Stripped threads re-threading size questions

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I have a set of 4 metric (I believe) bolts that have stripped the threads out of a diesel engine thermostat housing they were bolted into. The reason for the stripping was because the previous owner had used the wrong threaded replacement bolts which ruined the threads so it is not a mechanical fault. I need to drill out the stripped threads and tap the holes to fit a new set of bolts that are just big enough to facilitate this.

Major diameter of the current bolt threads measure 0.235" on my calipers. Shaft diameter measures 0.237"

Clearance is small so I need to be drilling out the least amount possible to keep the strength of the thermostat housing intact. I have done drilling and tapping before but It has been so long that I wanted to ask before cutting away my one likely chance at correctly re-threading and therefore saving my engine.

Current bolts are 3.63" long including the cap. but threads are only 0.87" Original bolts were threaded all the way up the shaft to the cap however.

I need help figuring out the following information:

1. New thread diameter and pitch to source new bolts and taps
2. New tap pilot drill bit size that will create a smooth wall out of the stripped threads
3. Clearance fit drill size that the new bolts will comfortably fit through.

SAE or Metric are both fine, the correct size and being able to find quality bolts, drills, and taps are what is important.

If anyone knows what high grade of bolt I should use for a rumbling diesel engine and a good place to source them along with taps and drills that would be great as well.


Any help with this situation will be much appreciated!
Thank you!
 
What brand and model of engine? Sounds like the original bolt was 6 MM thread size
Are the existing threads too damaged to clean up with a 6MM tap?
There are various thread repair inserts that could be used- Helicoil is one I recall
It doesn't sound like this is a tremendously high-strength application
 
Last edited:
Another vote for Helicoil.

If you have not used these before, now is the time. This is the right thing to do for this kind of fix. They're easy and strong.

EZ Lok inserts are also awesome, but they typically take a little larger drill out and tap than a Helicoil. Which may be good or bad depending on how much tearing out the hole has had and how much you can drill oversize and have enough material left.
 
If these holes go through to a water jacket, consideration for your installation method will need to ensure the Helicoil and replacement bolt are both sealed in the process.
 
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