This is how it was when I learned this; may be they now put stuff in diesel
that changes it but I don't think so:
- diesel fuel
flash point is well above 50 Celsius.
- standing diesel fuel
below that temperature (without being atomized by injectors inside cylinders etc.) will not ignite by spark or flame. Only when the liquid diesel fuel becomes warmer than 52 Celsius or so, it starts creating vapors that can ignite.
that is how it was and I believe still is.
p.s. Also, OP has an aluminium tank, not a steel
tank, so when drilling this there should not even be any sparks.
p.p.s. I believe many are confusing this with welding on a diesel tank, which process can heat the liquid diesel above it's flash point.