So I have the MX5 Parts service kit which comes with a set of NGK BKR6E-11 spark plugs. First time doing this, but seems straightforward so far…
However, I had a couple of questions about the old spark plugs… From left to right, these are from front of engine to back. Exactly the same as the MX5 Parts ones - NGK BKR6E-11.
I am unsure about what model 5 you have, if it’s a NC (mk3) those are the wrong plugs.
Open your new plugs and you’ll see the difference the central electrode should look like a piece of wire almost, change them any way.
If you have a NA or NB they should be correct, but as with tyres don’t have a mis-matched set.
Plugs should last 60k miles so your not saving anything by not changing them all in one go.
The plugs on the Mk3 are specced to last 60,000 miles.
The plugs on the earlier cars are specced to last anywhere between 12,000 miles and 27,000 miles or replace if they appear worn at each service.
The only way to test plugs is put them in a pressure chamber and check the sparking performance there and virtually no garages have the kit these days.
Therefore if you have new plugs handy do not waste your time posting photos of the plugs here, either have them tested which is now virtually impossible or just change them.
If you want to worry about something you could worry about some of the plugs appearing to be running a bit leaner than others and pay £100 for a Tereaclean or similar to clean the injectors.
As I do not worry about that, I would change the plugs and get out with the hood down if it is not raining.
I have a NB 2.5 (2002) and yes, I plan to replace all as a set.
My concern was more over whether my engine was running fine or not since two of the spark plugs are a different colour - but from the sounds of it its not an issue or anything to unduly worry about…