If you go to the store and ask for "1/4" washers"- the OD is 3/4". The hole in them - the ID - is actually a bit larger than 1/4" (~5/16")
Bolts, nuts, washers, etc. are manufactured to meet various standards. I don't have the actual standards on hand, but you can see the sizes in this chart
. You can find similar info for nuts, bolts and other hardware.
Thanks for the link. Interesting. A little follow-on on this subject - for both competitors and event supervisors..
The rules call for "3/4 inch diameter flat washers"
From the table in the link, USS spec "1/4 inch" washers are not quite 3/4" OD; at 0.734", 0.016" smaller. Note, too, that SAE spec washers are a lot smaller- 1/8th" smaller; they are obviously not the correct/intended one. Don't buy them. It might be possible at a fastener specialty shop to find a 0.750 washer, but it seems reasonable to assume USS spec washers are what is intended.
You'll see when you go to Home Depot, they have, in bins, both 1/4 cut washers, and 1/4 flat washers. They're USS. They also have little bags of SAE 1/4.
Anybody bothered to look closely at a washer?
Interestingly, when you throw the caliper on the cut and flat, the flat is slightly larger; 18.6 mm, vs 18.7mm; 0.0039 inches larger; does that 4/1000th of an inch make a difference? I don't think so; can't imagine a design/build where if you used one, and at the competition had to use the other, it would be a problem. But the difference is there.
There's something more interesting, that could have... a small but measurable performance implication- if you're trying to be seriously competitive; playing with small variables, looking for little increments of improvement. Take a close look at your washer(s). Both the "cut" and "flat" washers are not flat. Get a good straight edge; steel ruler, razor blade; put the flat sides against the edge, and look with a bright light behind. On one side, there are tiny lips, around the OD, and around the center hole; maybe 1 or two thousandths above the flat surface. It appears that the washers have been made by stamping them out of a sheet of steel with a die. On the "front/upper" side, the edges (outside and center hole) are pushed down/rounded; let's call this the rounded side. On the "back/lower" side, the metal is pushed out, forming sharp edges, and the noted "lips" let's call this the sharp side. If you put the sharp side against a straight edge, and assume the little sharp lip will push into wood if it's tightened against an attachment plate, the "contact diameter" is 18.6 (or 18.7)mm. If you flip it over, though, the contact diameter is only 15.4mm. Not a lot, but significantly less.
So, how does it matter that the sides are different?
First, looking at the tension member attachment to one bolt; conventional attachment plate held to the wall by bolt/washer; tension member(s) glued into the plate. Could be one tension member, could be two on either side of the washer. With the tension member(s) out beyond the edge of the washer, when loaded, there will be a force trying to bow the plate-pull the side(s) with the tension member(s) attached away from the wall. With the rounded side of the washer against the plate, it will be able to bow more. How much, depends on how far beyond the washer the T-member is mounted, and how stiff the plate is...? 1/2mm....2mm. That'll cause two things; effective lengthening of the member(s), letting the boom/compression member droop, and a twisting force applied to the T-member, and it's glue joint in the plate. It will fail at a lighter load than it would if the washer were mounted with the sharp side to the plate.
Second, if you're using 2 T-members mounted on 2 bolts, an additional factor- if the two washers are mounted with opposite sides against the plate, in addition to the distortions at the one mounted rounded side to plate, the asymmetric pull of the two tension members will put torsional (twisting) force into the compression member(s), again, leading to earlier failure.
Enough on washer minutia.....