I want to tap a hole in a surface that I think is steel.

According to the chart I need a 6.8mm drill bit but screwfix doesn't sell a 6.8mm hss drill bit. Can you use a 7mm instead?

For future reference sake, what if the chart said 3.3mm, can I use a 3mm drill bit?

  • Is this a coarse thread, or a fine thread? Apr 28, 2019 at 18:32
  • Instead of a 7mm drill bit, purchase an "H" size drill bit. It's 6.8mm and easier to find everywhere.
    – Dotes
    Apr 29, 2019 at 2:16

1 Answer 1


You need a better supplier.

If you use a 7mm drill bit the threads won’t be properly formed and will have less strength. If the load is critical then it might well fail.

For the 3mm instead of 3.3mm you run the risk of breaking the tap off in the hole, which can be a *real pain to remove as the steel used for the tap is very hard.

Engineering suppliers carry an extensive range of drill bits and Zeus tables used to work out drilling and tapping hole sizes.

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