Page last updated July 17, 2008



Bolt Hole Rethreading / Tapping


Let cool according to manufactures recommendations and file down with a dremmel and finally with a sanding block until it is level with the rest of the block.

The picture to the left is only an example. I found that the brazed material is to soft and drill bits tend to walk into this area, even with a drill press. If you can find very short bits, it still becomes a problem. Again, if you are attempting to repair a failed tap procedure, it is best to work the remaining bolt out or flatten it using various grinders / cutters before you attempt to drill and braze the area. If you have enough of the head still available, it might be possible to drill it out with a press, trial and error.

If the repair is completly brazed with new metal, then drilling with a press should not be a problem. When it has dried according to manufacturer recommendations, position the drill press or move the block to the drill press so that you can measure a good hole for the correct angle and depth. Transfer these measurements over to the brazed area. Apply some cutting oil to the tip of the drill and the area to be drilled.


Apply preasure ensuring that angle is maintained and that you don't drill to far into head. You should place a piece of tape on drill bit indicating how far you must go or set the drill press stop to achieve matching depth for other bolt holes.

Apply firm preasure to bit, appplying cutting oil periodically and watching head and drill press ensuring no deviations from required angle occurrs.

When hole is complete, use your tap set to get the proper tap for the bit you used.

You will need a T-bar for the thread tap, Don't use a wrench as this will apply uneven pressure to one side causing problems. A T-Bar allows it to go evenly down and striaght. Let the tap cut the threads. Back out every 1/2 turn and blow out shaving with compressed air (compressor works best) and also blow tap to clean it of shavings

Re-oil hole and tap and continue. When complete, blow out the hole and test fit of bolt into hole, hopefully everything went OK. This all depends on a drill press. Without it, you risk big problems.

JB WELD You ask??

Well, I am going to try to repair a powerhead using JB Weld. It has a cracked bolt hole on the power head. You can see the repair. This is one that went in crooked and I could not tell. This was before I purchased a drill press. I have since removed powerhead and you can see the new threads.

I had to drill deeper into powerhead but it was fully capable of receiving a deeper bore. Notice also the additional grinding of metal in order to allow the tap to go straight in. This repair was so much easier with a drill press, I should have done the job with a press to begin with, but a lesson for all who read this. I believe the repair to be a perfect candidate for the JB Weld as I am reluctant to apply brazing material due to the extreme amount of area that needs to be covered. I have the chipped off piece and will use JB Weld to "glue" the piece back and and will using it so also fill in the area that was drill at an angle.

JB Weld should not be used on intake or exhaust manifolds. This area is a water jacket . Water circultes around this area and is covered by the exhaust plate. The actual exhaust manifold is right below it but has a channel of water protecting it. Hopefully, this area does not exceed 500 Degrees. I doubt it, but I will post an update. The JB Weld will not actually hold any pressure, it will be used as a filler to produce a flat area for the gasket to seal against. Notice the deep threads.

I mixed the JB Weld according to instructions on box and applied a light coat of oil to a long bolt. I placed bolt into position and with a syringe, applied the JB Weld into all crevices. If you want it to be creamy so that it flows, you can heat under a light bulb for a few minutes. You can see the results to the left after I initially sanded down with dremmel. The end result was smoother and flat. Hopefully, it will work. The area is cooled by water so it should not get too hot. Lots of vibration and there will be plenty of surface for the miracle stuff to bond too. Well see!!


I have put engine back together and though I failed to take pictures (Or I have not located them), the engine started up fantastically. It initially leaked a few drops of water from the above bolt. I was not sure if the bolt was long enough to grab the deeper grooves so I I went ahead a placed a slightly longer bolt than standard and It grabbed well and the leaked stopped.

I was careful not to overtorque gasket and coated all gaskets with gasket sealant. After waiting a full day for all remaining gaskets to set in, I started it back up and NO LEAKS.

Ran at full throttle same day and redid timing with timing light and adjusted idle while in the open water. Everything worked wonderfully and after 4 hours, no leaks.

I have to say that I was dreadful about the above bolt, and though I am not willing to take the exhaust cover off to check and see if the JB Weld is still holding, It works currently and does not leak even under full load at a pretty good skip down a choppy lake.

JB Weld may work for you, give it a try. If it fails, all you have to do then is remove the old material and try something else.


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