I have never had to use one of these to get a tank unit nut loose to do a sender/pump unit until I came across one in a plastic tank. The nut on this tank has 6 locking areas versus the normal 3 you see on steel tanks. Normally I just tap on the side of the nut with a chisel to loosen it, no problem. Not on this one... I pounded pretty hard and it would not budge. I would think since the tank is plastic, it absorbs the shock and does not accomplish anything. Rather than ruining the nut trying to beat it loose, I looked to Kent-Moore for a tool (they build all of GM's special tools). The tool above (Kent-Moore J-39765 ask them for a catalog) does the job with ease, and the report of the locking areas releasing tells you its needed to get it off.

I also bought these to deal with the locking fuel lines at the sender, Snap On FIH 9055-1 and FIH 9055-2, one is 5/16", the other is 3/8".