Gunsmithing tap size and drill bits

[gknittel]

Hello.. I'm going to drill and tap a CZ 452 receiver to accept 6-48 screws to mount a scope base.

I'm buying a couple of 6-48 HSS taps with the following specs:
Central core: 2.82mm
Diameter A: 3.16mm
Diameter B: 3.505mm
Thread height: 0.344mm

Now I have to figure out what drill bit is the right one.

Brownells catalog recommends #31 drill bit. But I see it's 0.120" diameter (3.048mm) and the tap is bigger (3.16mm). Is this ok?

Also on Brownells' catalog I see 1/8" drill bit (3.175mm). This is 0.015mm bigger than the tap. Isn't this a better choice over the #31?


All suggestions will be very much appreciated.

 

[mattja99]

Re: tap size and drill bits

Just off the cuff, the difference may be the depth of the threads. If in complete doubt, once you get a drill bit and tap in hand hold the bit in front of the tap, you should only be able to see the threads on the tap behind the drill bit. The tap packaging will usually recommend a drill bit size. Brownells is probably spot on.

 

[300sniper]

Re: tap size and drill bits

don't use anything larger than a #31.

 

[Raven 6]

Re: tap size and drill bits

No, the drill is supposed to be smaller so you will have threads when you tap it. If the drill is bigger the tap will fall though. Go with what Brownells recommends. Try not to think in metric since this is an English thread.

Mark

 

[gknittel]

Re: tap size and drill bits

Thanks alot!