GROMMET TYPE HEAD GASKET FA2023002 AT001 High Boost Capacity Head Gasket FA20 engine tuning through turbo and supercharging is becoming more and more popular. In these cases, many people will eventually experience the issue of head gasket failure. To address this, HKS releases a new grommet type head gasket which takes things a step further than the existing stopper type gaskets. Piston Gasket Thicknee mm Compression Ratio Stock Stock 0. 6 12. 5 HKS 1 HKS 0. 8 10. 8 HKS 2 HKS 1.