The Water Passage. This is going to be a long post.... On the back of the J series motor, there is a coolant manifold that Honda refers to as the water passage. Water coming out from the water pump passes through one middle of the water passage, through the thermostat, and out to the radiator, and then it comes back in to another section of the manifold from the radiator return where it is routed in to the back of each head. It also contains EGR passages that run from the head to Intake, the EGR valve mounts to it, The Air-Assist valve is mounted to it, it holds a temp sensor, and it has misc coolant input/output connections for Heater core, transmission cooler, and IAC valve.
This is what a stock one looks like off the motor (not mine)
While not really mandatory, there are several things that I am changing about my water passage. First was the replacement of the EGR valve with a blockoff plate. (This was shown back near the beginning, but here it is again to show all related things together:
The second change relates to the big coolant return pipe seen in the top right of the stock part photo. It is pointed back toward the firewall. While there is clearance between the firewall and pipe itself, it doesn't really leave the room needed to get a hose on there. Given that the Minitech kit is budget minded, the instructions call for you to "massage" the firewall with a body hammer to create needed room. This didn't sit well with me, so I picked up a new pipe. Speedway motors had what looked to be exactly what it needed, so I ordered it:
It was exactly what I was looking for. Given my welding is limited to ugly steel welds with my Harbor Freight flux wire welder, I took it to a welder near me with aluminum TIG experience. a Day later and the wallet a little lighter, The old pipe was removed and my new one was cut to size and welded on:
Once it was back, I did a quick test fit. The bend of the pipe is more than 1/2" off the firewall, and now there is plenty of room to easily get a hose on there.
Next up was deleting the air assist valve. It is mounted to the top of the water passage and is designed to let extra air in around the fuel injectors when cold to improve emissions during warm-up. Not needed. Prank parts makes a nice blockoff plate for it.
Here it is alongside the factory part:
And here it is installed.
Now I need to cap off unneeded coolant nipples. I got some silicon blanking caps and some Gates Powergrip shrink to fit hose clamps and capped them all off.
While I was at it, I capped off the line intended for the transmission cooler that comes off the water pipe between the pump and passage.
Also of note since I brought up the Gates Powergrips, I picked up enough to do all of the radiator and heater hoses too, and a bunch extra for on hand spares. Zoro.com is really the place to get them, as they charge about half what autoparts stores and places like jegs charge, plus free shipping when you spend $49.
Last up is dealing with the Temp gauge sensor. Honda has a temperature sensor that feeds the ECU, and then the ECU feeds the gauge, but that doesn't work for the Miata gauge. An easy fix is simply bringing over the Miata gauge sensor and finding a good place to install it.
The water passage has a nice spot for it that is tapped on some J series engines, but not mine.
The Miata sensor is 1/8" BSP tapered thread, so I picked up a tap and went to town. A few minutes later, we had this:
Also done but not photographed was the addition of a new thermostat and gasket. You may have noticed from a prior pic that the thermostat housing bolts were making contact with the water passage. The long bolts are made to work with the OEM thermostat cover, but are just way to long for the Minitech one. I turned to my bin of old Miata nuts and bolts and found some shorter ones that worked perfectly. *Side note- I have come to the conclusion that the biggest benefit to parting out a Miata is the wonderful supply of metric hardware it gives you. It saves me from a trip to the store and a regular basis.
I need to go back and add some PTFE tape to the sensor threads, but other than that, the water passage is ready to be reinstalled on the engine.
All done!