Hi Jon: First - it is not an "expansion" tank nor an "overflow" tank - it is a gasoline vapor recovery tank. You can tell that because it is mounted higher in the car than the filler neck. If it was an overflow tank, it would be mounted at or lower than the filler neck. It's sole purpose is to contain gasoline vapors within a closed systems, thus preventing them from venting to the atmosphere, until they can be either condensed back into liquid form where they drain back to the tank, or sent to the engine to be burned. Gasoline vapor control started in California {CARB standards} and was required for new cars sold there in 1970. I do not know if California required that system earlier than 1970, but it is possible they did. The Fed's added the requirement to the Federal Emissions Standards for all 50 States in 1971. Early cars sold in Canada in 1970 did not have it, nor did they have the air injection systems required in the US. Sometime after March of 1970 - Nissan started producing all 240Z's sent to North America with the same standard emission controls so Canada got them, necessary there or not. The earliest 240Z gas caps were the vented type {many 240Z's produced in 1969 did not have the vapor recovery systems at all} - which allowed gasoline vapors to escape when the gasoline in the tank expanded from heat, and allowed air to enter the tank as gasoline was pumped out of the tank, or contracted with cooling. With the sealed evaporative emissions control system, vapors are collected in the vapor recovery tank, and vapors/or liquid is pushed to the engine crankcase {then vented into the intake manifold by PCV valve}; when contraction occurs air is drawn into the tank from the air cleaner. This is controlled by the Flow Guide Valve on the left front inner-fender. If you eliminate the gasoline vapor recovery system - to eliminate the possibility of gasoline vapors escaping into the passenger cabin - then you should also make sure your gas tank cap is vented. This is most important - as failing to do so can give the symptoms of a weak fuel pump or vapor lock. The fuel pumps on the L24 are capable of pumping liquid from the tank, to the point that the tank can collapse. The fact that it doesn't happen often has to do with the old gas caps failing to seal as designed when new. If you are unlucky enough to have a really good sealing gas cap - then you'll see the symptoms mentioned above. Someone - some time ago Posted pictures of both types of Z gas caps on the Web. Someone else Posted pictures of his collapsed gas tank as well. FWIW, Carl B