We have come to believe from experience that circular or disc ("coin") fishing sinkers offer an excellent compromise in terms of convenience, efficiency, performance, value, and ease of installation - especially in a build-workshop or clinic environment. This is because circular lead (Pb) sinkers are relatively inexpensive, can be installed with a drill, can work with a variety of designs (including "wings" / "planks" that aren't too thin), and can be lightened or reshaped if necessary. There are at least two types of round / discus sinkers: "river sinkers" that are recessed on the sides, and "flat sinkers" with flat sides. Of the two, the round "flat" sinkers (see image) are the most efficient for derby purposes because they take up less space for the same amount of weight.
Molds under the brand name Do-it allow for the manufacture of standardized round sinkers using molten lead. But I don't really recommend DTer's buying molds and pouring molten lead, because there are vendors that make and sell inexpensive sinkers from these molds. Even better, some vendors will pour custom sinkers without the brass eyelet installed upon request, making them the malleable (but bulkier) equivalent of the original, non-slotted tungsten (W) "rounds". Buying customized sinkers online in quantity is certainly less hazardous, and likely cheaper, for ordinary derby construction purposes than DIY molten-lead pouring. In the interest of promoting safety and economy, I have a vendor recommendation over in the "Derby Products and Services" forum, although there are other sources; this vendor told me that omitting the eyelet was actually a common customization request.
With all that said, the main purpose of this thread is to document the dimensions of the Do-it mold's round flat sinkers so that people can know how to design their cars around them, and have the proper tools on hand to install them. Keep in mind that the listed weight of these sinkers is not exact; it is close but the actual weight can vary from piece to piece or batch to batch by, say, hundredths of an ounce. The one sinker listed here that was consistently different from its advertised value was the very wide 3 oz flat, which ranges 2.8 - 2.9 ounces (and may not be from a Do-it brand mold specifically). The 3 ounce weight is the thinnest of all the discs, but its diameter is equal to the width of the standard pinewood block, so few designs will easily accommodate it.
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List Wt (oz) Diameter (") Thickness (") Diameter (mm) Comment 1/4 9/16 7/32 14.3 3/8 21/32 7/32 16.6 1/2 3/4 7/32 19.2 Press fit in 3/4" hole 3/4 7/8 7/32 22.4 Press fit in 7/8" hole 1 63/64 7/32 25.0 Loose fit in 1" hole 1-1/4 1-1/128 9/32 25.6 Press fit in 1" hole 1-1/2 1-1/32 5/16 26.2 1-3/4 1-1/16 11/32 27.2 2 1-1/8 11/32 28.3 Snug fit 1-1/8" hole 3 1-3/4 3/16 44.3 Relaxed fit in 1-3/4" hole, ~2.85 oz actual
In our workshops, we find that the 2 ounce size is probably most popular; it is the cheapest per pound and thus the best value, and can be drilled out if 2 ounces is too much. We can recommend the 1/2, 3/4, 1, 1-1/4, 2, and 3 ounce sizes for those owning a Forstner bit set in 1/8" increments. The others are going to require specially sized Forstner bits, which if purchased individually, drives up the overall cost of using this kind of weight.
Spray painting these rounds could make them safer to handle, and they could be painted different colors to "color code" them according to weight and hole size. But painting might also affect their fit (some fit pretty tightly already!), depending on the paint.
If somebody wants to experiment with these without making a bulk purchase online, some sporting goods stores sell round sinker weights depending on where you live (and these can be significantly discounted outside of fishing season). Of course, these may not be the same "flat" type as described above, and they will have the eyelet installed.
To remove a brass eyelet, insert a long nail into the eyelet and twist. Usually the eyelet snaps off cleanly flush, but if it breaks, use pliers to pull off or cut away any jutting brass wire. If the eyelet removal process malforms the sinker, you can tap the malleable lead back into its original shape using a small hammer.