The proper way to repair that is to drill out the old threads and tap it to the next larger size (1/2?). Use a brass threaded bushing to reduce it back to stock size. Of course you cannot do this mounted on the engine.

You could clean the thread really well, then smear them with JB Weld and the screw the water neck in to reform the threads. The water neck threads should be really clean and coated with mold release. Probably shouldn't try this on the engine either.

My oldest RPM Performer mainfold that is port matched had the same problem. The threads were there but loose. I found a fitting that had some extra long male threads. Since those threads are tapered, I cut off a couple which put me farther up the taper where the threads were bigger. I used the paste pipe dope and it threaded in real nice and tightened up well. That was two years ago and never a leak. Might be worth a try for you.

Tonite I got a 1/2"-14 NPT bushing that the 3/8"-18 NPT heater hose fitting will screw into.  Tomorrow I'm going to pick up a 1/2"-14 NPT pipe tap.  That tap is supposed to require a 45/64 drill bit, but I have a Silver and Demming set that includes a 11/16" drill, which is only 1/64" smaller.  So I'll post the results.  Thanks.

jkordzi wrote:

Tonite I got a 1/2"-14 NPT bushing that the 3/8"-18 NPT heater hose fitting will screw into.  Tomorrow I'm going to pick up a 1/2"-14 NPT pipe tap.  That tap is supposed to require a 45/64 drill bit, but I have a Silver and Demming set that includes a 11/16" drill, which is only 1/64" smaller.  So I'll post the results.  Thanks.

Picked up a NPT pipe tap set at HF that included the 1/2"-14 tap.  My engine wasn't installed but I think I don't think it would have mattered.  I kept the intake manifold on the engine and removed the water inlet and thermostat and jammed some paper towles into the front water passage in the head so cuttings wouldn't go into it.  I then used the 11/16" drill bit in a 1/2" drill and followed up with the tap.  I used cutting oil and initially tapped down until all the threads were cut, then went one turn at a time until the 1/2" bushing engaged all the threads, but no more.  Vacuumed it out thru the water inlet opening.  Worked great.