Spark plugs with 3/16 x 40 thread.

[HSM01]

I just purchased a 5/8" -12tpi LH nut from an eBay supplier. China was on the package. It was for a Delta Unisaw. It only went 1/3 of the way on. Nothing could be done as there is no way to retain the arbor while threading. The next attempt was from Amazon, and it fit perfectly. It was also sourced from China.
I returned the junk one. It's the only way the seller will learn. He did offer me a 25% discount, LOL. I only mention this because it seems to be the same issue, only in a much larger size.

 

[timo_gross]

So I guess the conclusion to all this discussion is that their label of '10-40' probably is meaningless, at least as it pertains to thread sizing which is kind of the whole objective. You might consider making a comment or suggestion in their purchase feedback area 1) to help another model engineer from falling in the same hole 2) maybe they will update their label marking &/or description one day if they realize its confusing.

I have seen this issue a few times on AliExpress & similar sellers. As mentioned, sometimes its a language barrier. Sometimes they really don't know what they are producing or replicating. Other times its a bit more mischievous & they view it as a way to increase sales, which really just causes more aggravation for all in the end. Good luck. I hope it 'sparks' for you. Keep us informed.

Mixed options here:

• No idea idea what they sell! (surplus re-seller, socks today sparkplug tomorrow)
• language barrier (e.g. soldering and welding is the same word in chinese / & AI translates everything to welding ).
• purposeful wrong description. (1000 items sold are gone and paid for. Who goes through the trouble to send it back? )
• Someone simply grabbed the wrong item when they shipped it.
• The description on tha package is already wrong, so seller uses it wouthout knowing
• .....

Good idea to make a screenshot of the description, as it might change while the shipment is on its way .
I had a case where it took the seller 1 day to correct their description after I contacted them. (some specialized seller do care and know what they sell, sometimes even worth a asking if they can get items not listed.)