When doing a K series swap on Fits, CRZs and other Hondas you can sometimes run into an issue were one of the heater core lines on your K series engine is 3/4” but your heater core fitting at the firewall is only 5/8” so your heater core hose will be too small and won’t fit. This aluminum 5/8" to 3/4" hose adapter solves that issue. If you are using a K20z3, K24a2, or K24a4 you will need one of these adapters and a small piece of 3/4” hose.