I just read about 10 minutes ago on the Internet while looking up the problem and one person said that the white clips to the coil packs had to be at the front. The write up I was following is flawed (I installed my Denso Iridium (1 step colder) spark plugs today. DIY included!! - Subaru Impreza WRX STI Forums: IWSTI.com - " The coils with the shorter wires go to the spark plugs closer to the firewall.") This is wrong and thus my misfiring.

The question is, do I try the new plugs or just take them back. The extra length has me concerned even though the person at Subaru says it's OK.

The ones I mentioned are the recommended type on the NGK web site http://www.ngk.com.au/spark-plugs-part-finder Go see for yourself unless you already have.

if you have supplied the vin number to them, and they have searched them correctly then you shouldnt have a problem,
although the extra length is a bit of a concern so i would be a bit cautious

Post up a pic of the two side by side. I'd say if there was going to be damage, it would have occurred already.

The amount of times that I have seen the wrong part in a box may surprise you.

I have, thanks. I have also contacted NGK's Perth's representative to see what he says.

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Wouldn't surprise me at all, I have seen it many a time
Although one plug in wrong box I would say is possible but not 4

To me they look like H6 spark plug as they are longer than the 4 cylinder ones
I wouldn't be putting then back in till 100% confirmed

WOW

They are quite a bit longer arent they. I wouldnt have put them in at all before double/ triple checking. I hope all is ok and they havent caused any damage although you did say there was no sign of impact on them.

The long plugs are for 2.5 litre engines.
Confusion arises when people without the correct product knowledge use catalogues intended for the US market where the 2.5 was fitted to earlier models than the Aus version.
The wrong plugs are often fitted to WRX's and it causes havoc. Your car is on the changeover, could be a MY06 (thus EJ25) There are also 2.5 engines fitted with 2.0 heads to add to tyhe confusion.

NGK confirmed they are not the right plugs and lucky the extra length did not cause damage by the piston hitting it.

That being said, my car has a loud whistling noise out of the engine now. I'm about to drive out to Subaru to get the plugs replaced - hopefully it will be non-eventful.

Thanks all for your help, very much appreciated.