View Full Version : DIY Drilling & Tapping 870 Receiver

October 11, 2004, 05:54 PM

I would like to install XS ghost ring sights on an 870 shotgun. The rear sight requires two 8-40 holes drilled and tapped into the receiver. One problem I see is the rear sight requires the rearmost 8-40 hole to be drilled on the slope of the receiver. The XS sight instructions call this a compound surface radius. This is located 1" from junction of the receiver and stock.

What type of vise or jig would you recommend for securing the receiver under the drill press?

Thank you for the help.

October 11, 2004, 06:01 PM
of a jig, I would start with a small end mill (slowly), then a small center drill, then the correct size drill for tapping. Just make sure receiver is perpendicular to spindle before you start, and make absolutely sure gun can't move.

Harry Bonar
January 18, 2005, 09:45 PM
Dear sir:
ALWAYS drill with a smaller drill than your tap drill. Like for 6x48 I use a drill 3 drill sizes than the 31 and then ream with the 31; this way even if your flutes are off on the 31, it will be a #31 hole.
Many 31 and 28 drills DO NOT drill a 31 or 28 dia. hole.
If there is any chatter use a piece of denin under the reaming drill!

January 19, 2005, 01:36 PM
A drilled hole is always larger than the bit diameter. Anywhere from a few thou on up. For an accurate hole for fine pitch screw threads (read shallow threads) ou need to dril undersize at least one, and sometimes two, numbered sizes and ream to the finished dimension required.
It is not as much of an issue with larger and deeper threads. They ofen can survive a slightly oversize hole and have adequate engagement.

January 20, 2005, 01:47 AM
BTW, it wasn't mentioned here so if you're a beginner, the type of reamer is probably worth mentioning. It's a chucking reamer. That's how you get from a drilled hole (which is usually triangular in shape rather than round) to a "proper" hole.

Chucking reamers come in undersize, oversize, dowel pin and regular. Let's say you want a 0.500" hole. An undersized reamer will ream to 0.499". An oversized reamer will ream to 0.501". Dowel pin will ream to 0.4995" and a regular will ream to 0.500".