Today’s automotive bolts are deceptively complex, and mistakes can be costly. Why do we have fine and coarse thread bolts? What is a torque-to-yield fastener? Knowing these answers may help prevent a disaster.
What is fine and coarse thread?
The number of threads per inch or millimeter, varies among fasteners. Manufacturers call this pitch. In standard treads, pitch is the number of threads in an inch. For instance a half-inch bolt may have 13 threads per inch, which is a coarse thread. Having 20 threads per inch, makes a half-inch bolt fine thread. This is expressed as 1/2-13 or 1/2-20 and sometimes USS for coarse and SAE for fine thread.
Metric bolts also have combinations of fine and coarse threads. The distance, in millimeters, between the thread is the classification. For instance 12mm-1.5 means 1.5 millimeters between the treads. With fine thread 1.25 millimeters may be the spacing between threads. This is expressed as m12-1.25 where m shows a metric fastener, 12 is the nominal diameter in millimeters and 1.25 is the thread pitch.
Why do we have fine and coarse threads?
A fine-thread provides greater ultimate strength and more clamping force with equal torque. Tightening is also a bit more precise and some feel they offer greater resistance from self-loosening. Fine-threads are most often on a bolt or stud, with a nut.
When threaded into a softer material, such as an engine block, coarse threads are far less likely to strip. Most automotive-screws are coarse-threaded. This is also why head studs have coarse threads on the engine side and fine threads on the nut. This provides both benefits, but at a higher cost.
Does length really matter?
The length of a bolt and the number of threads has a bearing on proper tension. Most stretching occurs in the threaded portion of the bolt. Automotive engineers size bolts, to provide adequate stretch and substituting a different length can cause problems. For instance, installing a longer bolt may place too few threads in the grip area. This can cause the bolt to fracture, normally at the first thread. Reducing the threads in the grip area affects the stretch of the bolt. For bolts six inches and less in length, the threads from the head to the nut, should equal twice the bolt diameter, and 1/4 inch. As an example a ½ inch bolt should have 1.25 inches of threads, between the head and the nut.
The same is true for screws that use factory-installed spacers. Bolts that appear longer than needed, are often used to avoid breakage. They may need the longer length to provide adequate stretch. The technician should never omit a factory-installed spacer.
Torque to yield fasteners
Many applications today are too precise to rely on torque for tightening. Head bolts are an example. An aluminum head expands far more than the cast-iron engine block. The intake side is cooler than the exhaust and cylinder pressure is extremely high. Without exact bolt tension, the head gasket cannot last. Most head bolts are a torque to yield type fastener.
A torque-to-yield or T-T-Y arrangement, requires a special bolt. This bolt stretches beyond its elastic limits. This additional stretch helps maintain consistent tension, with head gasket shrinkage. For instance, consider a 7/16 inch T-T-Y bolt, stretched .070 inches. The clamping force equals 11,900 pounds. A traditional 9/16 inch bolt may stretch .030 inches to equal the same tension.
The difference is, when the head gasket compresses 25% or about .010 inches. This equals about 1/7th of the stretch on the T-T-Y bolt and almost 1/3 of the stretch on the conventional bolt. Tension with the T-T-Y fastener is still 10,200 pounds compared with 7,993 for the standard bolt.
With tightening of T-T-Y, all bolts are first pre tensioned to a specified torque setting, in a uniform pattern. From there we rotate each bolt, a specified degree. For instance, we first tighten the bolts to a torque of 35 foot-pounds. Next each may receive an additional 90-degree rotation. Often they specify a second turn of X degrees. The degree of turn is fixed and the same for every bolt. This is not dependent on turning resistance and gives more accurate tension. Though one bolt receives more torque than another, the clamping force is more equal.
T-T-Y fasteners do not return to their original state when loosened. We must replace and never reuse these bolts. Many applications, from engines to suspensions, now use this type fastener. The technician must be aware of this and replace any removed T-T-Y fastener.
With so many fastener variations, an assortment of accurate torque-wrenches are no longer a luxury for a vehicle enthusiast. Accurate service data, for the vehicle is also mandatory. Replacing fasteners with exact matches and following the proper procedures can help prevent a disaster.