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My BJ7 had an overflow reservoir fitted by the P.O. I checked it many times and it never had any water in .. probably didn't have the right type of cap. I would keep the water level about an inch down from the top and it stayed there. If you fill to the top the radiator will overflow until it reaches the "inch down" level. That's been my experience anyway.
Cheers,
John
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Hi John,
An overflow catch tank is not the same as a coolant recovery tank. As the water expands due to heat, it opens the pressure cap release valve, & flows into the catch tank. This coolant is not returned to the engine.
The CR tank siphons the water BACK into the radiator anytime the engine cools a bit, keeps the radiator always brim full. This requires a separate one way valve in the cap to let coolant or air in but not out. This one way in valve is found on most caps. It prevents the radiator & hoses from collapsing from vacuum as the engine cools. The regular 7 pound blow off valve works as normal to let excess coolant into the CR tank as things heat up. The NAPA # 703-1411 cap has these two valves, a good top seal, & fits the long neck Healey radiators. It takes a good top cap seal & having the CR in - out located below the coolant level in the CR tank. The average Healey will transfer about a quart of coolant back & forth. If there are any suction leaks between the radiator itself & the CR tank in/out, air instead of coolant will be drawn back into the radiator.
The advantage to the CR tank is that the system is always completely full, no air in the coolant, & no oxidation in the cooling system. A secondary, or primary advantage if you are environmentally inclined, is keeping the cooling systems from burping antifreeze on the ground, parking, or wherever.
D