Thread: URGENT! before I drill my firewall need advice :)

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One thing I meant to add was, don't try to countersink with a larger drill.
A true Countersink is designed to run true.
You'll find them with 3-4 cutting edges - most common - as well as with a single cutting edge.
Get the single cutting edge version because it's the smoothest running one.

Countersinking with a large drill will have the drill chattering and in some cases walk right off the hole you're attempting to countersink which creates damage as it goes.
The chattering also leads to a ragged countersink in most cases.


However, and this is just for the heck of it.
You can countersink for common allen head bolts with a regular drill when you're making auminum brackets and the like.
Shown in the pic below.

The drill used to countersink is just a few thousandths larger than the head of the allen bolt.
Most times I go deep enough so the allen head is almost flush with the surface of the aluminum.
Other times, part way.
Makes for a nice finished product.

You may be wondering how it works out since the inner countersink has an angled surface, but what happens is, a few tighten/loosen cycles will create a flat ledge for the allen bolt and it will go no deeper.

I like this look and would rather do it this way than use FH (FlatHead) allen bolts sunk flush with the surface.