Radium Universal Coolant Expansion Tank, 750 mL Billet, 6AN

Radium Engineering Universal Coolant Expansion Tank, 750 mL Billet Aluminum - Part #20-0270-00

Move the radiator, delete the overflow bottle, or run a K-swap with a low-mounted radiator and you'll find out why Honda put the fill neck at the highest point of the system: air collects at the top, and if the top is a hose, it never bleeds. Radium's universal expansion tank puts a pressurized fill point and an air bleed back at the highest spot in the bay, holds 750 mL of reserve, and does it in a two-piece billet tank with a sight tube and AN ports instead of a plastic bottle and hose clamps.

What It Is and Why It Works

An expansion tank is the pressurized part of the cooling system, not an overflow catch. Coolant expands into it as the engine heats, drains back as it cools, and the radiator stays completely full at every temperature, which means the top row of tubes is always working and the water pump never sees a slug of air. The tank body is two pieces of CNC billet 6061, sealed with O-rings, with three ORB ports on the bottom that ship with 6AN male adapters and three 90-degree push-lock hose ends for the feed, the return, and the bleed. A nickel-plated sight tube on the side shows the level without pulling the cap. The neck is a billet 32mm Type-A receiver, so it takes a standard small-neck Honda cap (the cap is sold separately; Radium's 18-0076-A series fits). A 1/4 inch barb on top is the pressure bleed for a line back to the radiator or the highest coolant passage on the head. Anodized bracket and stainless hardware mount it to any flat surface or tube.

Specs

Fitment

Not vehicle-specific. Common on K-swapped EG, EK, and DC2 chassis with half-size radiators, tube-frame and race cars, and anything running a radiator below the engine's highest coolant point.

What to Know Before You Buy

Mount it at the highest point of the cooling system, or it's just a bottle. The bleed line from the top barb goes to the highest point on the engine or radiator that traps air; the bottom ports go to a low point on the system so coolant can circulate through the tank. Buy a Type-A cap with it, and pick the pressure to match the rest of the system, not higher; a 1.3 bar cap on a stock K-series radiator is fine, a 1.5 bar cap on 20-year-old plastic end tanks is a gamble. Push-lock hose ends take push-lock hose only, not braided AN. If you keep a factory overflow bottle, this replaces it, not adds to it.