"Steel-tipped" inlet needles are necessary when using exotic racing fuels or alcohol or when using benzine or acetone additives. a . 097" seat size should be used for small four barrel carburetors; a . 110" seat size should be used for carburetors up to 735 CFM; larger seat sizes should be used with carburetors 750 CFM and larger. "Adjustable" needle and seat designs allow external float adjustment, without removing the fuel bowl. The "non-adjustable" needle and seat design does not provide for external float adjustment. The fuel bowl must be removed for this procedure. All Holley model 2010, 2300, 4010, 4011, 4150, 4160, 4165, 4175 and 4500 DOMINATOR carburetors will take either one version or the other. Center hung fuel bowls use only the "adjustable" style needle and seat, as do all model 2010, 4010, 4011 and 4500 DOMINATOR carburetors. Side hung fuel bowls have come in a variety of styles over the years and have used both the "adjustable" and "non-adjustable" styles needle and seats.Pit Stop USA offers Holley Needle & Seat - "Steel" Needle and Seat (Adjustable w/ .150" Seat) 6-519-2 at low everyday prices. Be sure to check out all of our products from Holley Performance Products including Holley Performance Products Needle and Seat.