90 degree countersink

[Uncle Alvah]

I am confused about the purpose and the necessity of using a 90 degree countersink after drilling for the flash hole liner.

From the instructions I got from TOW for their White Lightning liner.

. Counter sink the outside of the hole slightly with a 90 degree countersink.

Necessary step or optional?

[Ky-Flinter]

Alvah,

Necessary.  The liner has a corresponding bevel, so the liner can be tightened against the countersink.  If you don't countersink the hole, there's nothing for the liner to tighten against.  And since the liner isn't installed to the designed depth, you may cut into the inside cone when you file the liner flush with the barrel.

-Ron

[JLBSparks]

90 degree CSs are usually used for metric bolts/threads, 82 degree CSs are used for standard threads.

   -Joe

[Uncle Alvah]

Makes good sense, thanks!
Gonna order me one up here right now.

[D. Taylor Sapergia]

I'm with Joe on this issue.  Standard countersinks are 82.5 degrees, as far as I know.  You can buy special ones at 90 deg. from sources like Lee Valley tools, and some of their screws and hinges are designed around that angle of countersink.  I have always used standard 82.5 degree cutters and have had 100% success.  The reason for the countersink is well said by previous posters.

[Dennis Glazener]

I have used a larger drill bit to countersink for the whitelighting liner. It worked fine for me.
Dennis