I checked that first with my metric thread gauges. The plug that comes in the hole from the factory does indeed measure M10 but just try getting a pipe that size in this (USA) country! A 1/8 inch NPT thread is almost exactly the same thing -- that's why I ended up using it. It fits perfectly, as illustrated by the photo I posted in an earlier reply to this thread, and is much easier to adapt to the tubing that comes with the boost gauge.

I just remembered that I bought a M10-to-NPT adapter with my boost gauge. I could not get the M10 adapter to thread into the tapped hole in the intake manifold. Not wanting to strip threads, I didn't push it but instead looked in my plumbing box for similar fittings. After some experimenting, I discovered that a brass fitting with a 1/8 NPT tapered pipe thread went in perfectly. I can't explain why this is so, but it is what I found. YMMV.

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970