Rejuvenating this thread because I had a recent experience that might help folks.
Having the single aim to save money, I bought some NGK plugs from a seller on ebay (quickdog-autopart at a very good price). Sometime later, I remembered that counterfeit plugs were a potential problem so I bought another set from a local Advance Auto Parts store figuring that whether or not the ebay plugs were conterfeit, I'd at least have one good set of plugs and if they were counterfeit, then I should be able to get some sort of refund.
I looked at several sites and videos explaining all the ways to tell if an NGK plug is counterfeit. No doubt some are valid such as poor printing quality on the insulator but many probably were not such as printing on the box. The ebay plugs had some possible evidence of being counterfeit and quickdog-autopart is based in China where most of the counterfeit plugs come from (whoda thunk!). but I didn't feel certain enough to accuse someone of selling fakes so I called NGK tech support (in the US: 877-473-6767; I expect there is a similar number for other countries). The NGK lady asked me the lot number stamped on the hex of the plug and my lot numbers were good. "Is the lot number determinative?" I asked. "Yes", she said. I asked her about some of my other concerns, for example:
The picture of the spark plug on the box has a blue cast to it where other NGK boxes were more true to color.
The box says they are made in Japan but the plugs had a stamping that it was made in the US with Japanese parts.
The ground electrode had an angle to it where some said it was supposed to be a curved piece.
She said none of that mattered. For example the boxes were printed in Japan and the plugs were made in the US.
So I dodged a bullet. The ebay seller had sold me genuine NGK plugs at a VERY good price. Would I buy from him again? I don't know. He was certainly honest with me so I have no reason to believe that he wouldn't be honest in the future. On the other hand, I might figure the extra price I paid at Advance Auto would be worth not having to worry about them at all.
If you got your plugs at Amazon or ebay (counterfeits are commonly bought from them) and you have any concerns, I strongly advise you to call NGK at the number I gave above. The websites and videos that try to explain how to tell a fake are well meaning but they are not a substitute for finding out directly from NGK.
Be careful! A counterfeit plug can cause a lot of very expensive damage to your engine - especially if your drive your baby hard.
