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 and filter plate are thinner than the the original filter elbow. Ref: SS51, SIE0313.