A countersunk socket head cap screw in stainless steel, 3/8 inch UNC by 1 inch long. The countersunk head sits flush with the surface once tightened, which is what you want on tank mounting flanges, cover plates, floor panels and any face where a protruding bolt head would foul something or catch a hand. The internal hex drive lets you get proper torque into it with a plain Allen key even in a recess where a spanner will not fit. Stainless resists rust in exposed and underbody positions where a plated steel screw would soon stain. The 1 inch length suits medium-thickness assemblies; check your grip length so the thread engages fully without bottoming out.