I have a sanborn magna force 5hp air comp. The belt is missing and would like to know the length for replacement

The web address below will provide you with the information necessary to order a belt for your compressor. Check the model number first however to insure correct part.
http://www.mastertoolrepair.com/165tgc51b850tgb8p165tg51h850tgh8p-p-1454.html

I have replace the V belt on my compressor with an automotive type belt to save money (and better quality). I used a tape measure around both pulleys to get the length. Check the motor/compressor for adjustability as this will help when tightening the belt. If the motor/pump is already positioned all the way out, re-adjust to closed position and remeasure. Now for width of the belt; automotive belts are sold by length and width and use a common numbering system of five digits. The first two numbers printed on the belt refer to the width and the last three refer to length. Measure the width of the pulley grove. My pulley measured about 1/2" . Auto belts were available in 13/16", 15/16" and 17/16" widths. These belts are marked with first two numbers as 13 or 15 or 17. Next use the measurement around the two pulleys. My compressor measured 47 inches around the pulleys. So the automotive belt number for my compressor is 15470. If the measurement around the pulleys was 471/2" then the belt number would have been 15475. The last number refers to 1/2" either a (0) or a (5) Most autopart stores will ask you for the year and model of car for your belt but ask to see the belts with tape measure in hand and check for size using the numbering system for a guide. If your compressor uses a flat style belt you can buy from www.grainger.com. Measure around the pulleys the same way for length. Good Luck with your project.