Thread size question 1985 300DT manifold

[Jeremy5848]

When I started thinking about an EGT gauge for my 300D, I discovered that the exhaust manifold had a nice tapped hole at the rear, just before the trap cat. The hole (see the first picture) was 10x1 mm and had a short plug in it.

I bought a pyrometer and thermocouple and a brass adapter to convert the 10 mm tapped hole to 1/8 inch pipe threads for the thermocouple. When I tried to screw the adapter into the exhaust manifold, it wouldn't go in, even though both the plug and the adapter had the same diameter and thread size.

By coincidence, the intake manifold also has a 10x1 mm tapped hole (see second picture) with a plug. Well, the adapter wouldn't go into that hole either. When I lined up the two plugs and the adapter (see the third picture) and stared at them, I realized that the threads on the adapter were slanted more than those on the plugs. I've never before seen threads that were the same but wouldn't fit each others' holes.

What's going on here and how do I adapt the thermocouple? I really don't want to retap the hole in the exhaust manifold and risk sending chips of steel into the tubo. Someone here a while ago suggested packing the threads of the tap with grease to hold the chips but that makes me nervous. I don't want to pull the exhaust manifold either -- way too much work. How do I make the existing tapped hole work for me?

Jeremy