The 1983 Trek 520 is a General-duty Touring model with 430mm chainstays, Reynolds 501 tubing, a 28/45/50 triple crankset, 27 inch wheels and sidepull brakes. This is an ideal medium duty touring rig for moderate loads. The 430mm chainstays are 10mm shorter than than most modern touring bikes, not an issue for most touring riders, but a limitation for large panniers. The sidepull brakes will fit 27x1 1/8 tires, smaller than ideal for heavy loads and gravel roads. The 501 tubing has thicker walls and is a little stiffer than standard 531, but not as stiff as 531C. See: http://www.vintage-trek.com/TrekBroc...art1.htm#page7

In 1983 the 720 was Treks Long-distance touring bike with longer 470mm chainstays and Reynolds 531C tubing, a 28/45/50 triple crankset, 27 inch wheels, larger 27x1 1/4 tires and Cantilever brakes. See: http://www.vintage-trek.com/images/t...3/83Trek14.jpg