Why do head bolts stretch?

Stretching a bolt is like stretching a rubber band. It allows the fastener to exert force against a surface to hold the parts together. This is especially important with gasketed surfaces because the gaskets have a certain amount of give that must be overcome by the fasteners to maintain a tight seal.

What causes head bolts to loosen?

What Causes Loose Bolts? … In a bolted joint, tightening the nut actually stretches the bolt a small amount, like pulling on a stiff spring. This stretching, or tension, results in an opposing clamp force that holds the two sections of the joint together. If the bolt comes loose, this clamp force weakens.

What is the purpose of a stretch bolt?

Stretch bolts are designed so that they can be elongated beyond their elastic limit into the plastic region without problems. When a specific tightening torque has been reached, the bolts are turned further through a defined angle, which pre-loads them into the plastic region.

Can head bolts loosen over time?

Agreed, but performing head gasket maintenance is a rare occurrence and loosening of the head gasket bolts is even rarer. Most of the time that head gasket failures happen is due to process problems on the part of the gasket manufacturer.

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Do you have to replace stretch bolts?

If you remove any torque to yield bolts (Stretch bolts), you SHOULD replace them with new bolts and torque to specification. stretch to ensure proper securing when tightened down. (Bolts with Specification: Torque plus Angle).

How many times can you stretch a bolt?

How many times can you stretch a bolt? Typically, the amount of stretching may be as little as 0.001-in. In most cases there is no way to know whether or not the fastener has yielded, so, it is advisable to never reuse fasteners in critical applications.

Does Double nut prevent loosening?

If the thick nut is overtightened, there is the risk of thread stripping or the tensile fracture of the bolt between the two nuts. The reason why the two nut system is effective in resisting self loosening is due to the way the threads are jammed together (hence the term jam nut being frequently used for the thin nut).

Can cylinder head bolts come loose?

Some head gaskets will deteriorate over time causing the head bolts to become rather loose,heating up and cooling down over the years will cause the studs to stretch just like in a car with High mileage engine. It’s always good to re-torque your head bolts every couple years on all older machinery.

Should you tighten the nut or the bolt?

There is no simple answer to this as in some instances, it is perfectly acceptable to tighten the bolt head while holding the nut in place. In other situations, it’s acceptable to tighten the nut.

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How do you tell if a bolt is stretched?

Like the others have said…if it gives you that “1/4 turn” line, it’s a stretch bolt. For example, the brake caliper bolts are listed as “always replace” in the Bently…but it’s just a regular bolt with the threadlocker stuff on it.

What are stretch bolts made from?

Stretch bolts are made of steel which has a relativly long horizontal section of the load/ extension curve past the yield point. In use they are tightened by stretching them a specific amount, by measuring the rotation after the joint is nipped.

Why do torque bolts yield?

Many vehicle manufacturers use torque to yield bolts; especially on engines with aluminum heads and or in conjunction with (MLS) head gaskets. Because, they provide more consistent clamping loads, across the entire head-to-block mating surface.

What happens if you overtorque head bolts?

When you overtorque above 15% of recommended which in this case would be about 95 ft/lbs you basically turn the fastener into a rubber band. Some of the fasteners will stretch and you will have uneven torque. Add heat and pressure and head gasket blows and the head can even warp.

What happens if you dont torque head bolts?

In most cases you don’t need to retorque head bolts or studs. If the bolts or studs aren’t tight you will lose compression or get coolant in your cylinders. You can use bolts or studs based on application, but studs tend to give more consistent torque values. Never reuse Torque-To-Yield (TTY) bolts.

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How do you not over tighten bolts?

To keep a fastener from loosening over time due to vibration and other external factors, a threadlocker solution, locking washer, locking nut or a combination of the three should be used. Things to Consider When Torquing Fasteners: Fastener Materials.