Re: NGK vs. AC Delco spark plugs
The XR5 looks more like a R43S than a R43. On a Z28 the tip of the plug might touch the piston, if you still have domed pistons. You might want to see if you can cross reference the R43 plug and see if it crosses to something other than the extended tip XR5. Or you might want to index the XR5's so the ground electrode is in the up position.
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John Brown
This isn't rocket surgery.....
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