Aluminum stub stack for SU HS4 and HIF4/38 carburetors. Proven by repeated independent tests to provide 3 to 5 more horsepower on a 1275 by smoothing the airflow into the mouth of the carburetor. Effective, in spite of its rough cast finish. Originally manufactured by K&N UK to fit inside their filters. No additional gasket is used, as it fits inside the air filter. The original bolts may be used, as the stub stack & filter plate are thinner than the the original filter elbow.