(Well if this isn't the most obscure topic to discuss I don't know what is... Welcome to the internet.)
So nobody asked this question, and nobody cares about the answer but here we go anyway...
Iron man. Hero; built, not born. The patron saint of engineers. Machinery, gearing, actuators... fasteners! Ah, fasteners; holding the world together, but dividing us so deeply.
Depending on which side of the pond you live on, your opinion on metric vs imperial (US standard) fasteners might vary, almost to the point of fuming hatred. But like it or not, both systems exist. So if a genius playboy philanthropist were to create a mechanical suit of armor, what would he use to hold it all together?
One might think, 'Tony Stark is an American, and therefore uses imperial fasteners.'
Another might think, 'Tony Stark is very worldly and standardized, and must then use metric fasteners.'
The answer may not be so cut and dry. So for sake of discussion and entertainment, let's give this problem a think! (We'll also only be looking at the MCU Tony Stark and not the comics, because we don't have all day)
Why is this even a question?!? Well, this isn't a debate about which system of measurement is better, (that's between you and your doctor). But generally, if you're a machinist in North America, your shop is geared in some capacity to handle both metric and imperial hardware. So what would Tony Stark use during his design process?
To best guess this, we're going to have to look at his past works. Granted, in the home machine shop, you have the discretion to use whatever you want; you usually use what you're more familiar with.
if you're a machinist in North America, your shop is geared in some capacity to handle both metric and imperial hardware
So for starters, Tony Stark went to and graduated from MIT. In US engineering schools they typically teach in mixed units, oftentimes steering more towards the metric system. However, the more applied engineering courses tend to favor the imperial system. I can't speak for MIT directly, but I'd wager they are similar to other engineering campuses.
After graduating, Tony had a large history of design at Stark Industries, a large military contractor. As the "Golden Goose" of the company, he was integral if not the primary designer for a lot of high profile military and aerospace projects. International companies, even if they're US based, tend to use metric units in case a foreign contractor is employed as well as allowing easy integration with foreign users. However, this is mostly done for dimensional specifications, not for hardware.
Looking at Stark Industries, it draws a lot of inspiration from the real life company Lockheed Martin, even going so far as fictionally developing the F-22 and F-16 (both really developed by Lockheed). While Lockheed Martin's space division operates exclusively in metric (after a certain Mars mission unit conversion incident), their regular manufacturing and military contracting is done primarily in imperial (1). A fictional US based weapons manufacturer would likely conform to the same principles. Similarly, another large US contractor, SpaceX (whose owner is often touted as the real life Tony Stark), designs everything in metric but still uses imperial hardware (2).
One of the reasons behind this is that US government contracts, especially in energy and defense, require that all hardware be made in the USA. You may laugh at this, but seriously, there is a very large counterfeit hardware business around the world (which you may also scoff at, but it's true!). Additionally, military contracts often have extra military specifications for hardware (hence mil-spec), which foreign manufacturer's aren't required to follow. While US manufacturers do make metric hardware, it's vastly more expensive (and rarer) than imperial. It's just not made in bulk in the US compared to imperial hardware. Of course there is great hardware made outside of the US, but by sticking with US fasteners, you guarantee a certain level of performance (If you're going to outer space, do you want a bolt that might hold or will hold?).
Secondly, it is still the primary practice in industry to expect imperial units when delegating to fellow US subcontractors. Everyone can handle metric to some degree, but it increases the chances that there will be a conversion screw up. Tony has a fair amount of manufacturing capability at his disposal, but I'm confident he has done his fair share of subcontracting.
...seriously, there is a very large counterfeit hardware business around the world.
The last clue that we have is the way Tony interfaces with his tech. The Iron Man HUD (Heads up display) displays his elevation in meters, but Jarvis tells him elevations in feet (SR-71 record elevation for example). This confirms that Tony is comfortable using mixed units on the regular. It also implies that since Jarvis audibly relays imperial units, Tony is perhaps more at home with the imperial system.
While I have no doubt that the Iron Man armor is dimensionally designed using mixed units, fastener-wise I think not. Based on Tony's large work history as a US government contractor, machining standards in US manufacturing, and the ease of quality hardware acquirement in the States, it is a safe assumption that Iron Man is held together by imperial hardware.
If you made it this far, thanks for reading! I don't know how this topic popped into my brain, but it was interesting (and ridiculous) enough that I felt compelled to write about it. If you like silly thought experiments like this, or have a piece of evidence to add to the discussion, let me know by liking, commenting, and sharing this!