pump fittings

You will need something like this:
. . . . . . .

This is a reducer, but you may see it listed as coupler or adapter. Probably not at HD or Lowes, but you can find them at a top quality hardware supply store (like Grainger). The easiest way is to search Amazon for "stainless reducer npt" (npt is national pipe thread, which will give you a tapered, waterproof connection). The one above can be found here in many sizes:
http://www.amazon.com/Parker-8MSC8N-316-Stainless-Fitting-Adapter/dp/B005CD2QDY/ref=sr_1_8?s=industrial&ie=UTF8&qid=1374075153&sr=1-8&keywords=reducer stainless npt

It's pretty easy to get 1/2" and 3/8" sizes . . . once you get to 1" there is less to pick from and costs go up. You may have to "rube goldberg" it a little, use a nipple (double male) and/or coupler (double female), or some combination to get your desired ID size. The good news is you can get it in 316 stainless which is the good stuff. Most all homebrew suppliers have 1/2" or 3/8" threaded hose barbs, usually in male or female configuration.

You will also need food quality, high-temp, silicone tubing . . . just add it to your basket at Amazon:
http://www.amazon.com/White-SiliconeTubing-ID-Wall-Length/dp/B003TJ9YQU/ref=sr_1_1?ie=UTF8&qid=1374076658&sr=8-1&keywords=food grade silicone tubing
This is a tiny bit pricey, but it's good quality and mine seems to last a long time (still on original tubing). Sometimes you can find it on eBay a mite cheaper.

 

okay, so are you thinking of turning your immersion chiller into a HERMS system? Otherwise, I'm not sure why you would want to run your brewing pump into your immersion chiller. You should use a separate submersible pump if you're thinking of re-circulating ice water so that you have water fittings for that pump and brewing fittings for this pump. Hope that makes sense. PortLargo looks like he has you covered on the adapter front.
Also I'd recommend some sort of quick disconnects for it like cam lock or QDs. It makes brew day a lot easier & safer if using a pump.

 

Still not sure if you're planning on sending cold water through your Immersion Cooler or hot wort. If you use your IC just for cooling you can get a cheap (and pretty good) submersible pump from Amazon cheaper than the adapters I linked to above (~20-25 bucks). The pump doesn't even have to be clean and you can use el cheapo vinyl tubing to circulate the cold water because it will never touch the wort.

If you are circulating hot wort, you really want the pump to be food grade and rated for temps at least to 212F. This is where you'll connect everything with food grade silicon tubing and of course sanitize it to death.

When I start to chill my wort I use the two pumps I described above. One is pumping nasty old cold water and one is circulating (whirlpooling) hot wort. I believe this is what Jamil describes on one of his websites.

If I have confused you let me know and I will post a photo to try and clear up. BTW, this is a pretty effective setup.

 

You're only going to attach the fittings once, so stripping the threads shouldn't be an issue - just be careful on the first try. I'm of the understanding (I could be wrong) that plastic fittings do not provide a clear path and can easily clog with hop gunk and other matter. Metal fittings don't have this problem.