Description Reviews The MINI Expansion Tank Cap controls the pressure so that the cooling system will work correctly. From time to time we see this fail, which can cause an overheat or loss of coolant. If you feel that this part has failed, a simple test is to start to open the cap after the car is hot and not running. The pressure should release quickly into the tank. If it doesn’t, that means there is no pressure and this cap has failed. Fits all Gen 2 MINI's R55, R56, R57, R58, R59, R60, & R61 (as well as many BMW's and even a few Bentley's!). Factory MINI Part that comes with the MINI warranty!