Hilton

I chose the 3/8" one because its good down to 5ftlbs.. which means water pump bolts and cam cover bolts @ 7ftlbs are reliable, and they're the ones which worry me more as there's little warning/tolerance for over-torquing and shearing the buggers completely.

Like Jim, I've also encountered Audi lower track arm torque-by-angle bolts, and didn't dare use the Snap-On 3/8" as it would have pushed it way above its stated torque range. However, for the majority of use, its great, as it covers everything fron 7ftlb M6 up to 85ftlb M12-fine threads.

And yes, the battery cover is a PITA.

Doug_B_928

Interesting. So more large size bolts have torque angle requirements (making the 1/2 " a good choice) but the 3/8" range covers head studs and most other automotive needs. Hmmm...

Adk46

Underlying all this is an interesting and unexpected feature of bolts that are used to cap a pressure vessel: if tightened beyond a certain point, the stress on the bolt does not change as the pressure in the vessel is varied between zero and its operating point. This eliminates fatigue failure. Tighten too little, and the bolt will experience cyclic loads, inviting fatigue. (Dissimilar materials and temperature profiles make this more complicated, or impossible.)

The "desired tightness" is, of course, the actual tension on the bolt. Torque is an indirect measurement of that, correct only when you have the mythical friction-less situation, or you've managed to calibrate a reproducible set of parameters governing friction.

If the thing being clamped is very rigid compared to the bolts, then twisting a certain number of degrees will stretch the bolt a certain amount and therefore to a particular stress - simple math involving threads-per-inch, length and Young's modulus. A crushable gasket or washer spoils this approach, however.

I imagine a gasket designer would like his gasket to be crushed a certain distance in order to work properly. The engine's structural designer wishes a particular preload on his bolts. Two things to set with only one ****, and it's a lousy ****. The two designers reached some joint agreement as the drawings were prepared. Where I worked, anyone curious would go back to the formal Design Record Book for the details. It seems that where we are today we must - gasp! - rely on individuals with skill and experience.

That's certainly not me, but I enjoy thinking: A sophisticated approach would involve monitoring a chart of torque (Y) and angular displacement (X). This would have a variable ramp as the items came into full contact, then a straight rising section where everything is elastic, then a plateau while the gasket is getting crushed, then a rapidly rising, fairly straight section after the gasket has gotten pretty stiff and you're seriously increasing the tension on the bolt. Life would be good if you were provided this complex curve, with instructions on where to stop, based on shape, not any absolute values. You really want a Bluetooth wrench that communicated with your smart phone, on which you downloaded the appropriate curve. Siri would provide encouraging words, then a gentle "That's enough, Dave."

FredR

Curt,

Some interesting comments there that seem to fly in the face of what I have experienced in the oil business over many years. If the stress on the bolts does not change then logic suggests the bolt has exceed the yield point/elastic limit and thus would be in the plastic nether region. Systems I have worked with do not allow any stress to exceed 90% of yield under any circumstances [usually hydrotest] so your comment leaves me wondering if I have missed something along the way. As I am aware a well designed system operates in a controlled stress range typically around 60 to 70% of yield where Young's law holds true.

The gasket is no different in that it is an elastic member and its ability to seal is a function of the pressure it has to seal against and the clamping forces required to maintain such. As one can imagine, the cylinder head sealing process is extremely cyclical so logic suggests the gasket has to have very good elastic properties to form a robust seal.

In my experience gaskets fail when they lose their elastic properties [age hardening]. During refinery start ups the gasket that fail are not usually the new ones we put in during a shutdown but the ones we did not touch- typically because the gaskets had age hardened and the thermal cycling from hot condition to ambient was enough to crack the seal to a point the gasket could not recover from. A typical reaction to this was to flog the joint but invariably all this did was stretch the bolts more and in some cases led to complete failure and "Kabooom". Thus why a good operator does not allow flogging of joints under pressure.

Thus once a joint goes rarely can you recover from it without changing gaskets or whatever else needs doing to rectify the problem. On the positive side, most joints will operate successfully over quite a wide range of stress conditions- if they did not then the tightening procedures used would probably not be accurate. Thus a difficult joint like the head gasket is more critical and thus it is specified to an initial torque and then a defined degree of rotation as the best analogue to get the joint to work in the stress range intended by its designers.

Having a device that tells you the applied rotation is neat but in reality, if you can tell the time on an analogue watch I suspect you really do not need this feature as a simple mark on the bolt will allow one to know the angle subtended. I do like the idea of a continuous readout of the torque being applied- I use what is left of my muscles/tendons to tell me whether the bolt is stretching in the manner it should.

Once you get to industrial strength fasteners you can only do this with appropriate measuring tools. A good torque wrench has sufficient leverage such that you can actually "feel" what is happening to the bolt up to the top end limit of the wrench. This is exactly what I experienced when tightening my crank pulley after the timing belt job. The torque required was 220 ft lbs [I seem to remember] and my wrench was good for about 240 ft lbs. I took the tightening in about 4 steps to ensure I could be sure it was going as planned and at the last step I was pretty much on the limit of what I could physically apply when it clicked for the set point. Doubtless a gorilla could apply more but for a "normal" sort of chap like myself it worked just nice.

Rgds

Fred

worf928

I don't see that as a problem since you are only supposed to use torque wrenches with your hand on the grip.

My issue was that the spring on the other side of the battery cap fatigues, seemingly, within days. Then any minor pressure on the cap or distortion due to pressure causes loss of electrical contact across the batteries and thus wrench reset in mid-swing. If you are lucky. Because when it resets you get a long beep. If you are unlucky it just turns off and you don't notice until your wrist-o-meter says 'that ain't right.'

Maybe yours is from the days when they were made by CDI or Precision Instruments and thus your spring is made from better stuff.

Eplebnista

Zoro tools is having a 30% off sale today on items over $200.00 if you use the code "BlizzardSale" at checkout.....

That would seem to be the best source today.

Ladybug83

Just an FYI to anyone eBay shopping today. For a Snap On wrench, The first 4 serial number digits on the wrench corresponds to the month and year of manufacturing. Of course, the history of use is important and is always an unknown when buying used, but at least with this info, you can focus on newer-used tools. I recently purchased a couple snap on torque wrenches (1/4 & 3/8) and had the calibration checked by my mechanics tool guy. Good to go. The wrenches I purchased were manufactured in '11 & '13 and had no visible signs of damage (big scratches, etc). However, I also have a 1/2 snap on made in '03 and it works great too. I guess adding it up, I paid @650 for all three wrenches (including shipping), great deal, but they are the Techwrenches, not the Techangles, which would be more appropriate for the head work. Just my limited experience
Josh

Ladybug83

Example : Snap On wrench with serial number 1213xxxxxx was manufactured in Dec 2013.

M. Requin

Love it. I want one. Until then I will continue using this on the end of my torque wrench:

Todd T

I use one of these, same one I've had since high school.
In my opinion these are the most accurate torque wrenches:

http://www.sears.com/craftsman-1-2-i...9&blockType=G9

When I torque a set of heads, first step is running a die over the bolts, even brand new ones are ugly and need to be cleaned, often nicked up from shipping.

I put a little bit of motor oil on the threads (same Lubrication Engineers 20w-50 that's in the engine).

Stock Head Gaskets I follow the WSM keeping an eye on the torque value on the wrench, it should continually raise as you tighten the bolt. If it doesn't something is wrong with the fastener or the threads are stripping.

For the degrees I simply mark the fastener with a paint mark at 12 o'clock


With Cometic MLS gaskets I do the same process, then loosen the fasteners and re-torque with the initial torque slightly higher than stock followed by the factory specs for the degrees.

Last step is removing all the paint marks.