This is the perfect weight for Pinewood Derby cars. Tungsten is a highly dense material which is almost as dense as pure gold. Tungsten is 1.7 times more dense than lead and 2.7 times as dense as zinc.  
When you have a wafer thin car or are just trying to push your weight farther to the rear of the car, Tungsten is the ticket!

Each package of Derby Champ tungsten contains two cylinders (3/8" in diameter, a little over 3/8" in length, and weigh ½ ounce each). They fit perfectly in to the pre-drilled weight holes found our blocks, wedges, and pre-cut cars.  

NOTE: You are buying 1 ounce of tungsten. Most pinewood car derby cars require 2 or 3 ounces of additional weight  
* Tungsten is also non-toxic and environmentally friendly.