I'm not sure when the plugs were last replaced as I bought this from someone who didn't look like would attentively maintain the car. It could have gone a good 60k miles without changing. Even then, the car has been running great, getting 40+ mpg.

However, I replaced them today and the old ones look like this:




The tip is rounded and the stem is waaaaay out of the clearance limit. I think the recommended clearance for my car ('93 DX) is 1.1 mm. It appears that the stem has bent away from the tip quite a bit.

Is this normal after this many miles? I was only expecting rounding of the tip or corrosion on the metal parts.

 

I think the tips are just worn away, creating a large gap. You may see a small but noticeable increase in performance. Did you inspect the distributor cap and rotor for similar wear and corrosion? You might also consider replacing the plug wires.

 

i compared these to the new plugs. the tips seemed similar... only rounded more on the used one. could be that some of the material has worn away on both ends. The stems on the new plugs look much "thicker" at the end meeting the tip.

Yes!!! I did notice a much smoother engine and slightly more power/feedback than it's been lately.

I checked the plug wires with a volt/ohm meter and the resistance was in the range of 6k-12k Ohms (way below the maximum limit). They also looked very clean. I replaced the rotor/cap before when it started failing by making that aweful squealing noise.

 

Were the old bad plugs the correct ones for your Civic?

 

yeah, the same NGK ZFR5F-11 plugs