The word interchangeable is being used fairly "loosely" here. I think the phrase "will typically work" is a better fit. (All puns intended.) #10-32 and 5 x 0.8 mm are not identical, and therefore are not truely interchangeable. The #10-32 will usually be just a tad sloppy in a metric hole. It's typically not a problem, but if the fastener and hole are on opposite ends of their manufacturer's tolerance ranges, stripped components or parts that tend to loosen up can result. This was not an uncommon problem when Blackburn shipped its cages and racks with #10-32 screws that we threaded into metric bottle bosses and dropout eyelets. We learned that it was best to keep a stock of 5 x 0.8 bolts in various lengths around the shop and save those cute stainless socket head screws for other uses. We also started putting blue Loktite on Blackburn bottle cage screws.
Steve Barner, whose stash of slotted 5 x 0.8 mm toeclip bolts and nuts is finally running low after 20 years out of the bike assembly biz in Bolton, Vermont
> Date: Thu, 10 Jul 2003 20:32:42 -0700 (PDT)
> From: Fred Rafael Rednor <fred_rednor@yahoo.com>
> To: Sheldon Brown <CaptBike@sheldonbrown.com>,
> HM & SS Sachs <sachs@erols.com>, classicrendezvous@bikelist.org
> Subject: Re: 16 mm = 5/8" was Re: [CR]that bolt!
>
> > The common 5 x .8 bolts used for bottle cages,
> > fender/rack mounts and a myriad of other bike
> > applications are often functionally
> > interchangeable with #10-32.
> >
> > However, I would point out that the use of the
> > term "imperial" is ambiguous if you don't specify
> > which empire it applies to...
> Yes, the interchangeability of #10-32 and 5 x .8 mm was used
> to good effect by Phil Wood in years gone by - i.e in the days
> when he used 7/16" Allen bolts in place of 8mm.
---snip---
> Fred "Up With People" Rednor
> Arlington, Virginia