Screw size for older Lyman Receiver sight

Ian

Well-Known Member
The challenge with going to 8-40 for me hasn't so much been drilling and re-threading the hole as much as it has been enlarging the counterbore on the mount/base. Difficult to do without a vertical mill and a way to indicate dead-concentric with the original screw hole and counterbore.
 

Pistolero

Well-Known Member
HMMM. IME, a slightly oversized drill, if the part is held in a drillpress vise, because it WILL tend to
be grabby and the vise is held but permitted to be free to move slighty laterally, will self center very well. And
the hole bottom not being flat, should not be an issue, the outer perimeter if the screw should hold it
down fine.

If you drill it up, partial depth, don't "unflatten" the bottom of the counterbore, then use a mill cutter, which
will now fit in, not touch the sides, you can then deepen the counterbore and maintain a flat bottom.

I will add that I have not done this on a receiver site, but have on other, similar items.

Bill
 

Jeff H

NW Ohio
HMMM. IME, a slightly oversized drill, if the part is held in a drillpress vise, because it WILL tend to
be grabby........Bill
If you care to dedicate a twist drill to this specific task, you can grind a much steeper angle on the leading edges of the drill and it will be much less grabby in plastics. I started using the same drills to enlarge holes in aluminum and it works very well. I go by eye, but believe I'm putting only one or two degrees from horizontal on the edge. I've done this with twist drills for enlarging holes in aluminum, not so much for making a new one. I fill the drill flutes with Crisco and it lubes while holding the smaller chips and keeping them from getting between the drill and walls of the hole. It's more of a pain to clean out of drills and taps, but an old toothbrush does the trick.

This will still leave an angled bottom in the hole.
 

Charles Graff

Moderator Emeritus
The challenge with going to 8-40 for me hasn't so much been drilling and re-threading the hole as much as it has been enlarging the counterbore on the mount/base. Difficult to do without a vertical mill and a way to indicate dead-concentric with the original screw hole and counterbore.
I guess, I am just simple minded, but the sight base could be held tight on the receiver with one 6-48 screw and then a 8-40 tap drill can be run into base and through the receiver, making for concentric holes. Tap and tighten with 8-40 screw, remove 6-48 screw and do it again. With the receiver firmly locked to DP table, replace tap drill with a proper size small end mill and do the counter sink.
 
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Ian

Well-Known Member
"Firmly locked to DP table" is kind of an oxymoron with my flimsy drill press, but otherwise a sound method.
 

Charles Graff

Moderator Emeritus
I have a good 17" Jet floor model drill press bolted to the floor. She don't move! Before I bought it, I checked with Lee Jurras on Jet quality. He was making 1,000 yard bench rifles at the time. He told me that Jet machine tools were all they used in his shop.
 
Reactions: Ian

Ian

Well-Known Member
Jet is a great brand, very expensive too but you get what you pay for. I got to use a big floor model recently and it's even better than our old Rockwell in the shop here. I use a $200 Ryobi bench model at home and it is the definition of flimsy.