Why

Do some head bolts use torque and then finish with a set number of degrees ?

Because the bolt stretch is much more accurate this way.
When you are tightening a bolt, the clamping force is directly proportionnal to the bolt stretch.

When you do torque only, the variables such as amount of oil,under head surface condition, threads conditions and threads fits, give to much uncertainity on the friction factor between the bolt and the parts, and you may end up with different clamping force.

The torque and turn solution also has variables during the first torque sequence, but the torque being smaller, you have less error.
The torque and turn method also allow to have the final tightening right so the bolt is stretched up to the striction point (where it plastically elongates and will not return to initial length). That is the biggest advantage, but also why you never reuse torque and turn bolts.

This allow for very consistent clamping force, because the force does not vary much between beginning of plastic stretching and breakage. That point is totally unobtainable with torque only, because you may break half of the bolts in the head due to uncertainity of the friction factor.

If you reuse the torque and turn bolts, you may stretch them too much this time and go in the breakage zone.

All of the above also explains why the one time use made to stretch bolts have a smaller shank diameter.

Typical steel traction curve:



The part will go back to original shape only if submitted to loads in the almost vertical "straight" curve.

When you tighten stretch bolts, you turn until you are on the top of the curve, where displacement (strain) can vary a lot while having same pressure exerced inside the part (Stress).


This way your 4960 gasket is happy!

Thanks for the information. Cummins has a specified length
and a gauge to use to check the bolts to see if they are
stretched to much to reuse they also give a length in
millimeters and inches .

These are typically torque-to-yield fasteners. Once the steel in the bolt has begun to yield, the clamping force will be consistent even as the parts that are being clamped expand and contract from heating and cooling. Torque-to-yield fasteners are not normally re-used.

Torque specs on non-torque-to-yield fasteners are sometimes specified as torque plus degrees; in this case the idea is to get a consistent clamping force regardless of inconsistent thread friction.
Fel Pro Torque to Yield bolts

Right. I first encountered this with my L10. Apparently they had some issues, as, if I remember correctly, new short bolts came with the head gasket set. I think we gauged the longer bolts, and replaced some.

This truck engine was clearly abused, as after the head gasket blew, the machine shop declared the head junk. In fact they said this head took the record for cracks! They found a used head and rebuilt it.

The torquing instructions were not clear. So I torqued it the way I would any other engine- working my way up to the specified torque, then going over it several times. I don't remember how much of a turn was used after. Maybe 1/4, 1/3? Whatever it was, it has worked well ever since. I don't use the truck a lot, but when loaded, I don't baby it either.

I've got the gauge for the n14. Has a go/no go for the head and main bolt length, and the rod bolt diameter, if I remember right.

SV
No secret, I'm cheap.
I replaced the head gasket on farmall C ,
Used old head bolts.
I had to retorqued the bolts 5 times.
Each time, I would check torque after warming up the engine.
Still good after 15 years. Still cheap.

Because it takes most of tbe friction variables out of the equation. The method was used extensively by Caterpillar was a light torque followed by advancing the nut 1 or 2 flats. An even more accurate method is bolt stretch. We had a 500 CI Chevy race engine that had the rod bolts set by stretch and measured with a micrometer.

And they make torque wrenches in foot pounds and then you can change it into degrees of movement for an accurate amount of degrees.

Is not the initial torquing subject to the same
rotational force variables you describe? For the
degree torque to be accurate, I would think the
initial torque would have to place the bolt in the
correct position. Why is 80 lb/ft initial torque considered
accurate and 95 lb/ft finish torque not?

Volkswagen diesels of the 70's-80's were noted for loosing head gaskets. Aluminum heads. It was every 60k for me. When they switched to torque then turn bolts those problems went away.

Yes---the yield strength is usually determined where the curve reaches a 45 degree angle---after that you are in the tensile strength range and stretch becomes permanent

Good article. Thanks

That would be nice

If I remember correctly, this concept came with the introduction of electronically controlled wrenching. The idea is that you can record torque and rotation at the same time. When the torque stops climbing with further rotation, it automatically senses that and finishes the task. The OEM then gets a record of the process to use for quality control and for warranty purposes. One of the bigger worries is getting a batch of defective screws. This helps to detect them. (You can test a sample, but not all of them.) A couple things to remember are that long screws are easier to keep tight than short screws. And soft metal, or soft washers are bad news.

All the technical explanations have merit and some of them are even accurate. The real reason Cat developed this method is expediency in the field, which is where most heavy work is performed. It is fairly easy for one person with a normal torque wrench to put 60 to 90 foot pounds of torque on a bolt. When the bolt may be difficult to get to, or require a bunch more torque, requires lots more leverage and maybe even a 4 to 1 or 10 to 1 torque multiplier. But by tightening to a nominal torque and then marking and turn torqueing an impact wrench can be used by one person in confined space.

Why is 80 lb/ft initial torque considered
accurate and 95 lb/ft finish torque not?

Click to expand...

Because if you are shooting for 95 lb.ft and you are using torque only, you have to be in the "vertical and straight" part of the steel traction curve above.
Every friction change is going to give you a very different clamping force. As you know, it does not take very much angle to increase the torque. The bolt barely turns. This is due to being int the "vertical and straight" part of the curve.

You have absolutely same lack of inacuracy during pre-torque if you are shooting for a pretorque of 80ft.lbs. Your inaccuracy due to friction means that you have an error of angle of maybe 15 to 30 degres.
But you are going to ridicule this error of 15 to 30 degres by turning the bolt 120 degres or similar during angular thightening. This way, you are in the stretching zone of the bolt. (curvy almost horizontal).
Now, lets say that you had an error of 15 to 30 degres at the beginning, it would not matter, because you are on a "horizontal" curve, so strain is very similar.

I hope it helps. Someone may explain better.