1. Remove bolt
2. Chase threads with a tap
3. Spray out threads with brake cleaner or carb cleaner, something like that, so it's nice and clean
4. get a new bolt from the hardware store, make sure it's the right length, more thread engagement is good as long as it doesn't bottom out
5. Use thread sealer on the threads and the install the bolt. You can find thread sealer at the autoparts store or on Amazon along with many other places, there are several brands. There's even high temp thread sealer

AJ/FormS said

And get rid of the splitwasher. They make bellevilles that work excellent as secondary back-ups. Do not use as a primary cuz the coolant will be trapped in the passage and eventually corrode it up so bad it will be impossible to take it apart. You will have to cut it apart, destroying the T-cover and probably the pump too.

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