Crain Auto Lock Tube. Crain 501 Auto-Lok tube adjust your stretcher poles in a flash with our Auto-Lok Tube! Just push back on your tubes to extend, push buttons and pull back to shorten. A proven time saver! Adds up to 4 feet 8 inches of length to your stretcher in convenient 1 ½ inch increments. Works with other Junior Power Stretchers with standard size poles. Closed length: 2 foot 10 inches. The range of useful length adjustment: 2 foot 10 ½ inches to 4 foot 8 inches.