Frommy experience, the closest drill bit size to a 5/16″-18 tap is 17/64, which lines up with an F drill bit. You can also use basic math to determine the relative drill bit size. We concluded that it was the closest bit because 17/64 is so close to the 5/16′′-18 (DH=.2583) tap when divided. Lastly the recommended tap drill size is provided for each standard tap size. This size drill bit should be used for drilling the initial hole that will then be tapped. Downloadable versions: 9/16-18: 0.5625: 14.2875: 716-20 unf 20 .4375 .4050 .3762 .3834 25/64 1/2-13 unc 13 .5000 .4500 .4056 .4167 27/64 1/2-20 unf 20 .5000 .4675 .4387 .4459 29/64 9/16-12 unc 12 .5625 Usersreported that the ANSI 9/16 - 18 UNF Internal Tap drill size is larger than the MAX minor diameter allowed. Status: This incident is being investigated for a Thisitem: Drill America 9/16"-18 UNF High Speed Steel Plug Tap, (Pack of 1) $16.71. Only 16 left in stock - order soon. Ships from and sold by Amazon.com. Product Dimensions ‏ : Onceyou know your wire diameter, you need to find out what is the premissable Measurement for your pitch diameter (Depending on the class). Use our Over Wire Calculator to find out! 1 7/16 - 18 UNEF : Basic Thread Dimensions (Pitch, minor & major diameters, depth, lead), Tolerance range for all applicable classes and much more. Onceyou know your wire diameter, you need to find out what is the premissable Measurement for your pitch diameter (Depending on the class). Use our Over Wire Calculator to find out! 1 9/16 - 18 UNEF : Basic Thread Dimensions (Pitch, minor & major diameters, depth, lead), Tolerance range for all applicable classes and much more. CombinationDrill Tap: 3/8-18, 4 Flutes, High Speed Steel. 37/64″ Drill, 1-1/16″ Thread Length, 3-1/2″ OAL. 0.2055″ Drill, 9/16″ Drill Length, 25/32″ Thread Length, 2-9/16″ OAL. List Price. $25.60 / ea. Combination Drill & Taps; Thread Size (Inch): 1/4-20; Thread Limit: H3; Thread Length (Inch): 1/2; Thread Direction Similarly different thread sizes and types, such as UNC, UNF, and NPT, have different requirements for tap drill sizes. A tap drill size chart is an invaluable resource for anyone working with standard threads and can save time and frustration by providing the 5/16-18 #11 (.193) 5/16-24 #10 (.205) 3/8-16 #9 (.254) 3/8-24 #8 (.257) 7/16 TapDrill Sizes this page is for information only Tap Drill size calculator (for inch size taps) Go to: | Inch tables | Metric tables | Acme | BSP & BSPT | Pg | Handy Hints INCH TAP Zj0O.