For stack pipe mounting brackets on Peterbilt Models 357, 375 through 379, and 385. Lot includes a complete set of four. Our premium USA made exhaust bushings include a heavy duty metal sleeve. Most of the ones sold are imports and include a less durable plastic sleeve. HYPER-FLEX The X-Treme Duty Polyurethane! HYPER-FLEX is Energy Suspension'sunique and technically advanced chemical formulation performance polyurethane. It is used on all of Energy's products. Some that have been awarded 9 U.S. Patents. HYPER-FLEX polyurethane provides longer component life (3 to 10 times over rubber), cutting the cost of vehicle operation! Clearly, durability is superior to most rubber components that are used for trucks and service vehicles– of all classes.