You swapped a K series into your S2000 or your Miata and you're staring at the factory steel hard line that's supposed to route the coolant return around the back of the engine. It doesn't fit. The firewall's in the wrong place. The heater core's not where Honda thought it'd be. You can't use the factory hard line and you need a different way to get coolant back to the block. This Hybrid Racing fitting slips into the opening on the thermostat housing and gives you a 5/8 inch hose barb. Now you're running a rubber hose straight from your heater core to this barb. No hard line. No trying to bend pipes that won't bend. You get two stainless steel brackets in the kit (one flat for PRB engines, one notched for RBB/RBC/R40 engines) and an O-ring. The fitting slips in, the O-ring seals it, and the bracket clamps it down so it's not vibrating loose or pushing out from coolant pressure. That's it. You can also use this on the factory chassis if you're turbocharging your RSX or Civic Si and you want to delete the factory hard line. Works on K swaps into EG, EK, or DC chassis too if you want a cleaner engine bay or you're going turbo.

Honda designed that steel hard line for a K series sitting transversely in an RSX or a Civic Si with the heater core in a specific spot on the firewall. When you put that same engine into an S2000, it's rotated 90 degrees, and the heater core's nowhere near where it used to be. The hard line doesn't reach. It hits the frame rail. It's in the way of your turbo manifold. You can't use it. Same thing happens when you swap a K into a Miata or an NSX or a Lotus. The engine's oriented differently, and the coolant routing Honda designed for a Civic doesn't work anymore. You need a barb on the thermostat housing so you can run a simple rubber hose from wherever your heater core ended up. That's what this fitting does. It's not just for crazy swaps either. We've seen people use this on EG and EK K swaps because they don't want the factory hard line cluttering up the engine bay. We've also seen it on turbo RSXs and turbo Civic Sis where the factory hard line's in the way of the hot side piping or the turbo itself. If you need a 5/8 inch barb on the thermostat housing for any reason, this fitting handles it.

You're getting two stainless steel brackets with this kit, and you're using one of them. The flat bracket's for PRB engines. That's K20A2, K20A3, K20Z1, and K24A1. The notched bracket's for RBB, RBC, and R40 engines. That's K20Z3, K24A2, K24A4, K24A8, K24Z1, K24Z2, K24Z3, and K24Z7. The reason there are two brackets is that the thermostat housing's shaped differently between those two engine families. The notched bracket's cut to clear the RBB/RBC/R40 housing shape. The fitting slips into the opening on the thermostat housing. You drop the O-ring in to seal it. Then you bolt the bracket down, and it clamps the fitting in place. The bracket keeps the fitting from vibrating out or getting pushed out when the coolant system's pressurized. Slip it in, seal it, clamp it down, run your hose, and you're done.