Most of you have probably heard of the "short nose crank problem" which has been heavily associated with early NA6's. Well as I have recently had the displeasure of discovering, this problem is not limited to early NA6 engines - it can happen to any NA or NB. I am not sure if there are differences with the NC and whether it could be an issue for them or not. I have had this explained to me by Danny (ORX626) from MX5Plus, and so will endeavor to pass on that information here and share my experience of this problem.
When I purchased my 2002 NB, it had done 103,000 kms, had full Mazda service history, and had the full 100,000 km service done by a Mazda dealer, which of course includes changing the timing belt. In a nutshell, the crank pulley and so too the crank pulley bolt must be removed to change the timing belt. That bolt MUST be replaced, and also then correctly fitted and torqued to the required specs. I also understand that using a product such as Loctite is a very good idea when fitting the new bolt. If that is all done, and correctly, this "short nose crank problem" should not be a problem for any engine, including those early NA6's. The longer nose crank is a bit more fool proof, but not immune!
The symptoms which my car displayed were varied, and intermittent at times. The problem gradually became worse over probably 8 months and 20,000kms. In hindsight, if I had paid more attention earlier, the problem may have been resolved without the need for the crank having to be replaced. The symptoms included hesitation, "flat spots" in power delivery throughout the rev range, higher than expected fuel consumption, and in the end, severe loss of power and stalling. It was particularly worse when using the air conditioning in summer, and at times I had trouble just taking off in first at anything below 3000 rpm. Over the course of the 8 or so months, I replaced the cam angle sensor, ignition leads and coils, spark plugs, and also ran a couple of bottles of injector cleaner through the fuel system. On the topic of fuel, I always use BP Ultimate in the car, and most often buy from the same local servo, which has a very high turn over of fuel, so I never really suspected that I had just got a poor batch of fuel. Don't automatically assume that your car has this "short nose crank problem" if you're experiencing some of the above, but it should be checked. The most obvious clue with mine was the wobble of the bottom pulley when idling. The pulley can't be moved by hand, you need to have the engine running for it to spin fast enough to become evident.
Below are some photos of my crank and lower timing belt pulley, showing the extent of the damage, and hopefully giving you a better idea of what this is all about. When it was first confirmed that this was the problem my car was having, (having removed the crank pulley for a proper inspection) a couple of options were presented to me. Firstly, try a temporary repair, with no guarantee of success or longevity; or secondly, replace the crank. The temporary repair was carried out as Mazda wanted $1500 for a new crank, so I figured we might as well give the first option a try. It worked to some extent, and certainly made a noticeable improvement and the car was much better to drive. This could have lasted and been effective for 5 minutes, or 5 years, there is no way to tell. Meanwhile a used crank was sourced for $300, so I bit the bullet and booked the car in for the crank to be fitted and engine rebuilt to some extent (new main bearings and rod bearings should be replaced as a minimum).
The temp repair though, by its nature, wasn't perfect. The timing was retarded by approx 1 to 1.5 degrees as a result of the worn keyway in the crank.
I'm probably jumping all over the place a bit here with my writing, but I will try and explain the problem. I will assume some engine knowledge here
Basically, the timing belt pulley fits over the end of the crank. There is a slot in the centre of the pulley, which lines up with a corresponding slot in the crank. These are exactly the same size, and together form the "keyway". There is then a "key" which is fitted into the hole formed by lining these two slots up, which locks them into place so that they can not move independently of each other - doing so would adjust the timing "on the move". This is what had happened to mine. The whole assembly came loose as the bolt either hadn't been torqued correctly, or the original bolt (which is designed to only be tightened/used once) was re-used when the pulley was replaced after having changed the timing belt. These pics which I eluded to earlier will hopefully explain things better.The below photos show the crank/timing belt pulley fitted over the end of the crank. The resultant slot formed, should be exactly the same on each part. The key which fits into the slot is simply a small square piece of steel which fits the hole. In the photos, the hole in the pulley remains to spec, but the crank has been flogged out resulting in a wider keyway, allowing the crank and pulley to move independently of each other. I don't have a photo of the key, it is sitting on my desk at work, and I'm at home on holidays!
Needless to say though, one of the squared edges had been rounded off as a result of the play and movement between these parts.A solution which was considered for a short while, was to machine the keyway in the crank and pulley to a larger matching size, and make a suitable key. I don't see why this would not work, although I suspect there would be a limit to how wide the keyway could be made without causing problems because of the diameter of the crank. I suspect that doing this would cost somewhere between the $300 I paid for the second hand crank, and what Mazda ask for a new one...
As you can see, there is the possibility for movement between those two parts - this is not good, and is the whole problem. The timing gets thrown out all over the place. Thankfully on mine it was retarded and not advanced, especially as mine is a turbo'd 5, so advancing the ignition randomly could have caused many more dire problems. Perhaps because of the load from a/c and power steering on the bottom belt pulley, this caused it to do damage in that direction, but I'm only theorising on that.
This is not a problem caused by poor engine treatment or thrashing the engine. The cause is "mechanics" who don't know what they're doing!
So, in summary, it could happen to you too! Be careful about who does your timing belt change, and make sure as best you can, that they replace that bolt with a new one! If it happens to your engine, don't be too disheartened - disassembling the engine gives a good excuse for doing fun things like having it balanced, fitting forged internals etc
A more cost effective option though would be to replace the engine entirely with a good used one. I hope this has been of use to some of you.
Cheers,
NMX516
