Cooling

 To tick off one of the jobs, I decided to have a look at cooling. The donor radiator is just too large, I needed something half the size. The book recommends a VW Polo radiator, and going back to 1996 I can see one that would fit quite nicely. 

The problem comes from plumbing it in. The MX5 cooling system is fairly straightforward;

One loop for the radiator, one for thermostat bypass, and one for heater. The bypass one can stay where it is, the heater one will be looped back. That leaves two pipes to get from the engine to the radiator.

But then I need a reservoir connection, and this is where it gets messy. The MX5 reservoir isn't pressurised, it's cap is just a 'lid'. The coolant pressure is controlled by the radiator cap, which has a tee off to the reservoir. Therefore it only gets water into the reservoir when the pressure is too high. The fill is done through the cap. 

The Polo radiator doesn't do that, it uses a pressurised expansion tank;

The system is filled by the tank, and the tank cap is a pressure release valve. 

All of that means that if I use the Mazda reservoir, it'll end up being under pressure rather than just dealing with the overflow. The lid won't stay closed, water will go everywhere, game over. I think I need to move to an expansion tank like on the polo. But then the plumbing gets challenging, because there is no tap for an expansion tank. 

Following the Polo cooling route, the filler is connected to one of the lower pipes, and the overflow to the top. It kind of makes sense, and hopefully I can do the same. I have found these connectors as well;

https://www.carbuilder.com/uk/black-nylon-stepped-t-piece-3228mm-1512mm

Which I suspect may come in very useful. If I use a T piece to join the heater pipes, then connect the tank to those pipes, that should deal with the fill. The overflow can then connect to a higher pipe. 

What I do need to do is get to a scrapyard and find an expansion tank. Thankfully the expansion tanks were used for pretty much every VAG car so it won't be difficult to find one. I just want to find one that isn't really big and bulky. 

I'll also do a bit of Googling to see what other MX5 based Haynes have done, they must have ended up in this situation as well. 

(Edit)

Skov to the rescue again! Through my build I have frequently referenced his build, even to the point where I use the same blogger.com host. In case I lose the bookmark, here is his blog;

https://mx5-haynes-roadster.blogspot.com/

It's actually completely done, his last post was a farewell as the car got sold. So sad... :(

Anyhow, as expected he's done pretty much what I'm doing, using Polo donor parts. Here I can see pictures of the Polo expansion tank;

https://mx5-haynes-roadster.blogspot.com/2012/09/cooling-system.html

Along with clear photos of how/where it's plumbed in. He has done exactly what I am planning to do, plumb it in at the rear of the engine on the heater pipes. The only difference is that his radiator does have the extra connection back to the expansion tank. I'll need to plumb that in with a T piece off the upper radiator hose.