Hi all I was changing plugs in my 06 lacrosse, i noticed a vast difference between the ones I bought and the ones that were in car. Upon returning to o Riley’* to double check we confirmed I was given right plugs for 3.6, the guy at counter said the plugs installed were for 3.8???? How could that be? Should I continue with wrongs plugs?

It appears Oreilly is right. You didn't provide actual part numbers so I can't be absolutely certain, but looking at the length of the threaded section, I agree with OReilly.

- The 2006 Buick Lacrosse 3.6 has the longer one, similar to the clean ACDelco unit in your picture
- The 2006 Buick Lacrosse 3.8 has the shorter one, similar to the dirty Champion unit in your picture

So now for the fun part: The improper plugs have ~35% less threads than the proper plugs. Since somebody has changed the plugs to the short ones, and a bunch of miles were put on the car with the short ones, there is likely a bunch of carbon build-up in the 35% of the threads that were unused with the incorrect plugs installed. If you go in there like Rambo and crank the proper plugs in there, you take a chance of messing up the threads when the spark plug starts trying to screw through all that carbon. It would be best to chase the threads with a professional-quality tap and professional-quality procedures. These are expensive, and now somewhat rare, aluminum heads so you need to be careful not to mess them up getting yourself in a hurry. A bad tap, inexperienced user, going too far, etc. could all take things too far and turn this little job into a "scrap-the-car" cost for repair. If you do this, make sure to clean the threads up really well of debris etc. with a vacuum and then some carb cleaner. Get it all together, start it to make sure it works okay for a minute, DON'T drive it, and then change the oil and filter.

Another alternative would be to use more short plugs. I've never done that before, but it worked before, right? Not something I would do though. The biggest worry I'd have with this is that with less threads engaged, it would be easier to strip the threads out of the aluminum heads, even at the proper torque. You might already have a head-start on this as the previous mechanic was so inattentive that they didn't know (and/or care about) the difference between plugs, so likely they may also not have known (and/or cared about) the proper torque specification, let alone with less threads engaged. The fuel burn process is probably also hindered by having the spark recessed into the head, and the spark plugs can't cool as much.

Either way, use anti-seize on the threads of whatever plugs you install. Don't slather it on super thick, just enough to coat the threads. I can't tell for sure, but I don't think there was any used on that old plug in your picture.

Maybe run the part numbers of all of these plugs by us here in this thread. May as well make sure there are no additional issues that could have been avoided by double-checking part numbers.

And now that I've typed all of that, it occurs to me to ask: Are you absolutely positive that you have a 3.6 in this car? Putting a long plug in a 3.8 will cause catastrophic damage. I think I see a 3.6 alternator in your picture, but that'* not the most accurate way for me to confirm.

CathedralCub
Wow, Thank you so much for the reply, it is greatly appreciated! Is there a serial number I can look on block to in reference, to guarantee I do have a 3.6, I can put plug numbers on here this evening when I get home,

I attached pic of motor I believe yes it is 3.6? Champion plugs that were in car are 7940, the plugs that I bought were ac delco 17’*

Yeah, looks like a 3.6 to me. Definitely not a 3.8!