Sorry, I used a thread pitch gage to check the threads, definitely metric. Then used a tap to check. No corrosion, no new threads cut, just a nice perfect fit for M22X1.5. I checked with the 3/4 and 7/8 taps and it was a no-go. That was on my headers.

 

I don't get it then.

edit: I guess it really doesn't matter, as long as the opening at the entrance to the fitting is sealed.

 

I'll trust you know what you are stating, and know how to check thread pitch and have actually measured the OD. Some relevant info however.

7/8" = .875"
22 mm. = .866"
7/8 X (18) = obviously 18 tpi
22 X (1.5) = 17 tpi

As stated above the metric plug is small enuf (sloppy) to allow the 1 tpi interference to actually screw in for a certain depth.

There are (were) several on another forum who were quite sure the thread (oem) was metric. It took a near act of the Almighty to convince them otherwise. My craft (trade) requires that I be familiar with threads and thread pitches. I also stand on what I stated previously, ain't such a thing (AFAIK) as a metric inverted flair thread. OEM is NOT metric! Why does the aftermarket install the holes in the first place? Aftermarket does not have their own AIR kits, so the answer is to fit OEM plumbing.

 

Hear, hear, I've been redeemed.

 

You and I both know the correct plug would need to seal on the shoulder, iow, at the bottom, just like the oem inverted flair fitting. The incorrect metric is forced to seal by the wedging interference of the incorrect pitch thread, cuz with the incorrect pitch, (by 1 tpi as noted above) it won't make it to the bottom B4 the interference/wedging occurs.

Granted, others have installed metric and gotten away with it. To each his/her own.

Rather than by the correct pitch plugs, I just weld a plug into a short section of the oem tube, which uses the oem fitting nut to do the sealing.

Edit: I take that back. I didn't weld anything. I used a machined plug, with the correct amount of pr. fit, and just pressed them in. How soon I forget some things.

 

Agreed. I can't figure this out. I asked myself the same thing while I was measuring. Why would they do this?

As to fit, I was able to screw a metric plug all the way down until it bottomed. No interference at all and a nice easy turn with my fingers.

 

As long as the others do not find out we are the same guy using different computers, I, er, we may pull this one off.

As for different years having different sizes of fittings, check the part #s for the different year exhaust manifolds, I know for a fact, one particular model uses both the 7/8 and 3/4 size fittings, depending on the year, yet take the same manifold part #, and I have verified oem installed as also being the same part. The diff being a particular year (later) plumbing being smaller, which required a reducing bushing in the manifold. FFT

 

If the plug is short enough, there won't be thread interference like nateh said. I used 18 mm till the correct plugs came in.

 

I have 4 plugs available that I had made, they are 17-4 stainless
7/8 threads, with a hex head, they bottom out in the tapped hole and require no sealant. Email me.
I chased this problem for about two months and this fixed it.