2-Pass just means that the coolant passes through the radiator twice. In other words, for a 2-pass radiator, the coolant makes a 180 degree turn and goes back to the same side as the inlet. The flow pattern is like a U.
In a 4-pass radiator, the coolant starts out on one side, goes to the other side, turns back for the second pass, reaches the initial side, turns back to go for the 3rd pass, then reaches the other side to turn back and go for the 4th pass and reach the initial side. The flow pattern is like an M.
In a single pass radiator, the coolant starts out on one side and the outlet is on the opposite side. It is one pass through the "length" of the radiator.
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