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Hi, hoping some people here can help me. How do I determine screw size, pitch, etc to order some extra fasteners from McMaster Carr? I want to make sure I get the right thread pitch, diameter, and any other characteristics or features I need to be aware of.

I also want to drill and tap some screw holes, what tap size and pitch do I need to get? I am working with some fasteners for both an RPM bumper for a Traxxas Slash (2mm and 2.5mm hex head for the screws) and I will also be doing some of the same for rc10 gold pan applications like mounting an rpm gear cover on the motor plate which would require drilling and tapping a hole.

Unless I'm confused by your question. All you need to do is determine what hardware you plan on using. Then buy the drill/tap set for that screw.

Search the web for a tap drill chart. That will give you an endless amount of information regarding fasteners.

Long story short you use these as common threads in various lengths

2-56
4-40
5-40
6-32
8-32

2mm x .4mm
2.5mmx .45mm
3mm x .5mm
4mm x .7mm

Most Americans cars will use standard screws and rest of world is typically metric

Thanks, does thread pitch matter or is there no variation? What does the 5 and 40 stand for, with a 5-40 screw?

What's the 50% and 75% pitch thread thing? Where is 5-40 on this?

http://www.davehylands.com/Machinist/Charts/Standard-Tap-Guide.pdf

A 5-40 is a #5 screw with 40 threads per inch
Metric has a stated thread pitch as well, but in the rc world it isn't really spoken of because there isn't a "fine" pitch until M8 and larger really.

flipwils11 wrote:What's the 50% and 75% pitch thread thing? Where is 5-40 on this?

http://www.davehylands.com/Machinist/Charts/Standard-Tap-Guide.pdf

If you go down to 5 in the first column, there is a 40 and a 44 listed. 44 is the fine thread, we don't use it.
The 50% and 75% must relate to how much thread it cuts, but I have never seen that be an option. Must be a machinist thing

I think when speaking of metric, the only major difference between the threads of a 3mm screw, for example, is if it's a machine screw or self-tapping screw.

You can make your own tapping screw in a given size on the cheap. Choose a good screw to make your tapping screw, the harder the better. Then using a Dremel and fine cut-off disk, cut two notches on opposite ends of the threaded end; i.e. at 12 and 6 o'clock. You want the deeper part of the notch to be at the end, and the notch decreases in depth as you move up from the end. Look at a tapping bit and you'll see what I mean. This image I googled is a bit messy & not very straight (not mine), but kind of shows what I'm talking about. I don't own a tap & die set but I've tapped my own holes (mostly 3mm) easily enough in nylon, plastic & aluminum using my own homemade tapping screws. Also, if you decide to make your own from a donor screw, choose one with a head you can really get good torque on; hex head would be best; Philips, not so much.
When you're ready to tap your hole, probably drill a hole half a millimeter smaller than the size you wish to tap with your newly-made tapping screw. Then insert the tapping screw and gradually twist it in and out, again and again, a bit deeper each time...and take your time. The notches in the screw catch the material you're drilling out. When you unscrew it, blow out the dust you've tapped. Eventually you'll have a nice, threaded hole in the size you need. Practice on a spare or castaway part of the same material to get a feel for it; it's pretty easy.

the thread depth has to do with the material being cut. harder/stronger materials need less depth of thread to get the rated strength, which makes it much easier on the tooling, the taps and dies are stressed less and break less often. The weakest part of the joint is the root of the thread of the screw anyway.

yes, standard hardware uses a # for the screw gauge, along with threads per inch.

Metric uses pitch, direct distance between each thread, and the screw OD.

these are just standardized, too, before they made standards, people made whatever thread they wanted, it was a frickin' mess.

5-40 is not a very common screw nowadays, but for our RC gear, associated in particular, it's used on the 6 gear top shaft diff adjustment nut, and was also used on b2 for mounting plates, I believe. Also used on Losi shock shafts.

most metric cars use 3mm as a standard, unless they use a 3mm COARSE thread for plastics, which is completely different beast as far as thread pitch.

since you are in the US, I would just pick up taps and dies from places like mcmaster, rather than try and make your own tap, they are dirt cheap, 5 bucks or so, for bright finish and will work well for plastics, aluminum, mild steel, etc.

tapping itself however, especially for smaller than #4 or 3mm screws needs a very sensitive touch. it pays to set up your tap and tap handle in something like a tapping station, or drill press, or even just a tapping block. it's very easy to start a tap crooked which will result in a broken tap guaranteed.