If you missed Part 1 of this post, you can click here to get all caught up.
THREADS THAT DON’T TAKE NUTS
And there ends the discussion of thread pitch, right? Nope. Not close. That ends the discussion of thread pitch on fasteners that take nuts. Now on to tapping screws, woodworking screws, lag screws, which have thread pitch (threads per inch) as well:
Please note one thing, while there are metric screws in all these categories, (for example a 4.2 x 50 Pan Phillips Sheet Metal Screw which is really just an 8 x 2 with a metric label slapped on) most are made to IFI inch standards, with the exception of the Wood and Lag Screws, and aren’t really metric. But they sure do cost more 😊
As with my other charts, this is an abbreviated version with many of the rows missing. Only the relatively common sizes are listed. As always, Hardware Everywhere can have anything you want made.
Tapping screws with points are, in theory, supposed to transition to “AB” from “A” but users have almost unanimously rejected the arbitrary decision by IFI and “A” tapping screws are overwhelmingly “A” in North America – well over 15 years after the decision was undertaken with the consultation of absolutely nobody. In the DIN world, AB is the only specified thread for tapping screws with points. In sizes #4 and smaller, no matter if the specification is A or AB the count is the same. In the chart I made I left the “A” column blank to avoid confusion.
Decking Screws should use the PBS thread (asymetrical) but I’ve seen them made incorrectly with the coarse DWS thread. Chipboard screws typically follow the PBS thread as well.
Let’s summarize. Do you want coarse thread or fine thread? The installation material is the most important factor when selecting a fastener thread profile. This is due to the resultant pullout resistance that is determined by the thread profile. Pullout can be defined as the maximum tensile load at which a fastener’s threaded connection remains intact in the application substrate. For most secondary attachment applications, the following rules are applicable:
- Fine thread fasteners should be used in conjunction with thick gauge metal applications. They should also be used in conjunction with thicker and more robust metal applications. Fine threads provide more connection points in thick gauge material, which aids in pullout performance. In addition, due to the small difference between the minor and major diameters of fine threads, it typically requires less effort to drive these types of fasteners, but the installation time is slower owing to the number of rotations required.
- Coarse thread fasteners should be used in conjunction with thin gauge metal applications. Coarse threads have a "clamping" function when used in thin gauge applications which increases pullout resistance. This type of "clamping" action, where the substrate is locked between threads would not be possible with fine threads. Coarse thread fasteners should be used in conjunction with more brittle materials, such as plywood and 2" x 4" lumber. Fibrous materials, such as plywood, tend to embed more readily in coarse thread profiles. This provides additional pullout resistance, as a larger surface area of the threads are in contact with the substrate.
- For drywall screws, coarse threads go in faster, but fine threads hold better in thin gauges of metal. Fine thread screws tend to be a bit stronger. There isn’t much difference in price or availability in the black/grey phosphate. Zinc/Clear (or Zinc/Yellow and even Dacromet® coated in some sizes) is typically only available in Coarse thread and is more money. When building decks, or in woodworking applications use coarse threads. (Way back in the day people used the brand names Stinger® to refer to fine thread and Grabber® to refer to coarse thread. When going into wood with drywall screws, go with the coarse thread.
- For sheet metal screws, coarse (A) threads go in faster and hold better when wood is involved, but fine threads (AB and B) hold better when joining two pieces of metal. Fine thread screws tend to be a bit stronger. There isn’t much difference in price or availability between A and AB, but B are much scarcer and far more costly with limited availability.
- For thread cutting screws, stick to coarse threads. They’re making their own threads anyway, so there’s not much reason to go fine thread. Unless your engineer insists, go coarse. If your engineer insists, question him/her anyway for a valid reason.
- For self-drilling screws, you don’t get a choice of threads in most sizes. However, some #10 come with 10-24 instead of 10-16 threads, usually in the ply-metal variety. These are application specific. In the 12, some sizes of reamer self-drills come 12-24. In the 14, some sizes of reamer self-drill come ¼-20 in HWH only – and a few even come in ¼-28. All these specialty parts have specific applications, and unless your engineer requests them, stick to the standards.
- For machine-threaded (i.e.: takes a nut) bolts of all kinds (and their matching nuts), coarse threads go in faster, are less expensive, and readily available. Fine threads should be avoided in almost all situations. Fine threads are recommended when using the bolt as an adjusting bolt when fine adjustments are needed. Fine threads are much scarcer and far more costly with limited availability.
Besides what we’ve discussed here, there are also Japanese Bottle, French, French Odd-Angled, Löwenherz, Swiss, Swiss Bottle, Swiss Horological, German, German Bottle threads, German Horological, Progress, Elgin, and Waltham threads in addition to others which are not listed here. Most (but not all) are obsolete, but they’re out there. Bottle threads? You see them every day when you unscrew a bottle of soda, a tube of toothpaste, the cap on your tire valve, or even the gas cap on most cars. Threads are everywhere!
If you want to turn into a thread geek, let me recommend the first book you should own. It’s called “Guide to World Screw Threads” and it was last updated in 1969. It was published by Industrial Press using ISBN number 0831-11092-9 and it’s still in print. Not only does it have common thread pitches for standard fasteners in US and Metric, it also has thread pitches for unusual sizes. And it has pretty much every thread pitch for every obscure fastener there is. If you want to know about the threads off a tiny screw in a Rolex watch, your answer is there. You should also have the Industrial Fasteners Institute Book. While this book is in the 11th edition and very expensive, if you’re concerned with thread pitches for US fasteners, any edition after the 7th (available used on Amazon) will suit you fine. For metric threads, you want the Mechanical Engineering Basic Standards 1 (DIN HANDBOOK 1) in the language of your choice; they come in a lot of languages so get the right one.