Sankei Coolant Expansion Reservoir Pressure Cap, 1.1 BAR - NSX, 1991-05

The pressure cap on engine coolant systems lowers the boiling point of the coolant while providing a safety mechanism to bleed off coolant pressure if the engine overheats. The factory expansion cap is rated for 1.1 kg/cm3, or 16 PSI. The cap should maintain this pressure, then bleed off pressure above this point. 

ScienceofSpeed previously reported new factory expansion caps failing. A batch of factory caps were tested. Factory caps are holding only 8-12 PSI. This creates an increased of coolant fluid boiling resulting in less efficient cooling of the engine and potential damage.

ScienceofSpeed tested a batch of expansion caps, and identified an aftermarket replacement cap that fits properly and holds 16 PSI of pressure. The Sankei cap is manufactured in Japan and features high-quality EPDM seals and has the correct pressure verified rating of 1.1 kg/cm3.