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spark plugs used to be cheap and therfore changed reg. now they are getting dear people cut cost where they can .still supose makes injectors not seem so dear
 
Depends what you need. The days of four N5s for a pound are gone. But if you go to a factors you can still get a box of ten standard plugs pretty cheap. Buying plugs from places like Halfrauds is bloody ridiculously expensive.
 
It can knacker the threading of the plug hole though. It was a common problem with the Rover 75. The plugs dropped about two threads into the engine, if left in too long and they carbon up would strip the threads on the way out. A common work around was to file down the last two threads on the plug.
 
First thing I'd do is buy a full set of the same, recommended, plugs. It _does_ matter what heat range the plugs run in, if it's running lean you need hotter plugs, if it's running rich, cooler .. If you try to mix heat ranges or types of plug you can cause all sorts of issues. At least that's how I remember it from dealing with petrol engines!
 

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