This is a tuned pipe adaptor (also called an exhaust manifold), which is used to adapt a tuned pipe to an engine. This adaptor is a piece of metal tubing that turns 90 degrees towards the rear of the engine. At the end of this adaptor you will install the piece of silicone tubing (called a tuned pipe coupler, one is INCLUDED with this adaptor). The tuned pipe will then fit into the silicon coupler. The tuned pipe is "tuned" by moving the pipe in or out of the silicon coupler until one achieves maximum power and RPM. This adaptor measures 1-5/8" from the center of the outlet pipe to the outside of the adaptor where it attaches to the engine. If one has a big cowl, Mac"s makes two more that fit these engines but extend out further from the engine. See MACG2591 and MACG2592. Outer Diameter: 3/4" (19mm) This tuned pipe adaptor will fit the following engines: O.S. .40 SF and .46 SF ASP .46 Royal .46 8/31/92 JBN *8) This will also fit the Thunder Tiger .46!!! 4/7/97 AST :-)