Pulled the spark plugs out my car, had a look at them and noticed there are two different types in there as per the photos. Four of the better type and two of the originals.

 

Have a look at the corrosion to the electrode on the left, the other was just as bad. The corroded ones look like the recommended NGK plug of heat rating 6 for a standard engine and the others were heat rating 7 which seemed to have coped alot better. Both are NGK.

 

The Iridium plugs, will probably clean up and could go back in tbh, they go for something like 50,000 to 100,000 miles, depending on mods etc. Our yellow Z had the same thing, except the 4 plugs that had been changed were the wrong plug for the engine, the 2 that hadn't been changed were the two under the balance bar.

 

Are you putting the same grade 7 Iridiums back in? I'd say they were changed to Iridiums and heat range 7 because of how modded the engine was, if you're not changing much I'd recommend doing the same. Lots of people on here say Iridiums aren't good in the VG, but I've been using heat range 6 in mine since I got it and they're fine, provided I use Shell VPower, any other fuel and I get a small amount of dett and it's just not quite right.

Yeah was planning on a couple of the Iridium 7's to replace those two ruined originals for now until I have a running engine. Will get a brand new set once its all ready to be used. If anyone has an old pair of Iridium plugs that still have some life that they can throw my way i'd be grateful.....