AMC 258 holds 10.5 quarts of coolant; Buick 225 V6 is close to 11 quarts including the heater core. Flush every two years or 30,000 miles. Stant 195°F thermostat and 16 PSI cap are the no-think replacements.
The CJ-5's cooling system is small, exposed, and works harder than the radiator on any modern truck. Air flow through the grille is poor at low speeds, the engine bay traps heat, and there's no fan shroud on a lot of years. Coolant degrades — the corrosion inhibitors deplete in two to three years and the system starts eating itself from the inside, even if the coolant still looks green. The fix is a flush every two years and a thermostat swap every other flush.
Capacity matters because the AMC 258 takes 10.5 quarts (about 2.6 gallons) of 50/50 mix to fully refill, and the Buick 225 Dauntless V6 is roughly the same. AMC V8 (304) variants run higher — about 14 quarts. The numbers include the radiator, block, heads, heater core, and hoses. Drain the radiator alone and you've only removed maybe 60% of the coolant — the block holds the rest until you pull the block drain plug.
The block drain plug on the AMC 258 lives on the driver's side of the block, above the oil pan rail, near the freeze plugs. It's a square-drive plug — 3/8" or 1/2" ratchet drive goes straight into the head. The Buick 225 has block drains on both sides. They are universally rusted shut. Plan to soak them with PB Blaster the night before, and be ready to break one off — most CJ-5 owners replace them with brass plugs on first removal.
Thermostat spec is 195°F for both engines (180°F if you live somewhere hot and run a stock radiator that struggles — Phoenix in August, for example). Normal operating range is 195–220°F. The radiator cap is 16 PSI; if yours is older than five years, replace it. A weak cap won't hold pressure and the coolant will boil at the block.
| Part | Vendor | Est. price |
|---|---|---|
| Stant 195°F thermostat (AMC sixes) | Stant | ~$9 |
| Thermostat housing gasket | Fel-Pro | ~$6 |
| Prestone Long Life 50/50 (1 gal) | Prestone | ~$18 |
| Stant 16 PSI radiator cap | Stant | ~$10 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.