The 8 inch port is the one you specify for the day you hope never comes: a clogged pickup, a failed float, a sensor that needs replacing from inside. At this diameter you get genuine arm access, which turns those jobs from tank-removal events into afternoon fixes. This version is black collar, black lid - the natural match for holding tanks, where dark hardware on a dark molding is exactly right.
On waste duty an opaque lid is not a compromise, it is the point: sealed, sober, and invisible until needed. The wide thread also makes it the fastest lid in the range to spin off with gloved hands.
Ronco installs the collar on your tank at order time at its Tustin, California plant, keeping the placement coordinated with your dump and vent locations - per-order tank finishing as practiced there since 1974. Fit it to anything in the RV holding tanks range or the marine shells under boat tanks; the full line is on the Ronco Plastics hub.
OneSource Plastics is an authorized Ronco Plastics dealer.