Number 10 stainless steel machine screws refer to a standard SAE diameter which is smaller than 1/4" diameter. They come in fine thread (32 threads per inch, or tpi) which would be designated 10 x 32, or coarse thread (24 threads per inch) which would be 10 x 24. Either threading will work as long as you have the appropriate stainless steel nuts.
You are likely going to have trouble finding stainless steel machine screws long enough for your seat hangers, though, depending on how much drop you want. Of course, the seat trusses you got from Ed's can be trimmed. The gunwales are usually an inch in height or so and using a finish washer will put the head of the machine screw a little above the top surface of the gunwale. The thickness of your seat frames is probably 3/4-1" or so and you need to have the end of the machine screw stick out at least 1/4" or so to get a nut on it.
That means for a 4" drop you will need machine screws that are at least 6" in length and they are hard to find. I have bought them from Jamestown Distributors in the past but they might have a minimum order requirement of 25 or so. You can find other online sources but they might have a minimum order requirement too, and might charge you a substantial shipping fee as well.
You might have better luck finding stainless steel carriage bolts in that length. If you are unfamiliar with these they are like machine screws with a somewhat wider rounded head but the head as no slots like a Phillips or standard slotted bolt head. Instead they have a short, squared segment on the bolt shaft just below the head that fits into a square hole and keeps the bolt from turning when you put the nut on. They do work but require you to cut or file a small square hole in the top of your gunwales to fit the carriage bolt into.
As for readily available pop rivets, many hardware stores carry steel pop rivets as well as aluminum, but your hand will get a real workout cutting the steel shafts with a hand pop riveter. It can be done, however. Actually, I have used the aluminum variety readily available in hardware stores and never experienced a problem, but I have only replaced a few rivets at a time, which would be the case in your situation.
If you use a stainless steel machine screw and finish washer the head of the machine screw will stick up a bit from the surface of your gunwale. It can scrape the gunwales of an inverted canoe if you slide it over the top of your boat in a boat-over-boat rescue.
Actually, the great majority of whitewater canoes with vinyl gunwales or vinyl-covered aluminum gunwales were set up this way, and I have never experienced any particular difficulty scraping my hands on the machine screw heads, but other people might have a different experience. Carriage bolt heads stick up a little too but have a lower profile and are rounded so they are a bit less intrusive.
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