Even if you’re an expert, it’s always a good idea to review the basics. Here, the essentials of selecting and installing threaded fasteners are covered.

Do you know how to measure bolts? Are stainless steel fasteners the best choice for your build? What lubricants should you use with your nuts and bolts?

Some of us builders know these answers as surely as we know our own names, while others are still learning fastener basics. Regardless of your level of expertise, this fastener primer will provide you with some practical information you can use in your shop today.

1.  Bolt Diameter

Bolt (or stud) diameter commonly refers to the “nominal” thread diameter. For instance, a 3⁄8-inch bolt shank should measure 3⁄8 inches (0.3750 inches) in diameter. Remember, bolt diameter refers to the thread or shank diameter, and this size has nothing to do with the size of the wrench required to service the bolt or nut. If you hear someone refer to major and minor diameters, here’s the explanation: major diameter refers to the diameter of the thread crest (the largest outside diameter of the thread) and minor diameter refers to the diameter of the root (the deepest part of the thread).

2.  Bolt Length

For most styles of bolts/screws, the published length of a bolt refers to the length of the shank/threads from the underside of the bolt head to the tip of the shank. This applies to styles such as hex-head bolts, 12-point bolts, socket-head cap screws and button-head screws. However, a flat-top screw (flat on the top and chamfered under the head) is always measured at its overall length.

3.  Fractional Pitch

“Pitch,” or “thread count” for a fractional threaded fastener is based on how many threads exist along a 1-inch length. For example, a bolt that is labeled as a 1⁄4-inch x 20 will have 20 threads per inch of shank length. A 5⁄16-inch x 18 bolt will feature 18 threads per inch of shank length. When dealing with fractional threads (many people have a habit of referring to this as “American” thread), the more course the thread, the lower the pitch number. As threads become finer, the pitch number increases. For instance, a 1⁄4-inch x 20 is commonly referred to as a course pitch, while a 1⁄4-inch x 28 is referred to as a fine pitch. The bigger the pitch number, the finer the threads become.

4. Metric Pitch

Metric fasteners use a different thread pitch format as compared to fractional fasteners. In the metric world, the thread pitch is actually a “thread spacing” index, which means that the pitch number indicates the distance, in millimeters, between adjacent threads. For instance, a bolt that’s 10mm x 1.25 will feature a distance of 1.25mm between threads. A 6mm x 1.0 bolt will feature 1.0mm between threads, etc. Metric threads get finer as the pitch number decreases. For example, a 1.0 thread is fine, while a 1.25 thread pitch is more course, a 1.50 pitch is even more course and a 1.75 pitch is still even more course. The bigger the pitch number, the more space between the threads. Unlike fractional thread pitch, the bigger the pitch number, the more course the threads become.

Stainless steel fasteners are excellent choices for street rod applications because of their appearance and lack of rusting. Also note the undersize hex heads on these exhaust header bolts. This allows increased socket wrench access.

5. Stainless Steel

Stainless steel (in its many different metallurgical grades) is a popular choice because it doesn’t rust or corrode. Naturally, many rodders prefer polished stainless from an appearance standpoint. A few precautions should be noted. Stainless steel is comprised of softer-than-steel alloys. If installed dry (without thread lubricant), you run the risk of thread galling, especially if you install the bolt or nut quickly (generating frictional heat). Depending on the application, it’s best to provide some type of lubricant to stainless threads. (Depending on the application, this might involve oil, moly lube, Teflon paste or even a thread-locking compound.) Just be careful not to be in a big hurry when installing stainless threaded fasteners. For example, never use an impact or any type of power driver to install or remove stainless steel fasteners. Also, to help ensure against potential galling, it’s a good idea to apply some type of thread lubricant to guard against metal-to-metal friction such as oil, moly or thread sealant, depending on the application.

Also, just because it’s stainless doesn’t mean that it’s appropriate for any area of the vehicle. Depending on its hardness grade/tensile strength, it may not be strong enough for a high-stress application such as wheel fasteners, engine main caps or cylinder head bolts and/or studs. If in doubt, buy only high-quality fasteners that are intended and part-numbered for specific performance applications, and follow the fastener maker’s rules and recommendations for application and tightening procedures. I’m a huge fan of using stainless fasteners, but you need to be aware of their various limitations.

6. Torque Value

Published torque values aren’t provided just to fill space on an instruction sheet or in a service manual. These values are important, so pay attention. While proper tightening values are important for every threaded fastener, this is especially critical when it comes to high-stress areas such as connecting rods, main caps and cylinder heads, and wheels. For every specific application, a certain clamping load is required for proper component retention and performance.

Also, various fasteners (based on diameter, length and tensile strength rating) are designed to slightly stretch when fully tightened. This is referred to as the fastener’s elastic nature. Especially in high-stress applications, the bolt is designed to stretch within its elastic design range to provide ongoing clamping load (similar to a rubber band). If not stretched enough, the clamping load will be insufficient. If stretched too far, the bolt can fatigue and won’t provide the proper clamping force. Even for fasteners that aren’t subjected to high loads, such as those for oil pans, paying attention to torque specs aids in even distribution of clamping load. By using a torque wrench and being consistent on all fasteners for a component, you’re able to spread the load and avoid component warping and/or gasket leaks. Buy a torque wrench, take care of it, keep it calibrated and use it.

When lubricating a bolt, apply lube to the underside of the bolt head, in addition to lubricating the threads. This reduces frictional variables during torquing for more-accurate clamping loads.

7.  Lubricants

The use of thread lubricants involves much more than simply providing a smooth installation and reducing the chance of rust seizure. This is a very complex subject, but here I’ll provide a brief overview. Torque values are established by fastener and parts makers based on the use of certain lubricants. For instance, in critical clamping load applications such as cylinder heads, main caps and connecting rods, published torque specifications may be based on the use of either engine oil or a specific moly lube. If a torque spec was established with the use of engine oil in mind, and you coat the threads with moly, you’ll reduce friction, which means that you’ll over-tighten, possibly exceeding the fastener’s design elastic stretch. If the spec was based on using moly and you use oil, you may under-tighten, resulting in less-than-intended clamping load. If a fastener maker provides specs for both oil and moly, be sure to follow the torque value that matches the type of lubricant that you plan to use.

Also, whenever applying lubricant to a fastener’s threads, you should also apply the same lube to the underside of the bolt head or contact side of the nut. Remember that the fastener experiences friction at both the threads and where the bolt head or nut contacts the component surface.

8.  Thread Engagement

How long is long enough? How short is too short? Blindly grabbing any bolt length that’s handy could spell disaster. If a bolt is too long, it can bottom-out in a blind hole, which means that it can’t provide the necessary clamping force. Also, if it’s too long in an open hole, depending on the specific application, the extra length may block off an important fluid passage. If the bolt is too short, you’ll have insufficient thread engagement, with the very real possibility that, depending on clamping load, the bolt could rip out the upper threads of the threaded hole (if not initially, down the road as heat and load place added stress on the threads).

If  in doubt, the rule of thumb dictates that the bolt should engage to a depth equal to or greater than it’s shank diameter. For instance, a 3⁄8-inch diameter bolt should engage into at least 3⁄8 inches of thread. Remember, we’re talking about minimum thread engagement. For certain applications, the minimum will increase. For example, when attaching outer tie rod ends to inner tie rods on a rack-and-pinion unit, it’s recommended to have at least 2.5 times thread engagement as compared to thread diameter. For instance, if the tie rod threads are 1⁄2 inches in diameter, you should have at least 11⁄4 inches of thread length engagement. The more threads that are engaged, the stronger the connection. Plus, considering that you’ll be making toe adjustments, it’s safer to have more threads engaged.

9.  Thread Treatments

Sometimes applying a chemical compound to bolt threads aids in the fastener’s function and performance. If the bolt hole enters a fluid passage (water, fuel or oil) or a vacuum passage, a thread sealant is required (unless the bolt features a built-in sealing O-ring). A common/popular choice is Teflon thread sealant, which is available in both tape and paste form.

If you use Teflon tape, be sure to wind the tape opposite the tightening rotation direction so that the exposed end of the tape faces away from the direction of rotation. If the tape is applied with the final end facing the direction of rotation, the act of installing the bolt can begin to peel the tape away. In other words, the end of the tape should follow the direction of rotation. If the end of the tape attacks the direction of rotation, you’ll ruin the tape.

Also, whenever sealing any threaded fastener where fuel is involved (for instance, an NPT fitting on a fuel system), it’s best to use Teflon paste instead of tape. The reason: If you’re not extremely careful, a sliver of Teflon tape can enter the fuel system, potentially clogging small orifices, such as jets, metering passages, injectors, etc.

All NPT (tapered pipe thread) threads must be treated with a thread sealant. Don’t rely on the tapered thread alone for sealing.

Thread-locking compounds aid a threaded fastener from loosening during exposure to heat and/or vibration and during metal expansion/contraction. A locking compound acts as a chemical lock washer.

You need to be aware of a few concerns with regard to thread-locking compounds. These are commonly anaerobic in nature, which means that they cure with the absence of air (once the bolt is installed, it begins to cure).

When installing cylinder head studs, it’s not necessary to use a locking compound, as fully engaged studs achieve clamping force via nut tightening so the studs don’t need to be super-tight. However, some folks (primarily racers who routinely disassemble/reassemble) like to lock head studs simply to make future head service quicker so that studs don’t back out during nut removal. However, if the engine block features fairly thin cylinder walls, an anaerobic compound can expand when it cures, potentially resulting in cracked cylinder walls. If you want to lock head studs in place and you’re concerned about the cylinder walls, a better choice is a two-part epoxy. If you need to remove the stud in the future, high heat will break it down enough to allow removal.

Also, pay attention to the grade, or strength, of the anaerobic compound. For most applications, a medium-strength thread locker is sufficient. During future removal, you’ll be able to break it loose with a hand wrench. If a high-strength compound is applied, this can make future bolt removal very difficult. High-strength compounds are reserved for those fasteners that you never plan to remove. However, if removal is needed, and you can’t break it loose without fear of snapping the bolt, you can apply concentrated heat to the bolt. This will break the compound down, essentially “melting” it.

Studs should be installed finger-tight with a light "snug." Never double-nut a stud or attempt to tighten the stud itself to a high torque value. The clamping force will be applied as the nut is tightened onto the stud.

10.  Installing Studs

The use of studs instead of bolts offers distinct advantages for various applications in terms of increased clamping force accuracy for enhanced appearance, for assembly convenience, or any combination thereof. However, all too often inexperienced assemblers make the mistake of over-tightening the stud into the parent material. You don’t need to over-tighten a stud into a component. The clamping force (and the recommended torque value) applies to the tightening of the nut onto the stud.

In other words, the “tightening” occurs by tightening the nut, not the stud itself—the stud only needs to be fully engaged into the threaded hole. It can be lightly snugged finger-tight or slightly preloaded (if the stud tip features a female hex, use a hex wrench to apply a light-to-moderate hand-tightening). For instance, a 7⁄16-inch or 1⁄2-inch diameter stud can be snugged down with about 8 lbs. /ft. simply to fully engage the threads. If you over-tighten, the stud will tend to splay (move at an angle) and you’ll have difficulty installing or removing the part. Rely on torquing the nut, not the stud. Generally, studs are installed “snugged” finger-tight, while some manufacturers may specify a slight preload (for example, 8–10 lbs. /ft.). Always follow the stud maker’s instructions. Once the nut is fully tightened and clamping force has been achieved, the stud isn’t going anywhere.

Also, the use of studs eases component installation, providing a guide for positioning. In some cases, studs are available that feature a radiused “bullet” nose. This allows you to drop the nut onto the tip without immediately engaging the threads. This greatly reduces the chance of cross-threading. This is a popular choice for mounting components such as carburetors, exhaust header flanges and distributor hold-downs.

Tags: Fasteners, Mike Mavrigian, Tech Tips