Countersink holes in scope bases

[bush1]

I have a Weaver scope base and will be drilling a new hole for a different location and an wondering what the countersink angle would be. I would guess that it would be 82 as that’s pretty much a UNF / UNS standard but the scope mount screws are 6-48 NS.

And idea what they would be? I have some aviation countersinks but they are 100 deg.
Brownells only has a 60 deg listed so that’s no help.

Types[edit]
Countersunk holes.svg
CS angle Type of thread
60°
82° American inch screws (UNC, UNF, UNS)
90° ISO metric
100° British imperial inch screws (BA, BSF, BSW, etc.),
Unified National (i.e. inch size) Aviation fasteners[1]
110°
120°

 

[guntech]

Brownells sell piloted tools for making the hole exactly what you want... Weaver 6-48 Counterbore

https://www.brownells.com/optics/ri...ng-tools/weaver-6-48-sight-screw-counterbore/

Overall 2½'; Body Dia. .218'; Pilot Dia. .137'; Pilot Length 1⁄8'; 45° Shoulder. Use Drill No. 28 for Pilot Hole.

 

[bush1]

^^^^^ Thanks ^^^^^

Never would have guessed it was a 90 deg countersink for these screws.

 

[Raton57]

I am facing the same problem because the scope base for my one rifle hasn't been made it 12 years and only by Tradions Weaver made a base for that peculiar rifle.

 

[guntech]

When I was smithing I had 4 counter bores... 2 for the 8x40 size in flat bottom and tapered heads, and 2 of the same in 6x48 size. Used often over the years...

 

[Jahnj0584]

All our customers use 90° csinks even when using imperial bolts. Don't overthink it. All will work in real life if cut to the correct Dia

 

[bush1]

All our customers use 90° csinks even when using imperial bolts. Don't overthink it. All will work in real life if cut to the correct Dia

I didn’t know it was a 90 deg until guntech pointed it out so I’ll get the proper counterbore. I was actually calling it a countersink and that’s why I couldn’t find it on the Brownells website as they list it as a counterbore…

 

[DANS40X]

I didn’t know it was a 90 deg until guntech pointed it out so I’ll get the proper counterbore. I was actually calling it a countersink and that’s why I couldn’t find it on the Brownells website as they list it as a counterbore…

You are dealing w/ Fillester head screws for Weaver bases.
They are machined deeper since the heads of the screws are domed.

Check the countersink angle w/ an 82 or 90 degree center drill...or the underside of the screw heads.

A counterbore has a square bottom usually machined for allen head cap screws.

 

[bush1]

You are dealing w/ Fillester head screws for Weaver bases.
They are machined deeper since the heads of the screws are domed.

Check the countersink angle w/ an 82 or 90 degree center drill...or the underside of the screw heads.

A counterbore has a square bottom usually machined for allen head cap screws.

Agreed, Weaver does have bases that use fillester head cap screws and these holes could be made with a proper square bottom counterbore as you mentioned but my bases are different.

Mine have 90 deg countersunk holes and use flat head 90 deg 6-48 screws. My only question was getting the proper countersink angle, 82, 90, 100, etc and guntech pointed out that Weaver bases are 90 deg and Brownells has a 90 deg counterbore specifically made for Weaver bases. Just placed the order for one.

 

[guntech]

You are dealing w/ Fillester head screws for Weaver bases.
They are machined deeper since the heads of the screws are domed.

Check the countersink angle w/ an 82 or 90 degree center drill...or the underside of the screw heads.

A counter bore has a square bottom usually machined for allen head cap screws.

Dan, I think you missed on this... Brownells offer 4 counter bores for 2 different sizes of mounting screws the 8x40 size and the 6x48 size... each size has 2 counter bore "profiles" 1 for a Weaver style tapered screw head and 1 for the flat bottomed Fillister style screw head. I bought my first ones from Brownell's 40 years ago.

 

[Potashminer]

I suspect some readers know this stuff already, but likely is some that did not know, or did not care - based on mix-matched that came on various used rifle that I have bought.

Picture below - showing a Parker Hale base (lower one) that uses "flat bottom for screw head shoulder", and a Weaver base (upper one) that uses taper shape into the hole - the correct screws beside each base - maybe have to zoom in to see the detail. The PH base has a third tapered hole in middle to accept the recoil stud that sticks out the bottom of the PH rings (usually). Both screws are threaded 6-48. It appears to me that PH may have only made one length of screw for their bases - counterbore drilled shallower or deeper as needed? But Weaver has at least 3 or more lengths of those screws that come with different bases.

 

[sean69]

If you run into something truly odd you can get piloted counter bores with interchangeable (different sized) pilots. (I like KBC Tools)

Spot drills come in 60, 80, 82 and 90 degree angles - with varying numbers of flutes.

What I have not been able to find myself are piloted countersinks with removable/interchangeable pilots. Closest I have seen are step drills (with a single step)