Transmission Pan Screws -- 99 Elantra

Hello,
    This past Saturday I was changing the transmission fluid on my sister's 99 Elantra (3rd change with 97k). While tightening all of the screws that hold
the transmission pan in, I ran into a problem with one of them which is longer than the other 12. I was hand tightening it and didn't feel any grip to it which seemed to be that the threads were stripped off. I then took the pan off and hand tightened the screw again and it held on without any problems. I then tried with the pan again but was only able to hand tighten it. Right now there are no leaks after 2 days (knock on wood), but I'm concerned that the screw is not holding. Does anyone know the length of this screw (HMA Service doesn't mention anything about the screw sizes, nor does it mention the 13th screw) and what pitch it has? I'm thinking of trying a just slightly longer screw to see if that helps.     I know that the first time the transmission fluid was changed, it was changed by a dealer (which also left 2 of the splash guard screws off) but the second time was changed by myself, and I don't remember having any problems.
Any advice would be greatly appreciated.
Nick
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All the screws are the same length from the factory. If one of them is longer now, it's because someone already stripped the hole and put in a longer screw so it would hold. You can try getting a screw that's just a little longer still, but if it's a blind hole, make sure you don't start trying to tighten beyond the bottom of the hole. In some places, this can damage fluid passages. If that doesn't work, the quick-fix (it's not pretty) is to get a 1/4" diameter screw instead and stuff into the hole. It's just a little bit bigger than the M6 that was there originally and isn't so big that it cannot cut it's own threads in the aluminum.
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