As Complete As Possible: How To Do An MX-5 2.5 Swap

There’s a fair amount of content on the web about swapping a 2.5-liter engine into an NC chassis MX-5, which comes from the factory with a 2.0-liter that shares its architecture with a number engines used in different Mazda and Ford products. The main difference between the two engines is that the 2.5 is a bit taller due to increased stroke. It also makes more torque and horsepower, which is the whole point of the swap.

You can find video series on YouTube and some information on different sites around the web, but they all leave some or much of the swap up to you to figure out as you go along. The information is there — verbally if not nominally — and the websites and videos tell you what to do, but that does not follow one of the basic tenets I learned in journalism school: show, don’t tell.

That’s what this story is all about. Here, we are going to show Speed News readers and any other home mechanic intrepid enough to take on a 2.5 swap how everything goes together, with photos as detailed as we could muster. The difference is that after reading this, you won’t have to figure things out for yourself — we hope. Everything you need to know to do the swap is explained here and photographed in detail.

My goal with this story was to create the best guide on the web as to what goes into swapping a 2.5 into an NC chassis Mazda MX-5. Whether I’ve accomplished that, well, I’ll leave that to you to decide.

That said, you will find this is a lengthy “how-to” feature, and given the number of photos needed to show everything, I think it’s probably the longest scroll in Speed News history.

I sourced an engine online from LKQ. It came out of a crashed 2017 Ford Fusion with 19,000 miles on it. These engines are in good supply now and for the foreseeable future, which makes this a great swap if you are running an NC chassis in NASA Super Touring. If you want to do the swap, you’ll need to find a 2009 or newer 2.5 pulled from a Mazda3, Mazda6, Ford Fusion, Mercury Milan or almost any Duratec four-cylinder engine that measures 2.5 liters.

Bear in mind, engines that come out of Mazdas are generally more expensive than those that come out of Fords. I’m told it’s because of higher-quality internal parts, but it just as easily could be due to greater numbers of Fords in salvage yards.

Because the 2.5 was predominantly used in front-wheel-drive applications, there are a number of components that need to come off the 2.0-liter coming out of the car before you can reinstall the 2.5 in the car.

The aluminum arms that connect the block to the engine mounts and subframe are different, so R&R them from the 2.0 to the 2.5 engine. While I was in there, I installed new engine mounts.

Again, because the 2.5 comes out of front-drive stuff, the oil pan is different and so is the oil-pump pickup. The pickup is not expensive, so I bought a new one and the O-ring that seals it to the pump housing. Even if the pan would fit in the NC subframe, you wouldn’t want to use it because it has minimal internal baffling. Oil would just slosh around freely during spirited and track driving, but that’s also because the front-drive pan needs to accommodate the large balancer under the engine.

When you remove the stock 2.5 pan, you’re going to find a large, gear-driven balancer assembly, which counteracts the primary and secondary vibrations inherent in four-cylinder engines due to varying piston speeds at the top and bottom of the stroke.

When you remove it, you’ll need to plug its oil feed hole. I used the balance shaft delete kit from Fab9 Tuning, which has a whole section of its website devoted to the parts needed to do the 2.5 swap. Torque for the retainer bolt isn’t specified anywhere that I could find, so I torqued it to 30 foot-pounds, which seemed right. Red Loctite on the threads will keep things in place.

You also need to remove the crank pulley/balancer from the 2.0 liter and use it on the 2.5, because it has a different number of teeth, and because the 2017 Ford Fusion it came out of also used two belts. The 2006 MX-5 uses one belt. You must use a new crank friction washer and a new pulley bolt and washer. The MZR engines don’t use keyways on the crank or cams, so these friction washers are essential to holding everything in place and can be torqued only once. Same goes for the crank pulley bolt. The parts list below includes the part number, or you can find them on the Fab9 Tuning site.

I had the cam washers and bolts on hand, but I never had to unbolt either cam, so I didn’t end up using them. The engine only had 19k on it, so I didn’t think they need replacing for safety’s sake, either.

I will outline how to set the timing in another feature. This story is all about the parts and labor needed to swap the 2.5 into the MX-5. We’ll also publish a clutch installation story using an ACT lightweight flywheel and clutch kit, and how to get everything back in the car.

The coil packs and the spark plugs are different. I reused the coil packs and installed new NGK plugs for a 2.0-liter in the 2.5.

Also, use the valve cover from the 2.0-liter. The PCV barb is in a different location on the 2.5 and, because I live in California, I needed the car to pass smog and appear stock. Catch cans were not an option. A new cam cover gasket also is a good idea while you’re in there.

To fuel the car, you will need to reuse the 2.0-liter fuel rail with the fuel injectors from the 2.5. I had my injectors serviced at NASA member-owned Injector Pulse before I installed them.

I also installed a brand-new-from Mazda engine wiring harness, which is still available. Spec MX-5 builders have been encountering problems with original equipment harnesses, so I figured I’d get ahead of the curve and replace it before it gave me fits trying to diagnose electrical problems later.

On the back of the head, the coolant outlet and coolant temperature sensor are different, so swap them from the 2.0-liter to the new 2.5. Use a new O-ring on the coolant outlet for good measure. While you’re back there, install the EGR valve on the back of the head and use a new gasket.

On the block, the thermostat housing and PCV system are different. I sourced a lower-temperature Mishimoto thermostat housing from Goodwin Racing, and used a Motorcraft PCV baffle from a Ford Ranger, which has the correct size hose barb, is thinner and allows the 2.0-liter plastic intake manifold chamber to sit properly. Some of the coolant hoses are no longer available from Mazda, so I got all that I could and ordered a silicone coolant hose kit from Goodwin Racing.

For the EGR, you want to bolt the pipe to the intake first, then when you attach the plastic intake to the aluminum intake, you can thread the pipe fitting into the EGR fitting on the cylinder head.

Because the cylinder heads are different — the intake ports are 5 mm larger from top to bottom — you need to open up the ports on the aluminum intake manifold. I made a template from poster board and took it to master cylinder head magician Todd McKenzie so he could use it to port the aluminum intake. It’s not a lot of porting work, but I am certain he is better at doing it than I am, so I didn’t mind paying for more qualified labor.

When bolted to the 2.5, the ported aluminum intake still has a inward “lip” on the top of the port where the intake and head meet, but so does the unmodified intake when bolted to the 2.0-liter head. It’s just ahead of where the fuel injector fits into the intake port on the head, so maybe the lip induces some “tumble” to the air column.

If I had an unlimited budget, I would have had more material welded to the top of the intake manifold where it meets the head so we could port-match it exactly, but that would have required removing the locating dowel pins and machining the mating surface, which would have added time and expense.

Most of the instructions say to modify the factory 2.0-liter intake gasket, but McKenzie suggested anaerobic sealer, a gasket maker that seals in the absence of air, which is ideal for two precise-machined mating surfaces. I torqued the intake to the head and then wiped off the sealer from inside the port runners.

I have read about people slotting and enlarging the holes where the bolts go through to fasten the intake to the head, but I found that there was enough “play” in the dowel pin holes that you could push the manifold up to its highest possible position on the head and bolt it down to maximize the port match. Slotting the bolt holes didn’t sit right with me because it seemed like it was creating a failure point.

Because the 2.5 engine is taller, you do need to use the 2.5 timing cover. But, because it’s from a front-drive car, it has large mounting boss and three studs for transverse mounting in the Ford Fusion it came out of. I removed the studs and cut the boss to accommodate the routing of the serpentine belt on the MX-5.

I did it as best I could, but because the photos online weren’t like my timing cover, I first cut too little, then too much. I ended up having to fill a hole in the cover with JB Weld, but in the end I got the clearance I needed. To drive the accessories, you’ll need a new belt, but you need to use the NC 2.0-liter belt tensioner. Because the MX-5 had 144k on the engine, I installed a new tensioner.

When everything is installed, you can fire up and run the car with the stock tune, but you will need to buy a tune for the car for it to run its best with the larger, more powerful engine. Here in California, I have to get it running with the stock tune, get it smogged and then install the tune. In two years, when it’s time to smog the car again, I’ll need to revert to the stock tune, clear the I/M readiness monitors, have it smog-tested, then reinstall the tune. It’s a bit of a hassle, but a small price to pay to live in the Golden State.

Parts and Part Numbers

Mazda crankshaft bolt                   LF01-11-406

Mazda crankshaft washer              L3H5-11-407

Mazda oil pickup                           LFE2-14-240A

Mazda oil pickup O-ring                L309-14-248

Mazda exhaust gasket                    L327-13-460B

Mazda intake gasket                      LFE2-13-135

Mazda water outlet O-ring             LFE2-15-169

Mazda EGR gasket                         LF01-20-305

Mazda oil filter housing gasket      LF02-14-342

Mazda engine harness                   NE51-67-020N

Mazda serpentine belt tensioner    LF17-15-980E

Motorcraft VVT solenoid pigtail    WPT1251

Ford VVT solenoid seal                  BR3Z-6C535-B

Ford oil separator PCV                  4L5Z-6A785-AA

Fab9 Tuning MZR counterbalance delete kit

Dayco belt                                      5060910/6PK2310