Technical: The 2.5 Duratec I4

There is no shortage of myths and rubbish written about the 2.5 litre 4 cylinder Duratec, here are some facts.

Whenever something goes wrong with an NC engine someone always says ‘fit a 2.5’ even in the UK where they are rare and expensive compared to the US, where they have read about how cheap they are.

There is no doubt it is The Daddy. The widest bore* and the most CC takes care of that. In standard form (with its original large bore inlet manifold) it’s got a very flat torque curve, some would say dull, lazy and even diesel like in its delivery, yet in a light car like an MX-5 it’s not a bad move and certainly a relaxing drive whilst being quick when it wants to be. You have to remember it’s come from a van and heavy large family car so won’t struggle to power a much lighter car along briskly and with ease.

Look at the torque spread there, it’s almost flat from 2500 to 5000. For the record a standard 2.0 VVT makes 190 Nm @ 6700 rpm.

*You may wonder why I noted the bore size, well the 2.3 has the same bore as a 2.0, but longer stroke making it a more ‘torquey’ engine of the two, but with less of a revvy nature although It can be massively improved with the right mods. The 2.0 is known as an over square engine whereby the bore diameter measures more than the stroke so will rev safely to a greater RPM. A good example of an oversquare engine which really does rev well is of course any F1 engine from the last 20yrs or so. Thinking of overboring a 2.3 block to make a 2.5? Or even massively over boring a 2.5? Forget it, the bore walls aren’t thick enough for that or re-sleeving either and boring a 2.5 to more than Mazdas recommended will lead to a weak engine and the problem of where to get a head gasket from. Be very wary of anyone who claims to be doing this.

The 2.5 engine does not have:

Bigger valves

A higher flowing head

A forged crank

Ductile steel liners

It has:

Cast iron liners and crank

+10bhp and at 700 less rpm

+30ftlbs of torque at a peak of 1000 less rpm

The Mazda version has forged steel rods, the Ford one doesn’t.

Balance shafts

Both the 2.3 and 2.5 have a balance shaft assembly hanging underneath, it’s usually removed when fitting the engine in another car due to its size, weight and it saps some of the power from the engine to drive it. These are fitted to counteract second order forces and vibrations which naturally occur in an I4 engine. The bigger the engine gets, the greater the vibration. Removing them won’t damage anything, re-balancing the engine will not get rid of the vibration, nor will a crank damper.

“The ports are bigger and flow much more” is often bandied about on the internet. ‘Bigger’ is a very general term when it comes to ports, you need to be more specific. We look at that in more detail in another blog, but I can tell you that only the inlet port entry is larger on a 2.5, the rest is just the same as the 2.0 and 2.3 VVT heads and that includes the governing parts of a port (the choke points) which are the valve throats and the ‘Min CSA’ which is the minimum cross sectional area of the port.

In my opinion, the best way to get the most out of a 2.5 for the road/occasional track on a reasonable budget would be a set of higher CR pistons, cams and a remap on 99 Ron fuel. 200+bhp, a shed load of torque and good MPG won’t be a problem with those applied to it.

Novice: How to read a dipstick

I didn’t think i’d ever have to write this, but here we go:

This is what a clean dipstick looks like, the two raised lines going across it are high and low level. This is what they look like after you’ve wiped it clean or when your engine has no oil in it (not uncommon) and it comes on a trailer to see me.

  1. Leave the car on level ground for about 12 hrs. This is to ensure the oil is cold and also it isn’t running down the tube which the dipstick goes through which will smear it all over the place.

2. Pull out the dipstick, wipe it clean and reinsert.

3. Pull it out again and holding it level, horizontally read off the level quickly before the oil runs all over the place.

4. If you can’t read it book an appointment at Specsavers.

If you mess it up do it again until you get it right. Every attempt is free, it’s ok, it isn’t costing you. Points 1 & 3 are very important and why so many people get it wrong. It will help you understand when you know the small hole in the cylinder head where the stick passes through also has oil running down it , so if the engine hasn’t been stood for X amount of time you’ll be pulling it back through that.

The dipstick hole is there somewhere…
Just there.

Now here’s a thing; if you treat yourself to the late model Mazda Special needs dipstick it will do exactly the same so really, you’ll be no better off.

Notes and pictures:

A dipstick is not a precision instrument, the oil level in an engine is not a precise thing. When it’s been stood a while the level will be higher than it is after it’s just shut down. Accelerate, decelerate or corner hard and it gets slopped about and thrown all over the place. As long as the level is somewhere around the max when stood level it’ll be happy, like this:

As you can see this engine has a little bit too much oil in it, this was not a problem, it was running just fine.
Sorry this is a little blurred, but you can clearly see the oil is at the correct level. As i’ve been holding it to take the pic some has begun to run past the line, this is why you should be quick and have good eyesight. If you can’t see a dipstick, what are you doing driving?
Here we have reversed the dipstick to double check it on the rear to be sure. You can file your own lines across it to help with reading and to reduce the chances of oil running past.

Novice/Intermediate: Knock knock and oil, the Achilles heel of the MZR/Duratec engine.

Knock knock, who is there? It’s the conrod and bearing, if you keep on starting the car or driving it’ll pop out to say hello.

If your engine is rattling or knocking try this test:

When the engine is idling if you rev the engine gently and it makes a rhythmic knock on the overrun or as its accelerating then 9 times out of 10 it’s been run low on oil, the bearings have made contact with the crank (metal on metal) and this has worn part of the bearing metal away making the clearance too large for oil to fill successfully. So that knocking noise is metal hitting metal. There are many different degrees of severity (the louder the knock, the more damage has been done) but that’s the basics of it. If you want to be 100% sure then take the oil filter off, cut the threaded end off, pull the paper element out and look in the pleats around the outside for non-ferrous bearing material, usually in the form of bright aluminium flakes.

This was quite a bad one.

If it’s really knocking hard or constantly DON’T try to run the engine any more to show your Facebook fans and don’t drive it. It will not get better until it’s been rebuilt, don’t waste your time and money changing the oil or any of that rubbish, but it will get worse and cost a lot more if you carry on running it. You probably know very little about engines, but I think you’ll agree that this is not how it’s supposed to look in there:

This is what is left in the sump pan when someone didn’t stop the engine in time.

If the engine is run under load and/or without oil what eventually happens is the bearing and the crank journal get so hot they fuse together with the friction, this grabs the thin shell bearing and spins it round in the rod (hence the term ‘spun a bearing’) so now it’s damaged the crank, bearing and connecting rod. The next stage is where it really kicks off and things start breaking apart, sometimes the rod can seize, snap off and punch its way out of the block.

More delights, the entire engine was scrap so much of it had been damaged.

NCs and Duratecs are known for rod bearing failure, but that is no fault of the bearing.

Why are so many MZR/Duratec engined cars (including the MX-5 Mk3) running out of oil?

Let’s look at an MZR piston:

As you can see there are 3 grooves in a normal piston with rings in them. The top ring is called the ‘compression ring’ as the name suggests it seals to the bore so the piston can support compression and pressure. The second ring backs up the top one and prevents any gasses escaping down the bore. Right at the bottom are the oil control rings. This is a 3 component pack which as the name suggests prevents oil from coming from down below up into the combustion chamber. Note the oil staining where the oil collected and scraped off the bores has run out of the cut outs (as it should).

Piston ring tension (its strength in being able to expand to fit to the bore) is very finely balanced, too much tension and friction will go up, mpg and power will therefore go down. Too little tension and you won’t get a good seal against the bore.

As you can see in the picture the top two rings are loose in the grooves and able to expand to seal against the bore, the bore is only fractionally bigger than the piston, it’s a surprisingly tight fit. Now note how the oil control rings (which have much less tension in them) have actually become stuck in the groove. They can no longer expand to fit against the bore and oil is getting past them. You can see the soft black deposits of burned oil between the rings.

So there it is in a nutshell, oil passes by the rings, gets into the combustion chamber and is burned off until there is barely any left in the sump, the bearings then run out of oil and contact the crank.

If your engine is using oil there is nothing you can do about it apart from strip it and rebuild it with some new parts.

Forget anything else you may have read on the internet such as:

Not letting the oil level get to the halfway point, overfilling it, changing the oil, revving the engine, putting solvents in, faulty PCV valve, replacing the valve stem oil seals etc, it’s all tosh. There is no other way around it whatsoever apart from filling it up with oil every few hours!

But why specifically does this happen?

Various reasons and theories:

Oil and in particular oil changes: Mazda (and any other manufacturer) are selling cars in a competitive market, what sells cars is low running costs so they often spec long service intervals and semi-synthetic oils. These are on the whole ok, but for the uninformed or unwary sometimes too long for this engine with its Achilles heel in the form of weak oil control rings.

What happens with the Duratec is an oil goes in which is usually not fully synthetic (Mazda dealers use semi as that is what is specified), it’s left in for Mazdas prescribed interval (12k miles), often longer and the car is used by some middle aged or old person to crawl to work and back, maybe to the Supermarket once a week. The engine never sees more than 4000rpm, has old lesser quality oil in it so the oil control rings on the pistons stick together in the groove. This of course makes a mockery of the perceived increased value of a ‘Full dealer service history’ car.

It is thought that the later engines were equipped with either rings with more tension on them or perhaps a modified piston to avoid this issue. There does also seem to be a lot of 1.8 engined cars up for sale with these issues. Whether it is actually more common to the 1.8 or not is not known, it could be that the lower value of them means that when serious problems arise they are just sold on rather than repaired.

The 2.5 is certainly not immune to this problem.

Oil change intervals are a guide, not a rule. Some car manufacturers issue guidelines on oil change intervals dependent on use as things are seldom black and white, sadly Mazda don’t.

If your car is used for a lot of stop/start journeys, short trips, trackdays etc then change your oil change schedule to suit.

If your engine is healthy and you want it to stay that way then do the opposite of all that makes it break; Oil and filter changes at 6 – 8000 miles, use fully synthetic oil and when safe to do so (and the engine is fully up to temp, after at least 20 mins of driving) redline it now and again. Straight and uphill motorway slip roads private roads and tracks are good for this, get it in the lowest gear you safely can and nail it all the way up to 70mph, just be careful and don’t have an accident. Mazda engines love to be revved, they were built for it and this will help keep the piston rings free.

The problem is if you’ve bought an early car already with a certain amount of miles on it then the damage could have been already done. You can’t rewind the clock, but you can repair it by opening it up.

Buyers guide or new owners? A few things you should know about the NC.

This is a bit of a (sometimes) tongue-in-cheek article about people who don’t do research and when online forums and FB pages become boring and repetitive, don’t take that too seriously, everyone has their place in the community and new people are coming on board all the time. Do take the facts seriously though, they are very real. It isn’t a ‘How to’ guide, there are plenty of those already out there if you take time to look, but most people can’t be arsed, it’s just easier to post up and bore the pants off everyone else with repetitive mundane questions, but never mind, this guide is here to save you 🙂

So you’re searching for or you’ve already you’ve got your newly purchased NC, you didn’t do any research so it’s likely to be rusting away merrily underneath and the engine is due to run out of oil due to the pistons rings being gummed up. Here are the things which everyone in the community knows something about, still regularly gets wrong and is fed up with talking about….


Take note of the underside, it’s important yet very few take any notice of the consequences and buy simply by what they see on top.

It’s an old Mazda, 2006 cars are 10 a penny, 13 yrs old at the time of writing and will have rust – somewhere. They were never wax injected at the factory and the underseal was very patchy, amazingly a lot of the underside was simply finished in primer and left! In my experience mileage tends not to matter where corrosion is concerned, it’s the age of the car and how or where it’s been used. It usually starts at the base of the inner rear wheel arch where it meets the cill, this is visible from the outside, even more so when you get behind the cill with the car lifted up or by using your (phone) camera with flash to see it. Often it just looks a bit rusty, but hit it with a screwdriver and often it’ll go straight through. On the positive side it’s quite easily repairable and shouldn’t cost too much either. The later the car, the less chance of it going too far. If you intend on stopping it you need to act now.

The bit you don’t normally see, but really you should.
These were for anchoring down the cars when shipped., They have no underseal around them, were often not capped off, rust like buggery and let water into the cill cavities.

Under the bootlid often looks like this, unsightly, but in the grand scheme of things not a big problem.

Rear wheel arch lips rust eventually because the arch liners hold dirt and soil against them in big clumps. What happens is these clods of earth or road debris get soaked with water (often salt water) and retain it like a sponge. Hours or days later they are still wet and holding it against the metalwork of the car. Solution? Get down and dirty, clean them out. Initially run your gloved fingers carefully around the inner lip and see how much is still there and push it out. Often it’s enough to grow some potatoes in, then get a hosepipe and point it outwards from inside the arch and wash them clean.

Rear subframes rust, not usually to the point of perforation, but sometimes on really bad ones. There are two brace bars which bolt on to the floor pan and subframe to stabilise it, they can rot away completely on top.

Not as bad as it looks as they’re still doing the job and easily repaired or replaced.
Out with the old, in with the new.

They tend not to rust much at the front, the subframe will, but like the rear one, it’s made from thick steel and the engine heat dries it out, wheel arches and around the wing indicator and (aluminium) bonnet sometimes corrode, but it’s rarer. Also where the glass divider in the door meets the doorskin is very common on earlier cars – easily treatable in the grand scheme of things. If the rear wheel arches are bubbling then it’s quite advanced and bad news, a proper repair job there will be a lot of money, mainly because paintwork is needed, in some cases it can be much of what the car is worth.


This electrical plug (bottom left) in the fusebox has brown grease around it, it’s supposed to look like that, don’t worry about it.

Engine Oil

The actual grade isn’t as important as you might think, but for general road use a 5/30 or 5/40 fully synthetic will do. If your oil changes coincide with Winter and Summer you can use a slightly thinner (0/30, 0/40) in Winter and thicker in Summer, just open a manual and take a look for heavens sake. If you do a lot of track work or have a boosted car then 5/50 will do a better job. Mazda were more interested in economy than anything else, both with MPG and oil changes, this leads us onto:


‘Just because it say OXO on the side of a bus doesn’t mean the sell them there’ I was told as an apprentice.

The engine is a Duratec (Ford nomenclature) or MZR (Mazda speak), it was designed by Mazda, not Ford, it isn’t a Ford engine, they just happened to be in partnership at the time so Ford used it in their cars and made parts for it too. If you want to upset a Ford Fanboi who has one tell him he’s got a Jap engine under the bonnet, when he says it isn’t ask him why it’s got a Mazda bellhousing pattern and no Ford ‘boxes fit it…..

It’s generally a very well designed and strong unit, it’s only when humans intervene (or don’t when they should!) that it can break and given you’ve probably bought a second hand old motor there is little you can do, apart from choose wisely and just hope, although if it’s smoking run a mile or expect an engine rebuild. If you’re buying privately then all you can do is quiz the owner about oil consumption and hope they don’t lie.

Read here about a very specific engine problem you need to know about:

There is a second reason for rod bearing failure and it’s less well known and concerns boosted engines really. If the ECU has had a bad aftermarket map put in it then something called ‘Detonation’ will occur in the combustion chamber, this is more prevalent with forced induction and engines which are burning oil. These are uncontrolled explosions in the combustion chamber which send shock waves through the pistons and rods. It squeezes the oil out of the rod bearings (top and bottom) so they come into contact with the crank damages them.

An engine burning oil will not be a healthy one. Your engine is designed to run on petrol and air, not oil! When oil is introduced the combustion chambers and exhaust ports will coke up, the engine will run into detonation territory as per above. The factory ECU will detect this and pull the timing meaning it will be down on power, if it can’t stop the detonation by doing that or every time it does it before the engine detects it the bearings will take a hammering – and that’s before it completely runs out of oil.

So after digesting all of that what do you think to the value of a car with ‘Full service history’ or even ‘Full dealer service history’? Well i’m sat here looking at the records of a car in the workshop, it has full dealer service history and had it’s oil changed every 12,000 miles or so. I’m rebuilding the engine because the rings are stuck together and it ran out of oil….


All 1.8 cars had an open diff, all 2.0 cars had an LSD fitted. It’s as simple as that 🙂

My carpets are wet.

‘Oh God not wet carpets again’, is the usual retort when the subject comes up for the third time that day. It’s an MX5, it will leak at some time or another, the cures are very simple though and widely documented online. The first one is a drip from behind or under the dashboard: Externally beneath the plastic scuttle cover at the base of the windscreen are two square plastic grommets for screws to go into (one on each side), pull out the stupid dried up foam washer, clean it out carefully then put some silicone sealant around and under them, job done. Another main leak is from the drains behind the seats outside the car at waist level being blocked, shove something down to clear it after reading or watching an online guide on how to do it and the pitfalls of internal flaps and so on. Personally I would not recommend using a trombone brush from above, but would tackle the job from below. The high level brake light and also the tail lights can also leak too (dried up seals). Trying to dry out a wet through car in the middle of Winter is going to be very difficult, particularly as the water is now under the carpet, has soaked the underlay and is sat against the floor pan. Prevention is better than cure, DO NOT think it won’t happen to your car, it will, they all leak at the front under the scuttle at one point.


All 1.8 cars have an open diff, all 2.0 cars have an LSD.

My 6 speed gearbox is difficult to use when cold

People have been putting the wrong oil in these gearboxes for years now. That’s because they really don’t understand gearboxes, happily we do 🙂 It should really be called a ‘box of conflict’ (no not Cornflakes) because that’s what it is. 3 basic and different criteria must be met by one liquid and people think there is only one or two. When you take out the magnetic drain plug and it’s covered in ferrous metal ‘fur’ that’s the product of every notchy and missed gearchange where metal hit metal.

We have our own special blend of oils here which clear up these issues completely (clunky, crunchy and synchromesh issues when cold), it is the product of speaking to gearbox specialists, oil manufacturers and Motorsport people and has a dye in it so we know it’s ours. You can read about it here:

If you drive on track and have issues with being blocked as you’re throwing it through the gates then it’ll work for you there too. It takes the pressure off the early gear selectors which are known to break so they last longer.

If you can’t do that then slow your changes down when cold and feel it through, that’s the best you can do.

If you have a six speed then the gearchange collar/bush at the base of the gear lever never fitted properly from the factory. Take note of that fact, it is not worn, it was just never manufactured very precisely. Put the car in gear, wobble gear lever left to right, a bit too much play for your liking? MX5 Parts and us supply a brass replacement, I can attest that it’s obscenely accurate, I can’t speak about any others. It sits in what is known as the ‘gearbox turret’, this is a reservoir which contains its own oil separate to the main gearbox. More on that here:

I put lowering springs on and the car hasn’t gone fully down.

The suspension arm bushes on the front upper arm have not been slackened off to reset them to the new ride height and are twisted, holding the car up. To some (lesser) degree this does happen at the rear too, especially when the bushes are seized onto the through bolts and have been twisted to set the geometry or the car lowered. The cure is simple at the front; slacken off the offending upper bolts and with the car sat on its wheels, the bushes will rotate, car drop down then tighten them up. To explain a bit more of what is happening: The inner bushes front and rear are of the torsion type, so are locked in place by the through bolts, when the arm moves up naturally and down it twists the bush which contributes to roll stiffness.

I can’t read my dipstick.

If you can’t put a metal rod into a pool of oil and read off the level you need help, but it won’t be long before electric cars will save you so don’t worry. Mazda are very clever, they will sell you an overpriced ‘Special needs dipstick’ which suffers from the same problems, or you can just learn how to use your existing dipstick, you dipstick. A handy guide here on what you may be doing wrong and how do do it right: Remember kids, don’t lick the dipstick, it’s not good. Use a rag.

The ‘oil pressure gauge’ does weird things.

No it doesn’t, there is just a misunderstanding about oil pressure. When oil is cold it’s thick so the gauge will read higher (and vice versa), when the engine revolutions are higher the oil pump revolves quicker so the oil pressure goes up. Again, the opposite applies, hot oil and low RPM = low oil pressure. Are you with me on this? Ah now here is where it gets interesting, the OP gauge in an NC is actually fake, it isn’t actually reading oil pressure at all, but mimics it perfectly for a normal healthy engine. What the ECU does is take a signal from the engine water temp sender and also reads engine RPM, it blends these together to make the gauge work exactly as an OP gauge should if the engine is healthy. Just don’t depend on it for actual oil pressure. We sell proper working gauge kits:

Whilst we’re on the topic of gauges the water temp gauge lies like a Pikey. You might wonder what the heck this is all about. All manufacturers do it, the average motorist has very little idea what is going on under the bonnet so has to be insulated from it. If people saw the temp going over 100’c when stood in traffic they would worry something was wrong and the driving experience would be spoiled. It’s possible they would take the car back to the dealer and ask ‘What is wrong with my car mofo? The gauge needle is all over the place!” The fact is an engine will see over 100’c stood in traffic on a hot day and it does it no harm, once on the move it will come down to about 80 odd or so, but the gauge will still be midway. In reality engine temp is up and own and all over the place, it’s quite normal the gauge is just set up not to report all of it as it’s somewhat damped. That doesn’t mean (like the OP gauge) don’t take any notice of it, when it shows cold it genuinely is cold, likewise if it goes right to the upper regions then it is really hot and something could be amiss.

Whilst we’re on the topic of oil, or lack of it, the engine is fitted with a low oil warning pressure switch and i’m not sure why yet! I’m sat in a car which is clearly knocking very loudly and there are no warning lights on the dash board. I wonder if the warning light comes on when you’re sat on the hard shoulder and your pistons are back on the motorway somewhere.

A lot of peoples relationships with cars is psychosomatic, when they’ve spent money on pointless braces, Delrin door blocks, new dipsticks, ‘performance’ air filters, loud exhaust, changed the oils etc then naturally they get a feel-good-factor, the car is of course now handling much better, the gearbox is smoother and it’s much more powerful. The aftermarket sellers know this, the car manufacturers know this. Another ‘trick’ Mazda use on their later NCs is piping engine noise into the cabin, seriously they do. If you look under the bonnet of these models you’ll see a pipe and valve arrangement taking ‘induction roar’ from the engine inlet to somewhere under the dashboard. As you can imagine being a miserable git, on my road car it’s blocked off.

It will have a fancy marketing name, but the large pipe at the bottom there is simply carrying engine noise into the car.

The clutch biting point is too low or early

On some cars it certainly is, it’s not uncommon to have the clutch biting when the pedal has barely left the carpet, these cars are terrible to drive and you’ll wonder (like me) how it got so far on in its life without anyone doing anything about it! There is a fix however and its simple. If you look under the dash up at the pedal box you’ll see the clutch pedal has a nut and bolt pushrod adjuster on the back of it, loosen the lock nut and turn it clockwise until the biting point is better. Please note DO NOT go too far and have the biting point halfway up the pedal stroke, you only need a little bit extra. When you’re done you have to get hold of the clutch lever sticking out of the side of the gearbox box and make sure it’s loose and still has some forward and backwards movement, if not the release bearing can run against the clutch diaphragm all the time which will wear it out prematurely and could cause clutch slip problems..

The battery goes flat

The batteries are small to keep the weight down, also the battery could be past its best. With the obvious out of the way, then yes these cars do have a battery drain issue fitted as standard. The earlier cars seem to be better in this respect and will last roughly 3 weeks to a month on a new battery, whilst the newer ones closer to two weeks if left unused for that length of time. Temperature plays its part too. There are various remedies for this, but if you disconnect the battery it will cause a couple of very minor issues when it’s reconnected: The engine warning light may come on (turn it on and off again usually clears it), the idle speed will be a little erratic for a while (it’ll sort itself out) and the traction control light will illuminate too. This latter one will go out when the steering is used fully from lock to lock and the ignition turned off and on again. None of these are anything to worry about and quite normal, the ECU is going through some self learning process. Radios are rarely coded, but if they are hopefully someone wrote the code in the handbook or paperwork somewhere…..

Sticking brake calipers

Very common on daily driven Winter cars. Either front or rear especially during or just after Winter. If you don’t notice it pulling to one side a bit, one of the discs not as clean as the others then maybe you will when it overheats, you feel a vibration on the motorway or when it finally fails its MOT. Sometimes the sliders have seized, sometimes the pistons themselves.

And there endeth the lesson, please put what you learned to good use!