These spacers help create a larger plenum volume, increase brick surface area usage, and the phenolic versions help with heat-soak into the blower lid. Either material can be drilled for spray bars on the 5/8" and thicker models. Comes with spacer, correct length hardware, and a rope gasket for the bottom (billet and certain modified units only). Does not include an additional brick gasket (2 total needed). See "Custom Modifications" for more options. We normally have 1/2" thickness in stock, most other heights are made to order and lead times can vary. All orders other than 1/2" billet ARE NON REFUNDABLE AND NON RETURNABLE due to material availability and pricing.