of socket extensions, Why not?

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