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Thread: How To: Repair Intake Manifold Bolt Holes

  1. #1

    How To: Repair Intake Manifold Bolt Holes

    If I'm not mistaken, one of the forum members may have encountered some challenges repairing his intake manifold bolt holes. If your intake manifold manifold bolt holes are frigged up, first option is of course Helicoils. If the holes are too frigged up for Helicoils, this is the method I use:

    Step 1: Tap the hole 1/8 NPT. NPT threads are tapered -- this is definitely a hole for lots of cutting oil. If the tap gets loaded up with aluminum you've had a bad day (clogged thread will remove all the other threads on the way out). Also back the tap off more frequently than you would with straight threads. About halfway down the hole take the tap completely out and remove accumulated shavings. I have a handy dandy template I use to ensure the tap is 22.5 degrees to the head. Definitely a hole for a quality Made in USA tap, not a POS Made in China tap:

    RepairBoltHole1.jpg RepairBoltHole2.jpg

    Step 2: Blow the hole out and install a 1/8 NPT pipe plug. I suppose you could use Locktite, but the threads are taper lock so I never do:

    RepairBoltHole3.jpg RepairBoltHole4.jpg

    Step 3: Drill out the pipe plug. Again, I use my template to ensure the bit is 22.5 degrees:

    RepairBoltHole5.jpg RepairBoltHole6.jpg

    Step 4: Tap the pipe plug M7. Again, I use my template to ensure the tap is 22.5 degrees:

    RepairBoltHole7.jpg RepairBoltHole8.jpg

    Step 5: Enjoy your new bolt hole:

    RepairBoltHole9.jpg

    If the threads in your pipe plug ever get frigged, just remove the pipe plug with an easy out and repeat Steps 3-5 with a new plug.

    Bill Robertson
    #5939

  2. #2
    I did the same thing on an AC compressor back plate. Worked great as long as I owned the car.

    You will get a little better results tapping aluminum with this:
    https://www.amazon.com/Ounce-AlumTap.../dp/B002R84X6U

    But you still have to keep clearing the shavings very often.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #3
    My posterboard template was getting a little shopworn, so I just made one out of luan:

    LuanTemplate.jpg

    3-in-1 works just fine. At $13 a bottle I would bankrupt myself (I use a crapload of oil when cutting threads -- what is this "a few drops" of which AlumTap speaks?).

    Bill Robertson
    #5939
    Last edited by Greasy DeLorean Mechanic; 03-05-2017 at 06:28 PM.

  4. #4
    What do you do if the 1/8" pipe plug doesn't seat fully? Pull it out and grind it down?

  5. #5
    Quote Originally Posted by Boglin View Post
    What do you do if the 1/8" pipe plug doesn't seat fully? Pull it out and grind it down?
    Ummmm, tap the hole a little deeper....

    Bill Robertson
    #5939

  6. #6
    I assure you -- this works:

    RepairedIntakeManifoldHoles.jpg

    Bill Robertson
    #5939

  7. #7
    Quote Originally Posted by Greasy DeLorean Mechanic View Post
    Ummmm, tap the hole a little deeper....

    Bill Robertson
    #5939
    I'm going to assume that the same depth as the original hole will be sufficient. You might forget that my original 2.85L PRV had coolant coming out of the #3 intake manifold bolt-hole when someone tried to re-tap that hole.

  8. #8
    Blame a drill bit, not a tap. There's no way a tap can bore its way through solid aluminum. When a tap bottoms out, if you keep turning it, it will just rip your fancy new threads out (in aluminum -- in steel you will probably snap the tap).

    Original bolt holes are plenty deep enough for a 1/8 NPT tap to go all the way down. If you look closely my plugs are recessed slightly (1-2 threads worth). And I wasn't even all the way to the bottom of the hole -- just all the way to the end of the tap.

    I think people get into all sorts of mischief drilling broken off fasteners. As soon as the drill bit clears the fastener -- STOP. Original fasteners are not threaded to the bottom of their holes -- there is space between the end of the fastener and the bottom of the hole. You can clearly feel it when the bit clears the fastener.

    Bill Robertson
    #5939

  9. #9
    Depth of original bolt holes: ~1"

    Depth of 1/8 NPT tap threads: .75"

    Depth of 1/8 NPT pipe plug: .3" (Only drilling is through the pipe plug -- if your original threads are too frigged up for a Helicoil, you won't need to dill a pilot hole for the NPT tap -- frigged threads themselves will be the pilot hole).

    Bill Robertson
    #5939
    Last edited by Greasy DeLorean Mechanic; 03-05-2017 at 07:38 PM.

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