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Many times the intake / exhaust manifold bolts seem to be deficient in the strength to prevent their breakage when removing the manifold nuts. Knowing this I always heat the nuts prior to removing them. Well almost always. Below is how I repaired the situation when I skipped the heat the nut step.

I used a spare manifold as a drill guide. The hole in the manifold was 7/16". So a 7/16" drill was used to "center-drill" the stud. Then a small drill, 3/16", was used to drill completely through the stud. be very careful to drill perpendicular to the block. Other wise the threads in the block will be damaged.

After drilling through the stud, then I drilled out to the tap drill size. I did this in small steps so that I was sure I wasn't removing the threads in the block. Finish drilling with the tap drill, 5/16". Clean the drill chips out using a pencil magnet.

Before tapping the hole, pick the remaining metal from the stud out of top threads in the block. Now tap the hole with a 3/8"-16 tap. I recommend using a 2 flute tap, as these are the strongest and I REALLY don't want to break a tap off in there. I finished with a bottom tap to assure there was sufficient thread depth.

When installing the stud , be sure to use some thread sealer if the hole is into a water jacket. Also, I recomment using brass nuts for this application.


Here is the broken stud. Luckily it isn't the one in the rear. That one might necessitate engine removal.

I have a drill set that I never use except for checking hole sizes. This hole is 7/16"

Mount the manifold and use it as a drill guide

Here I am using the 7/16" drill to find the center of the stud. Drill till the drill is at full diameter.

Not a great shot, but the hole isn't deep enough.

Better picture and not it is full diameter of the 7/16" drill. The next drill will find center of the 7/16" hole now.

Use a tap drill chart to find the tap drill for the stud. This one is 3/8"-16.

3/8"-16 calls for a 5/16" tap drill. This is the size to finish drill to.

After drilling the through the stud with a pilot drill (see text), drill finish size.

Using a magnet to remove the chips from the water jacket.

Here I am taping the hole . I recommend using a 2 flute tap. (see text)

Tools required. Tapping fluid, Bottom tap, Regular tap, 7/16" drill, 5/16" drill, Pilot drill (3/16")


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Copyright Richard N. Meagley Sr.
Last revised: May 25, 2007.