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I needed to get a new tap for installing 1/4-28 grease zerks. I don't have a set of numbered drill bits so I bought one of the inexpensive Irwin kits that comes with the proper sized drill bit. I wanted a plug tap and a bottom tap. The plug tap comes with a #3 drill bit (0.213). The bottom tap does not come with a drill bit but the image of the card says to use a #7 drill bit (0.201). This seemed very confusing as why would a 1/4-28 plug and bottom tap use different sized drill bits?

Today I finally received the bottom tap and the package lists a #3 drill bit. Look at the two on-line images below of the packaging. The package I received is on the right. Where did they get a bottom tap card that has "#7 drill bit" on it? Talk about confusing. :)

784737
 

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OK, here I go !! The "standard" tap drill for a 1/4" thread is BOTH #3 and #7 depending on threads per inch and based on 70%-75% thread. The 1/4-20 uses the #7 drill and the 1/4-28 uses the #3 drill. This is also assuming tapping by means of a machine tool or 3 taps, (taper tap..1st tap, plug tap..2nd tap, and then a bottom tap..3rd tap). The reason for the larger #3 drill with the 1/4-28 is that the OD (major diameter) of both threads are the same size, .250", but the minor diameter of the 1/4-28 is larger as the threads are "shorter" in height.

So, that's how it's supposed to be, but many of us are in the back 40 trying to chase a thread and few have a complete tap set as well as correct tap drills...never mind the machine tool with .0005/ft accuracy! There's DEFINITELY a discrepancy between the center pic and right pic...same tap size, different drill size. The "reason" MAY be starting a bottom tap in a drilled hole without previous threads could be difficult. Using a larger drill (#3 shown on the right pic) will make starting the tap a little easier. I've always had the "luxury" of having tap sets available in my home workshop so I can't comment on starting a bottom tap in a drilled hole. Bob
 

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Discussion Starter · #3 ·
I have metric and SAE tap sets but I don't have numbered drills or all the weird fractional sizes. Need to get a set one of these days. Starting a bottom tap in a freshly drilled hole is definitely a bit of a challenge.

I fall into the first category where I'm always taping something weird with either the item laying on the floor or chucked up in a vise and never quite able to "get in there". :)

I did just order a set of tap sockets as I can't remember the last time I had room to use a T-handle.
 
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RW is correct and my guess is someone in marketing looked at 1/4-20 instead of 1/4-28 but his theory on the larger holes for the bottoming tap is valid. In the end either would work fine but I would use the smaller one especially if drilling it by hand since it would tend to be oversize anyway.
 

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The 2 taps seem to have the same p/n unless these old eyes are deceiving me...
 

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Mark, I agree that part numbers look the same on center & right pic. Center pic has faded blue (probably older) and as Herm stated, 'someone looked at wrong thread for tap! The "new and improved" (on the right) packaging has the correct info. Bob
 
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