NC1 2.0 Bottom End Knock, Engine Rebuild Options or Conversion?

Just bought another MX5 NC, this one came really cheap because the bottom end is knocking, only done 60k miles so the rest of it is not bad. I’ve read all the tails about how/why this happens and lots of prices for a second hand engine. But I’m a competent DIYer and have rebuilt many engines before. So I have 2 questions.

  1. Does anyone have first hand experience at rebuilding an NC engine? It’s going to be more than just replacing the big end bearings and I’d expect to have to get the crank reground and the rods checked/replaced?

  2. Alternatively given the cost of second hand engines, has anyone done a Duratec/Zetec conversion, or even something more radical?

@DuratecNC

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Hi
I’m pretty sure the knocking will have damaged a rod too, we would always whilst stripped cure the oil burning to prevent it happening again.
Not to mention a new oil pump because it will be full of swart……re crank grind…we wouldn’t grind the crank as you would need thicker bearings and as they are NOT held in the rod and are made of soft material are a weakness.

Hope that helps

Hi,
Do you think it would have an oil burning issue at 65k miles? I have an identical car, same year with 60k on it. doesn’t use a drop.
Interesting you wouldn’t think of regrinding the crank, I would have thought that was a given but maybe the crank really is just that hard?
Cant find any information on the engine block components in either the Haynes or Official Mazda manual. Plenty of info on the head etc.
The Duratec conversion is interesting, but on the first info I’ve seen looks a little expensive, a multiple of what I paid for the car.

It depends entirely how it’s been treated getting to that 65k.

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If it’s not burning oil why has it run low and knocked the bottom end out??

I’m not saying the crank is ok I’m saying replace rather than grind.

Ford bottom ends don’t allow for vvt
No pockets in pistons etc

The previous owner just didn’t check the oil.
So why buy a new crank rather than regrind and fit oversize shells, is there something different about this engine?

As I said in my first post the shells are not held in place….by a locating peg and the shell is the weakest part so why put a bigger/thicker weaker part in???

Attention to detail is the key

Re your above post
It HAS burnt the oil otherwise it would still be full!

As rodders says the lack of a tang locating the bearing shells into the rod and cap is a potential failure point which is why most engine designs use a locater to prevent spun bearings. To save money these are sometimes omitted only to have them added when the designers realise their mistake. Mazda will not have been the first to make this error but usually it comes up in testing and one spun bearing is usually enough to change minds. I’m not sure if the later MZR engines had locator tangs fitted when they went to the forged crank.

I believe that was the case but I’m sure Rodders will advise on that, hopefully.

Yes they do……

That said an early engine rebuild properly is a good engine.

I’ve just rebuilt an engine for a customer from Southampton and that engine had been done on the cheap previously….
Obviously it didn’t last

In which case is it possible, but probably uneconomic, to fit the later crank and associated rods and bearings to an early engine?

Yes you can fit later rods and crank but due to how the piston’s fit the rods you would need them too

If you rebuild your original engine and keep up to speed on oil/filter maintenance and regularity, it should serve you very well. My NC1 has just passed 125k and doesn’t use any oil.
I do check it every time I take it for a run out, twice weekly, and I do change the oil and filter every 2k or 2 trackdays (8 x per annum)
It’s a cheap deal with oil and filters but expensive for engine rebuilds.

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OK, so the bearing shells don’t have a locator tang even from the factory. Well I’m not going to spend £2k on an engine rebuild for a car that may never even be worth £2k, just can’t see that fitting a new crank is going to be economical. So I will probably be looking for a second hand engine.

Facebook has them :ok_hand:

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I don’t Facebook, i’m in no rush

Ebay has a couple One with 81k on for £500 and a known good one for £1200.

Rebuild doesn’t seem bad compared

I think I would go for a 2.5 conversion at £3k ish

I paid low £100’s for the car, could probably break it for parts and put the money into a runner, would work out more cost effective.

Still don’t really understand why fitting big ends without a locating tang on a reground crank which probably only has 10thou taken off it is any worse than the factory spec, particularly as the bottom end failure is due to low oil rather than an inherent design failure? It’s not like I’m going to do track days, it won’t even be a daily driver doing maybe 2k miles/year and an oil change every MOT. I’m an oldie! But I’ll go with what others have said on the basis that they have rebuilt these engines in the past.

So I’ll most likely just break it for parts.

Spun big end bearings are not necessarily a result of oil starvation. I have seen them on new build engines on run in on test beds assembled to the correct spec by people who do this for a living. It’s not common but it does happen which is why most engines have tangs. Looks like even Mazda changed their minds on later versions of the MZR.