When replacing my car's coolant I overtightened one of the air relief plugs and stripped the internal threads. It's located on the inlet manifold. The torque setting for the plugs is only 61-69 in-lb so it doesn't take much to strip the threads. I assume I will need to tap new threads but I'm unsure of how exactly to do it. Do I need to drill the hole out first?

  • Welcome to Motor Vehicle Maintenance & Repair! Feb 25, 2021 at 12:17
  • Keep in mind that drilling out or trapping both risk introducing swarf into the intake manifold, from where it would be ingested into the motor. Feb 25, 2021 at 19:11
  • @DavidSupportsMonica the op is talking about a coolant pipe mounted on the inlet manifold - so not the air intake to the engine.
    – Solar Mike
    Feb 25, 2021 at 20:29
  • @SolarMike Thanks for the correction. If "air relief" means a valve used to purge air from the cooling system as part of bleeding or refilling, then indeed whatever swarf might be created in drilling or tapping isn't going into the motor, 'tho it might enter the cooling system. Feb 25, 2021 at 20:36

2 Answers 2


Depends on the material and amount of material available.

It can be possible to re-tap the existing thread but perhaps not in your case.

Another possibility is to fit a tapered plug if you can get one in a suitable size.

Then taking it out to the next plug size is possible as long as there is enough material, but this usually means drilling out and then tapping from scratch with a starting tap and then parallel tap.


A different idea from tapping the hole to a larger one would be to repair the threads which are there already using an epoxy like JBWeld or the like. You could either just epoxy the part in place if it really doesn't need to be removed again, or if needed, there are other methods/products you could use which would create new threads where the fitting could be screwed into. These types of products are still an epoxy (like JBWeld), but comes with a thread release agent which would allow the fitting to be removed after the epoxy has set up. Since the part does not require a lot of torque, using something like this is a lot less destructive than tapping a new hole in a place where you don't have much strength in the first place.

  • That sounds like it might be the easiest solution and it's unlikely I would need to remove the plug again. Can epoxy be used in situations where it will be in contact with hot water?
    – jrcollins
    Feb 25, 2021 at 20:11
  • @jrcollins - Absolutely. Just ensure whatever epoxy you get is rated for it. It should say on the packaging. JB Weld has a product called Water Weld which might work for your application. (NOTE: I have no affiliation with this product or company.) Feb 25, 2021 at 20:41
  • Another option is to use a heli coil but I think that might be overkill in this situation.
    – jrcollins
    Feb 25, 2021 at 21:06
  • @jrcollins - From the OPs description, I don't think there's enough meat for a heli-coil. Feb 25, 2021 at 21:09
  • It should be possible I think but as I said, probably overkill in this situation. The bolt tightens up ok until near the bottom of the hole so I tried adding another washer just now and that seems to work.
    – jrcollins
    Feb 25, 2021 at 21:30

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