That’s my next step… I’ve never done that before, I’m just an amateur driveway mechanic. But I guess it’s time to learn !!

 

Just remember that NPT threads are TAPERED. So, don't run that tap in too deep, or you will never get it to seal.

 

Trust me, I’m going to be very careful. This is one job I can’t screw up or it’s game over!

 

this is part of why i prefer brass. it is soft. also based off your previous picture the plug that went half way in is likely fine based off how far the old one was threaded in. as long as it seals the hole it is fine. they almost never come out unless the engine gets a rebuild.

 

The original plugs were completely seated. The new brass ones barely seat half way. This is what I get for following my Haynes manual for flushing the cooling system !

 

So I’m going to risk it and try to tap the hole myself. I certainly know I don’t want a metric set. I’ve learned it’s threaded for NPT, so the end is tapered. If I get an SAE tap set will that work? It looks like the ends of the SAE taps are tapered like NPT.

 

This is due to the way they are manufactured and it does happen. Don't tap it because if it gets replaced next time it may be to big. If you are not happy with the fit try another manufacture.

 

I tried several different manufacturers. At best, I’ll get half way. Most will only go a quarter of the way.

My next concern is getting the right tap. Could I use an SAE tap for this job, just being sure not to over penetrate and ruin the taper?

 

No,You need NPT tap. They are shorter than regular taps and look like this.

 

yup, bought some today. Bought new plugs. Checked the diameter and pitch of the new plugs and they match.

Only issue was I couldn’t find a t-handle for that big 1/4 tap. I ordered a sliding handle socket wrench.