Drilling a receiver

[crow#2]

How hard is the steel in a marlin 39a receiver. I need one hole drill for mounting scope. The gun only has one hole in the top now.Im not concerned about iriginal or all that. Also i figure being a 22 lr that two scres would be plenty for mounting scope base. Wouldnt just a irwin bit fro ace work. Screws are 8/40. I thought i could drill it with 1/8 in. Bit then use the size needed to tap the hole.

 

[RoninPA]

You really want the correct drill to make sure you can tap it correctly. For a 8-40 thread you need a #28 drill, but if you have a 9/64 drill you should be fine. A 1/8 drill is a bit too small and you could break the tap. Also, if you're tapping a blind hole you will want to use a bottom tap, if a through hole, a standard tap will be fine.

 

[Mobuck]

Getting the hole drilled and tapped perfectly perpendicular to the top surface of the receiver and perfectly centered is a daunting task.

 

[Kevin]

You are asking how hard the steel is. Even for a .22 I imagine the steel is pretty hard.

 

[NikA]

Remember how the last hole you drilled and tapped worked out? Sounds like you don't have the correct setup to take this on.

I would strongly recommend finding someone who does to help you out. Otherwise you're likely to significantly deface an otherwise nice rifle.

Even if you got the hole perpendicular to the receiver, how do you propose to make sure the mount is parallel to the bore?

 

[arfmel]

I hope you're not planning to try this without at least using a good quality drill press with a vise mounted on the drill press table.

 

[black1970]

Don't-buy another 22. You will ruin much of the value. If you do take it to a gunsmith and make it look like it was that way from the factory. I used to work at a used car lot. What does that tell you?

 

[needsmostuff]

Just a thought,,,,,
Check with Turnbull , They sell a Red Dot base that fits the two peep sight holes for Winchester. Who knows , they may fit on a Marlin.
I would vote against drilling an undrilled receiver. Sell the gun and buy one factory drilled. Factory drilled is much more common.

 

[idsubgun]

Don't be the guy we post photos about in my Marlin 39A group showing how F'ed up the screw holes are and how they ruined the value of the rifle.
Take it to a gunsmith!
I'm a retired machinist and a gunsmith. I would do that indicated in on my mill, NOT in a drill press. I wish I could go around and destroy all drill presses because of the garage hack jobs I've seen. And BTW, machine shops, and hopefully most gunsmiths, don't buy their drill bits at Ace. Ace drill bits are junk! Any drill bit from any big box store is junk! We buy our bits from tool supply houses.
If the hole you drill does NOT look factory, you just ruined that rifle!!!

The steel is not hard. But it is tough and will snap a small tap in a heartbeat if it's not done correctly using a quality tap and the proper tapping fluid.
And if it is a blind hole, you'll probably need at least two taps, maybe three. Sometimes you can't get a tapered tap to engage in a blind hole so you need a gun tap or a blind hole tap BUT they may need to be ground a bit to get that engagement. I rarely buy blind hole taps and grind my own from a "regular" tap. But you have to know how to grind a cutting edge(s) on a tap to do that. I do that because blind hole taps rarely cut enough threads in a short blind hole.

We won't get far into drill bit sizes and percentages of tap engagement, but it is okay to "sometimes" step up a size in drill bits to get less tap engagement. Suggested tap drill sizes are roughly 75% thread engagement.
All that said, take it to a gunsmith. It really isn't that much money to have it done correctly. Which means a correct tapped hole AND in the center of the receiver. If the hole isn't centered, you'll have gun owners from now until the end of time calling you names. LOL!!