Dear WRO,, Shein dental supply, which you access on the net, carries the belts. Run a piece of string through the pulleys and motor pulley, bring them together and measure this length. Thes belts are sold in lengths and your arm should have a slide adjustment to allow for 6-12 inches of belt slack. If they won't sell to you, call a dental supply in your area, I'm sure they'd be glad to help...........Ed