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Navigating the Hardware Store: Making Heads or Tails of Bolts and Nuts

Updated on November 22, 2014

Decoding the Fastener Aisle

It's a bright and sunny weekend and you've decided to tackle putting together that new patio set you bought a few weeks back. You spend a few hours struggling with the instructions, putting pieces together, and what happens....You're missing 1 bolt to complete it. We've all been there and it's a quick way to bring your productive weekend to a screeching halt. If you're a handyman or handywoman and have a good working knowledge of hardware and fasteners, this trip to the store will be quick and easy. If not, you are hoping and praying there is someone there that can help you figure out the one part you need out of an endless supply of fasteners.

This hub will teach you the basics of what all those weird numbers on the packages mean and how you can be sure you are getting the right fastener for your project. Some basic knowledge and a little preparation can save you multiple trips to the store and ensure you finish your project in a timely fashion.

Wood and Metal Screws

At this point, some of you are looking down the aisle saying, "What's the difference? A screw's a screw, right?" Wrong. While metal screws and wood screws do look very similar, there is a difference. Typically, a wood screw will have a thread pattern that is further apart than that of a metal screw. The coarser threads grab the wood and allow the screw to sink in easier. Most metal screws have a finer thread pattern and are available in standard tips, self-tapping tips, or self-drilling tips. A self-drilling screw has a "V" notch at the base, which cuts through metal to create a pilot hole for the screw and eliminates the need for pre-drilling. A self-tapping screw has this pattern on one section of the screw. This makes screwing into metal much easier with less pressure necessary from you. Wood screws will usually sink into most wood without the need for pre-drilling. However, if your project involves finished edges and placing screws near the ends of your boards, you should pre-drill your holes anyway to avoid the wood splitting and ruining the looks of your finished product.

Wood and metal screws are available in a variety of lengths from 1/4" long to 4" in most hardware stores with longer lengths available through specialty stores or special order. If you're looking for wood screws with a diameter larger than 1/4", you will need to look for the lag screws. They are usually found next to hex bolts and carriage bolts and are available in zinc plated, galvanized, and stainless steel varieties. In most cases, the length of the screw increases with the diameter of the screw.

Machine Screws and Bolts

In its simplest form, a bolt is a large machine screw that is threaded in a consistent pattern that allows a hex nut to thread onto it for purposes of wedging a material between them. The most common type of bolt is the hex bolt, which as you probably figured out, has a 6-sided head which fits nicely in your socket wrench. This makes tightening a cinch (no pun intended...well, maybe a little) and is commonly available in zinc-plated, galvanized, or stainless steel finishes. If your bolt will be exposed to the elements in any way, you need a galvanized finish at minimum. If you live near the coast, I recommend a stainless steel product to prevent rust from occurring and compromising the integrity of the fastener.

Machine screws are simply bolts with smaller diameters. They are available in the same finishes listed above and with a variety of screw heads as well. Pan-head is probably the most common type, but there is also oval head, wafer head, flat head, and many others. Please reference the chart for a visual representation of the different head types available and choose the one that's best suited for your project.

Most machine screws are available in both course thread and fine thread as well as metric sizes. The common size is course thread, but you may need a fine thread screw in certain applications. An example of a common SAE bolt size is 1/4-20x21/2. The "1/4" represents the diameter of the bolt measured in inches. The "20" represents thread count, which is the number of threads per inch. This thread count will typically get smaller as the diameter of the bolt increases, so while 20 is the standard course thread count for a 1/4" bolt, the 1/2" bolt is a 13. A 1/4" bolt in a fine thread has a count of 28 as opposed to 20. More threads per inch equal a finer threaded bolt. See, it's not that difficult. Finally, "2 1/2" is the length of the bolt in inches.

Another type of bolt you will probably see in the store is a carriage bolt. These bolts are typically used in construction applications for fastening beams together and are designed to grab and hold tight. The head of a carriage bolt is rounded, so no screwdriver, wrench or other device will back it out. All of the tightening is done on the nut side. These bolts are commonly available in 1/4" to 1" diameters with larger sizes available in specialty stores or special order. In the next section, we'll discuss how diameter is measured for all of the screw types we have covered thus far.

Measuring Fastener Diameters

When you look at a package of screws, you'll probably see something like "#12x2" if you're dealing with wood screws or metal screws. That first number measures the diameter of the screw and ranges from #2 being the smallest you will find up to about a #14 or 1/4" diameter as the largest in most retailers. The second number represented is obviously the length of the screw and is measured from shoulder to tip. This means the full length of the portion of the fastener that will penetrate the material you are working with. If you have a hex head screw, it does not count the head portion that sticks above the fastened material.

In hex bolts and machine screws, the diameter measurements are slightly different. You may see something like "1/2-13x4 1/2" on a hex bolt. The first number, or "1/2" in this case, represents the diameter of the bolt and will be measured in either SAE or Metric units. SAE stands for Society of Automotive Engineers and is the common measurement used in most American made parts. Sizes measured in SAE typically range from 1/8" up to 1" in diameter or larger.

Metric sizes are measured differently and utilize millimeters instead of inches as its unit of measure. A metric bolt size may be M4x45. In this case, the "M4" is the diameter of the bolt. The larger the number, the larger the diameter. "45" represents the length of the bolt and is measured in millimeters instead of inches. Many hardware and big box retailers will carry a decent selection of common metric fasteners, but for those odd sizes, you may be forced to a specialty store to find what you need.


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