Just changed the plugs in my 2006 M3 2.3L engine. It has 6 years and 70,000 km on it. Manual says to replace at that point for severe service. A couple of additions based on my experience.
- Use a 10 mm wrench to take the battery ground wire off before you start. You may also need needle nose pliers to fish the socket back out of a rather deep recess that the plugs fit into.
- The plugs I found in the engine were marked FoMoCo ILTR5A-13G. An internet check suggests these are NGK laser iridium. Three plugs measured about 0.054" on gap, and another at 0.051". There is a bit of a deposit on the ground electrode, so perhaps they are effectively larger than they measure. But compared to the specified 0.052" new from Mazda and NGK, negligible wear.
- I could not get the exact same plugs and went with the NGK Iridium IX, LTR5IX-11. I checked all four plugs as they came out of the package and got 0.040", or nearly 25% tighter than they should be for the Mazda. I regapped them all to 0.052", using a Champion tool, and being careful not to touch the center iridium electrode or the ceramic insulator. I think it would be a mistake putting in plugs that are that much too tight. Probably would cost some gas mileage and HP.
- Using an open end 8mm to remove the coil hold down bolt was a no go. However, I managed with a regular length 8mm socket, but putting the socket only on first and then inserting the ratchet just enough to engage the socket. Thought for a while I was going to have to buy a deep 8mm.
- The iridium electrode on the ILTR5A-13G OEM plug is finer than the NGK Iridium IX. It probably is a better plug, and if you have the choice between the two, I would go for it. I also read that the Laser Iridium has platinum on the ground electrode, for longer life. Probably the reason for the extra cost, if you can get it.
- I'm sure the engine would have run fine for at least another 30,000 km or a total of 100,000 km.
Hope that helps some,