Coolant Reroute: RHD version

Here’s my take on the coolant reroute.


The Problem

The 1.6L NA MX5 inherited its B6 engine from the existing Familia GT-X. The B6 engine in the Familia was mounted transversely, with coolant flow entering one end of the engine and leaving the other. The MX5 implementation of the B6 engine had it mounted longitudinally. In this arrangement the coolant flow entered and exited the front of the engine and as such, No.1 cylinder has more than enough cooling but the coolant flow is gets progressively worse towards the rear of the engine. Mazda fixed this issue in later MX5s by implementing a head gasket that encouraged a uniform coolant flow.

A popular modification of the MX5 is a coolant reroute. This shifts the coolant outlet from the front of the head to the rear of the head thus creating a uniform coolant flow within the engine. There are existing kits such as one offered by BEGI, however these typically reroute the coolant pipe along the intake side of the engine. This is a good solution if the MX5 is Left Hand Drive (LHD), but the RHD cars have clutch hydraulic lines that make it very difficult to reroute the coolant pipe along the intake side.


The Solution

A Google Images research session revealed rerouting the coolant pipes along the exhaust side of the engine has been done successfully. I decided that it would be easiest in my RHD MX5 to perform an exhaust side coolant reroute. No existing kits allowed for this, but between my father and myself we had the skills and equipment to fabricate our own.

The heater core in the NA MX5 sources hot coolant from the rear of the head. We must consider the following:

  • A coolant reroute will have the heater core hose and main radiator pipe sharing the outlet.
  • The thermostat will be relocated to the rear of the engine.
  • The heater core hose should be connected upstream of the thermostat for the heating function to remain independent of thermostat control.

I also wanted to reduce the number of custom parts (ie, use/retain the stock parts) as much as possible.


The Implementation

Engine pull time! I like to take the engine and gearbox out as a single unit. It’s not much extra work in the 1st gen MX5s and beats trying to reach the awkward bellhousing bolts. Once engine is out, it’s time to remove the thermostat housing and the extension. You will need to remove the valve cover and cam pulleys to reach the thermostat housing extension bolts. Moving to the rear of the engine, the housing to the heater core removed and some plywood to obtain a precise measurement of angles.

Ideally a Kia Sephia thermostat housing would be used, however with limited project time, it was decided to reuse the old thermostat housing. The thermoswitch mounting needs to be milled down and plugged.

It was originally intended for the plug to be welded, but the poor quality of the thermostat housing made welding fruitless, so it was later bonded with high-temperature, water insoluble adhesive.

The spacer is next. As I was a engineering student (at the time of building), I was able to get PTC Creo 3D CAD software for free. I designed the spacer and produced drawings for my father to build on the lathe and mill. The heater hose barb was then welded in. Two things to note:

  • The location of the lower thread for the thermostat housing prevented the spacer having a flat bottom. The bellhousing has a ridge that interferes with the spacer. This was filed down with a large radius to avoid stress concentrations.
  • New is a good time relocate the ECU temp sensor into the “cursed water plug” barb. Drill it, tap it, done. The sensor needs a new home, why not also use the sensor to plug up that cursed plug?

Also fabricated a flaring tool to facilitate a good seal with the radiator hose. The cap screw is tapered at the end. Screwing it in pushes the ball bearings out. Use a large spanner to rotate the tool.

I got carried away with welding the pipes, so I didn’t get any photos of the construction. Used two 90° bends, two 45° bends (one of which was cut in half), and a straight section of stainless. The pipe is mounted at the lifting eyelet. Also made a port for the radiator fan thermoswitch. The entire fabrication was done using purge welding and as a result, there were no stalactites inside the pipe (checked with a bore scope).

Moving to the front of the head, a blanking plate is required. I’ve seen people leave the thermostat housing extension on the head and blank it off there, but it looks bad and the extension no longer serves a purpose. Two barbs are needed: one for the throttle body/idle valve coolant feed, and one that returns to the water pump. I’m not sure of the purpose of that last one, I speculate it is to provide a minimum flow for the water pump, but I would have thought the heater loop achieved that already. It’s time like these where it’s best not to doubt Mazda and change their design.

The final product! Everything fits very well, and it doesn’t look too out of place. The power steering tensioner clears the 90° bend well. No wiring harness modifications needed. Uses standard upper radiator hose. Job well done.