Correct method to drill and tap aluminum?

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  • 243
    304 AMC
    • Apr 08, 2001
    • 2478

    Correct method to drill and tap aluminum?

    I want to use the early 80's Blazer/Truck centercaps on my aluminum slot wheels. The caps are bolted on from the front with 7/16" bolt head cap screws. They may be 3/8" bolt head but I can't remember exactly. I understand the tap is a different size than the bolt head but I can't remember what size it is for the bolts at the moment.

    What is the correct procedure for tapping aluminum?

    I have tapped lotsa holes in steel but the single attempt in aluminum resulted in a broken tap.
    David
    76 NT Cherokee
    83 CJ7
    05 Frontier CC 4x2
  • andy d
    Shade Tree Shaman
    • May 06, 2000
    • 7205

    #2
    1/4 20 screws generally have a 7/16s head. i just re-tapped some 3/8 holes and the tap said right on it to use a 5/16 hole. so i guess i would try a 3/16 hole for a 1/4 thread,if the tap starts to bind, try a 7/32 drill. you are using oil, right?
    \'88 gwag,pure stock

    Comment

    • tgreese
      • May 29, 2003
      • 11682

      #3
      Aluminum tends to clog the flutes of the tap, so use a lot of oil and run the tap out and clean the flutes every few turns. Here's a table of drill sizes for various taps: http://www.widell.com/drillsiz.html

      Here's another one: http://www.newmantools.com/tapdrill.htm

      [ November 24, 2003, 04:54 PM: Message edited by: tgreese ]
      Tim Reese
      Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
      Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
      Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
      GTI without the badges: '95 VW Golf Sport 2000cc 2D
      ECO Green: '15 FCA Jeep Cherokee KL Trailhawk

      Comment

      • Millerluck
        360 AMC
        • Sep 08, 2000
        • 3075

        #4
        13/64 is the tap drill size for a 1/4-20 tap.

        Don't use the taps they sell at the hardware store they are junk in my book.

        Get a good high speed steel tap fom an industrial supply store. A two flute spiral point tap is what you need to get. Spiral point taps keep the chips forced ahead of the tap so they are less likely to clog it and break it.

        One and a half times the diamater of the bolt thread depth is plenty of threads most times.

        Drill one and a half times the diamater of the tap deeper then you want of thread depth. This gives room for the point of the tap and a place for the chips to go.

        Easy math so you never need a thread chart....

        1/4-20 tap drill size?

        1 dev 20(thread pitch) = .05

        .250 (Tap dia) - .05 = .200

        Closest drill in a standerd drill set = 13/64

        0r

        3/8-16 tap drill size?

        1 dev 16 =.0625

        .375-.0625 =.3125 (5/16 drill)

        Good luck

        Larry Miller

        [ November 24, 2003, 05:43 PM: Message edited by: Millerluck ]
        88GW "Mississippi" 401,Edelbrock heads,and stuff,
        4" Skyjacker, BFG AT's,OBA,Air Horn.

        '89GW "Mutt" wood for now, 360, 1405, Edelbrock cam,
        4"Rusty's,3"body,33"BFG. Detroit Truetrack Warn Hubs

        The Metal Workshop
        http://www.themetalworkshop.com/index.html

        Comment

        • Bozz
          258 I6
          • Aug 19, 2003
          • 340

          #5
          You might try a machinist tap. It has only two cutting rows and the rest is open to get rid of the metal fine's.
          Dan<br />82 J10 SWB<br />New Jasper 360 engine, 3\" suspension lift, Procomp shocks, TFI upgrade, Mile Marker hubs, Edelbrock carb, Performer intake, 2 1/4\" dual Magnaflow exhaust, 32x11.5x15, Ultra wheels, Trac-loc rearend, Weatherguard toolbox.. <br />1998 Honda ST1100

          Comment

          • TexasJ10
            360 AMC
            • Jan 03, 2002
            • 2774

            #6
            There are guys on the Home Shop Machinist website that swear olive oil is the best tap lubricant you can get. I have tried it and it seems to work really well at keeping the tap from galling with aluminum threads and it seems to keep the new threads from breaking off. On aluminum you also have to make sure you get a straight start. Other wise the tap seems to be going in OK but it is just tearing up the threads you just made.
            * 1981 stepside, 360, 727, 208, almost stock daily driver.
            * 1982 Laredo j-10, 360, 727, in rough shape and in the process of being rebuilt with 401, NV4500, Klune,
            . NP205,d60 front, d70 rear, fender work and minimal lift. It will probably take 10 years
            * 1973 jcab mounted on 1983 j20 frame. 360/t18/208 d44/d60. Almost completed

            Comment

            • 243
              304 AMC
              • Apr 08, 2001
              • 2478

              #7
              Thanks for the tips!
              David
              76 NT Cherokee
              83 CJ7
              05 Frontier CC 4x2

              Comment

              • joe
                • Apr 28, 2000
                • 22392

                #8
                Even hdw store taps are ok for alum but as Tim mention above don't try to tap the hole in one continuous shot. Back it out some every couple turns to unload it and go slow.
                joe
                "Don't mind me. I'm just here for the alibi"

                Comment

                • Quartermaster
                  232 I6
                  • Feb 13, 2003
                  • 110

                  #9
                  In addidtion to all the tips mentioned above it may be noteworthy to know that the standard taps have a lead of about 3-5 threads. Meaning the thread is not going to be as deep as the hole and you must drill deeper (as mentioned above) to get the depth.

                  More math:
                  1/4-20 tap. 1/20=.05
                  .05 X a lead of 5 threads = .25 short on thread depth. A bottoming tap can be used also which has a lead of about 1-3 threads.
                  1988 Grand Wagoneer (nude) 360 bored .03 over, Edelbrock performer intake/Carb/Cam, TFI upgrade, Thorley Ceramic coated headers, 3" High flow Cat, 70 series flowmaster, Espo springs (No lift) and air bags in the rear (for trailer towing), CS144 upgrade.

                  Comment

                  • FSJ Thing
                    Propane Protagonist
                    • Aug 25, 2002
                    • 3824

                    #10
                    And then a finishing tap which has NO lead! Funny thing is that I can't find anywhere that sells bottoming and finishing taps...If I hadn't used them so much on my ship I'd doubt that they existed!
                    The Cherowagladabego Project. Updates coming as soon as I do something update worthy.

                    Bring back Junk Yard Genius! he may have peed in some of your cornflakes, but everything he told me was helpful!

                    Comment

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