Passenger Side Head Water Port Problem

[FLGS400]

So, I decided to put a mechanical temperature gauge in my 1991 C1500 with the TBI 5.7. I wanted to keep the stock temperature gauge functional, so I am using the 3/8 NPT port in the passenger side head, that was plugged. I bought the square socket to get it out and it rounded out the square. Needless to say, I tried a bunch of things over the past couple weeks, to get that thing out. I ended up having to drill it out and re-tap the hole. Now when I install the 3/8 NPT adapter for the probe, it screws all the way in to the back side of the 5/8 tread side by hand. I'm pretty sure that is is going to leak as that is way too far for an NPT fitting.

Before I damage this thing any worse, I was looking for opinions for you all of what the best sealant I could used on the threads would be. I would really like something that will seal the threads, and lock the fitting in place without applying too much force to the adapter fitting (like finger tight). I considered JB Weld and a Permatex thread lock/sealant (supposed to be of the "blue" thread lock quality), but thought I would ask first...

Anyone have this problem before?

 

[FLGS400]

Here are some photos of the hole as it is now...

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[95OBSMan]

Hey Rich!
I did a lot of those Auto meter oil gauges back in the day, the ones with the copper tubing (like you have) and the ones with the nylon tubing, anyways those copper/brass NPT fittings seemed to always leak, almost always do to me over tighting of the brass to cast iron, any ways I know your situation is a little different, I have used Loctite 545 red, its properties are different than the typical red thread locker. IIRC the temp rating is over 300 degrees which I would think should do for your application.

For what its worth and it was my truck, I'd go with the Loctite 545 red... The truck is looking just killer!!

 

[Scooterwrench]

You're gonna have to open it up for 1/2" NPT and use a 1/2" to 3/8" NPT reducer bushing.
I use Permatex aviation gasket sealer for pipe dope,it has never let me down.

 

[Schurkey]

Seems like you ran the tap in too deep.

For the record, you'd have done better by heating the plug (not the casting around the plug) until it glowed. (Oxy-Acetylene--Propane without extra O2 isn't hot enough) Let it cool, it would likely screw right out.

Remember, you'll HAVE to use the bushing that came with the temp gauge kit, it's got a shoulder on it that seals to the rib on the copper temp-sensing bulb. Regular reducer bushings don't have that shoulder.

If you drill 'n' tap that hole oversize, you'll maybe use a standard reducer bushing to get to stock size, then the Autometer bushing to seal to the bulb.

 

[FLGS400]

Thanks for the responses guys!

I think I'm going to try the Loctite 545 red first. If that doesn't seal, then it looks like I'm going to have to drill and tap it for 1/2" NPT. I'll let ya'll know how it goes. Getting ready for a trip to Bowling Green, KY for the Buick GS Nationals next week, so it'll probably just have to sit until after I get back.