Teams that mostly use metric bolts, what is your go to bolt? How many teams actually use metric bolts?
As a team that uses standard bolts we have chosen the 10-32 bolt (as that’s what most hardware is compatible with) as our go to bolt. 90% of the screws on the robot are 10-32.
I know this forum is about FRC team. Many of us mentors are also part of some other teams that compete in different venues.
Team Automotive Industry didn’t use metric fasteners when I was young but they do now. Team Aerospace Industry still doesn’t use metric fasteners. I hope to live long enough to see that change.
M5 is a nice choice for many FRC situations. Fits the 0.2 inch holes in some of the drilled tube stock that many FRC suppliers offer.
M5 is also a close size match to 10-32, so if a team builds for the latter then the former is an easy switch.
Certified metric user here, in our robots structure, we use mostly rivets, but wherre bolts are required we most commonly use M5.
We also use M6 in our shafts, where we tap the ends and use M6 button heads with washers to retain them.
This year, I would say we have more M6 on our bot than M5 for that reason.
As @Richard_Wallace and @EricH have mentioned, M5 is extremely close to #10-32, and so can be used with most COTS #10-32 clearance holes, given tolerances are correct on both products.
The allen key for #10-32 and M5 is also basically the same which is nice (3.969mm vs 4mm).
However, this similarity can also be a curse…
We have unfortunately had some threaded hardware destroyed due to overzealous students forcing an M5 bolt into a #10-32 hole 
Let me just say that I’m very greatful that Krakens have so many mounting holes 
Yes. 10-32 and M5 are close to the same size but not identical.
Also, they use very different standard nuts.
Best not to make things confusing… the metric knockoff of 1/4-20 looks very similar, but it’s not quite similar enough to thread in.
The first thing I did when I moved into my lab was find and throw away all of the nonstandard hardware. I’ve seen too many ruined optomechanical components (and entire optical tables) because someone mistook a normal bolt for a metric one, and wasn’t paying close enough attention to the torque they were applying.
Close enough that if you are only getting a nut’s worth of threads, you can usually get away with swapping them. Yesterday at work I found that the software guys managed to mount a radio with 2 M5 screws but 1 M5 and 1 10-32 nut. (Don’t go on vacation when you’re the only ME I guess) M5 screw in a 10-32 nut was tight, but it didn’t cause any problems. ( Won’t always be the case if you have more threads or a tighter thread class)
The one that has really caused me problems is M5 button heads loose in the shop. Some student will find them, dig through the wrenches until they find a 3mm, and you end up in the pit trying to take apart a mechanism, your hex wrench won’t fit and the screw is extra tight because of the larger diameter.
5987 uses exclusively M5 hardware. For the reasons other have stated, it’s a good “all purpose” size for FRC. 5mm rivets for the same hole size are also very easy to come by.
5951 uses primarily M4 hardware, with M5 and M6 sprinkled in where more strength is needed. Having more sizes means needing to stock more hardware and tools, but comes with the benefit of weight savings where strength isn’t such an issue.
Outside of FRC, I use a lot of metric bolts. They’re the cheapest to get anywhere, running around $0.01 to $0.05 where a US bolt might be two or three times more expensive (until you buy in the 100k qty). The hex socket sizes also feel a little larger on metric bolts for smaller screws.
I run a lot of M4 buttonheads in test fixtures. Plastic stuff uses M2 tapping screws, sometimes M2.5. I generally avoid anything smaller than M2 or larger than M4, as once it gets that big shipping costs from overseas will make US bolts cheaper. And anything under M2 is just too small (as with anything under #2 imperial).
For FRC products I try to use imperial screws, so that if a team loses something they can find a local replacement. The exception here is self-tapping screws, where you usually need to order it special from McMaster anyway.
Being in the Metric part of the world. I can’t express how may motor or gearbox plate thread are ruined by students who mistakenly put a M5 screw in a #10-32 hole. M5 and #10-32 are just like oil and coolant in the engine. They are a headache to keep apart, and they always manage to find their way to combine with one another.
And at least in Ford’s case, it was a very gradual change. My 1984 F150 is about 1/3rd metric (AOD transmission and most of the body fasteners) and the rest SAE (engine, differential, lugnuts, etc.), my 1995 Ranger (and a second one I owned previously) is about 2/3rd metric (same as the F150, but at least two of the three engine choices are metric; all transmission choices are metric, and oddly the differential yoke flange bolts of all things requires a 12-point 12mm socket/ratchet wrench), and my wife’s former 2003 Taurus was almost entirely metric (to include the lug nuts!). I hear GM was also bad about this?
Meanwhile in my day job (Toyota supplier) the only things that aren’t metric that I’ve seen (on body/exhaust parts we make) are related to gov’t/safety regulations (7/16 NF used for seat belt bolts). Everything M10+ is fine thread too! But Japan has largely been metric for a long, long time.
It’s been a few years since we needed metric bolts but when we stocked some for 775 and 550 motors we bought only the blue died bolts from McMaster. Great visual indicator that prevented a ton of headaches.
Automotive has historically had some “interesting” bolts as well, like metric sized diameter/thread pitch but non-metric heads, or vice-versa.
Another option is black oxide vs zinc-plated or stainless, although of course you need to be careful of strength differences.
