There are three types of procedures used for torquing: torque to yield/angle to turn; torque to maximum stretch yield; and torque to a specific torque number and hope it's correct.

Basically what it comes down to, is that torquing with just torque values such as foot-pounds or newton-meters is at best a guess at getting the torque correct. Depending on the type of fastener, materials used for both the bolt and the part it screws in to, cleanliness of the thread, lubrication used, TYPE of lubrication used, etc., the torque between two "identical" fasteners can actually vary by as much as 35% (or more) with the torque wrench having the same setting for both fasteners!
Many fasteners, such as wheel studs, water pump bolts, etc., are just there to make sure that the bolt is probably tight enough, and probably not too tight. These are the bolts that you reuse over and over without any problems.

Head bolts, as well as most of the bolts internal to the engine, are what are called stretch bolts (also known as torque-to-yield bolts). These are designed to be tightened to the point they physically start to stretch, usually by a few thousandths of an inch. This leaves them in a elastic condition, that allows very equal clamping forces to be applied to the part they're holding down.

When you tighten one of these bolts, the torque setting that is used is designed to get the bolt close to the point that it starts to stretch. The torque angle guage is then used to put the bolt PAST the point it starts to stretch and actually start to stretch it. You just can't accurately measure this with a regular torque wrench, thus the torque-to-yield/angle-to-turn method. The Maximum stretch yield method doesn't apply here, since you need A) A very accurate micrometer to measure the actual bolt stretch, and B) Access to both sides of the bolt, which you don't have on head bolts.
The torquing process is quite simple. You simply torque the bolt using a normal torque wrench to a pre-determined torque. Then, usually you'll warm things up by running it at idle, then let it cool back down and re-torque again with the torque wrench. After that part is done, the torque angle guage comes into play. The torque that the bolts are at after the regular wrench is theoretically just before the bolt starts to stretch. This accuracy depends of course on the factors mentioned above. To get the final torque AND stretch on the bolt, you tighten the bolt a given number of degrees of rotation. This is what the torque angle guage is for, accurately measuring the degrees of additional rotation (typically accurate to a degree or so).

The Reason For Not Re-Using Stretch Bolts

The reason is quite simple. Once you have stretched the bolt once, it never returns to its original size. This weakens the bolt, and if you try to re-use it, you take a risk of snapping the bolt in two, leaving the threaded part in the block, the top in your hand, and a complicated repair required. It's just not worth the risk to try to re-use old bolts.

You can get replacement bolts that are not designed to stretch at the torque required to hold the parts together.

There are two good reasons for using non stretch bolts. You want to re-use them in the future, and/or you have an engine that is either forced induction (745i running massive boost for example) or very high compression. Since the bolt is already at it's maximum stretch without losing strength, you don't want to put undue stress on it from a high compression engine and take the risk of a blown head gasket.
There is also a good reason for NOT using these bolts, and staying with the original OEM style stretch bolts. Non stretch bolts require retorquing after a few heat/cool cycles (usually specified at around 5-6 cycles), and may have to re-torque them anywhere from every 30k to 70k miles, depending on application. If you have a normally asperated engine, there's just no reason to have to do the extra work when normally you wouldn't touch the bolts again after they've been done properly with regular OEM stretch bolts. Also, stretch bolts tend to provide more consistent, even clamping pressure than the solid bolts do.

JR given what you have just said & the lack of accuracy that you get from torque wrenches. Is there a particular brand of torque wrench that is better than say a Craftsman wrench?

I myself do not know who makes the best or poor but also is forgotten is they need to be recalibrated every so often.
Also claims are the racket types are not as accurate.

What about the wrenches that have some kind of digital interface built into them are they any good?

I am not a expert in which are best but here is a quick and simple way to check your Torque Wrench for accuracy. Be aware, though, that this is not the best way to check it. That would be to send it to a company that specializes in calibrating test instruments. This will, however, let you know if the Torque Wrench is “close” to correct.

Stick an old socket on the wrench and clamp the socket in a vice. You want the wrench to rotate freely. Turn the handle to a horizontal position. Now, hang a known weight (You can use water for the weight by using a bucket and filling it using gallon water bottles. Every gallon of water you add weighs 8.3 lbs. You can factor in the weight of the bucket if you wish, but this is probably more accuracy than you really need. Remember; this is a “ball park” check.) on the wrench handle by using a loop of wire. Measure the distance between the wire and the pivot point of the wrench head, and divide by 12. Then multiply this by the weight applied. The result is the actual torque being applied to the wrench and can be compared with the reading on the Torque Wrench.

For example, if you hang a 60lb weight at 10 inches, your applied torque is:

60lbs. x 10inches/12inches per ft. = 49.9ft-lbs. If you have a click-type wrench, you would adjust the setting and see if it clicks when it's set near 50 ft-lbs.

A final word about the click-type wrenches. ALWAYS relieve the pressure on the springs by resetting the torque to zero when finished. The springs can take a “set” and lose ANY accuracy if you leave them under pressure.

Great info. I had no idea there was so much to torquing bolts. Bad news is I have been leaving my wrenches set on the last setting. Guess I'll have to go to the garage and fix that! Maybe get them calibrated now!

I just ordered a Torque Angle Guage (P/N TA360) from Snap-on Tools. It cost under $70.00. I will post any information I receive when it arrives. I've always trusted Snap-on Tools for their quality and came across a recommendation in one of the Head/Cam books I'm reading.

Peter Ross
Ashburn, VA
2002 Z06 JR tune, Donaldson Blackwing and Injectors
Team ZR-1 Member 06-407 prross@verizon.net

I was planning to send my Tq wrench to be recalibrated. I think I will do some before and after measurements. Great write up.