Um, lessee- what is a plausible made-up answer for this? Fender washer
(as the name implies) is so the bolt won't draw through the substrate,
so you want a tight fit, where the bolt can't pull through the washer.
Regular washers, you want them to be able to move around a little so as
to act like a little bit of a bearing when things flex? They can also
deform a little when tightened, to seat better.
IOW, hell I dunno. But now you have me curious. Where are our resident
Actual Engineers? I've only used fender washers with self-tapping bolts
and machine screws, not through bolts with nuts. (And it was usually on
actual fenders and other car body parts, to refasten through
wallowed-out holes in sheet metal.) I guess the washers for carriage
bolts fall somewhere in the middle- I remember them as being a pretty
tight fit, but they are smaller than fender washers.
Actual washer dimensioning can be all over the map.
SAE washers are typically 1/32" larger than the nominal bolt size.
Aerospace washers can be be very closely sized to the nominal bolt
size (like .510" for 1/2" bolt)
The cut washers you're seeing with greater than 5/16" hole is a so-
called 1/4" are probably junk imports.
Hardware buyers, unless controlled by some sort of spec, buy some
pretty crappy stuff.
QC can be close to non-existence and the original design intent
In my experience, for decent hardware, +1/32" is a good oversized hole
for a bolt, anything more is just plan too sloppy.
Maybe I didn't fully understand the point of your question.
My WAG would be that since fender washers are intended to be used with
screws with "conical" heads, the slightly larger hole allows for the
portion of the head between the substrate and the "outer" edge of the
washer. Otherwise they'd have to bevel the hole...
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