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I believe almost everyone working on a car in any capacity has heard this, many things should be tightened down in a criss-cross pattern.

Why? What is the rationale behind it?

I can imagine several plausible explanations myself, but I'd like to know what the actual reason behind it is. Why exactly is it that something will not seat correctly if it is tightened down in a clockwise pattern instead of criss-cross?

  • 4
    It's not just automotive use, but very common on things like vacuum flanges in the lab (and especially vacuum system windows). The torques are probably 100x lower than in many car applications,but the principle is the same. We'd often take up all the slack in such a pattern, then barely finger tight, then properly finger tight (that may be enough, or if not tighten once more). – Chris H Jun 18 at 12:55
  • because it wont be even. it'll be tilted to the tightest side, therefore it cannot move evenly on the other screws – Emobe Jun 20 at 10:57
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There is more to it than just a "criss-cross" pattern. You should also work your way out from the middle in stages of bolt torque.

If you imagine tightening a cylinder head down starting at the two very ends. You have a gasket between the head and the block that doesn't really want to be crushed. When you tighten the ends down, the middle of the head will be being pushed up in an arch shape by the gasket, bending the head. With the ends of the head now locked in place by the end bolts, you now tighten the ones in the middle forcing the arch down. This will cause movement on the end bolts. There is a good chance now that the gasket will fail to do its job.

If you work in stages from the middle out in a criss-cross pattern, the load is spread evenly without bending anything and gives the best chance of success.

Additional by @Paulster2 - If you started torquing a head on one side and worked your way around, you are actually pushing the gasket ahead of your torque. As the gasket gets pushed around to where you started at, you now have a thicker portion which will cause a lump under the head, which would cause the head to warp. By criss-crossing your torque, you've eliminated this

  • What if you're not tightening down something with a gasket? – AlphaCentauri Jun 19 at 20:40
  • @AlphaCentauri You still want it to come down evenly, not least to equalize stresses in the metal. You don't want the metal sitting there with a lot of strain on it, and then apply your service loads, and find the two stack to break the part :( – Harper Jun 20 at 0:54
12

In general things should be tightened in stages with some sort of crossed pattern depending on the number of fasteners in the circle.

Not doing so creates a bias that could result in false torque readings on the final fasteners. In other cases it can warp or damage the item being fastened, or perhaps prevent proper sealing of a gasket. Cylinder heads, wheels, timing covers, valve covers, intake manifolds, water pumps, oil and transmission pans ... the list is endless.

It's best to simply make it a habit, whether ultimately required or not. "It can't hurt!" - and if it's habitual you never have to worry about if you can get away with an exception.

7

Cross fastener tightening accomplishes:

  1. It keeps the components being fastened aligned, and not cross mounted. This is a particular risk when a component is interior to a flange.

  2. It helps keep the component from hanging up on threads or alignment pins, by assuring a somewhat parallel entry to the seating / final position.

  3. It helps prevent gasket or seal damage by somewhat uniformly torquing down parts. Some gaskets, like brass or aluminium do not tolerate being misaligned during assembly.

  4. It provides for more uniform torque distribution and tightening, and when accomplished with incremental torquing, provides nearly uniform torque through all the component fastenings without excessive torque at any small number of fasteners.

It is also a simple, easy to learn process, that only requires initial mechanical training to accomplish.

Nice artwork available here: http://www.boltscience.com/pages/tsequence.htm

2

Think of it like threading shoe laces. If you criss-cross the laces up the eyelets of the shoe, you get an even pressure across the top of the foot and the shoe stays on comfortably. If you laced the shoe up from one hole to the next down then up, then pulled tightly on the laces sticking out the first and last holes and tied them equally tightly, you'd deform the shoe, it wouldn't be comfortable and it wouldn't stay on your foot.

0

It is good practice. If you fasten clockwise, the side you start on is tightened down, but now the opposite side is at a slight angle and you are applying angular force to something meant to go on plumb.

0

My instructor told my the first time I torqued something down that this is done to squeeze the gaskets evenly. Otherwise you can damage you gasket and they wont seal properly.

Also, don't tighten the screws all the way from the beginning.

It should also help you when tightening screws which connect parts without a gasket in between, This because your torque-reading should be more accurate. Achieving accurate torque is important for uniform tension -> which leads to uniform friction between parts. Friction being the underlying force inhibiting the movement of the bolted parts.

Inadequate tension may also lead to some screws loosening in use.

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