In the process of going thru my 1.8 and replacing rubber pieces inside the engine bay. I feel like that I just did this not that long ago, and the once new rubber connecting hoses on the intake to the plenum are already rock hard and cracking.
I think I have seen pictures of people using silicone couplers, but can't find a source. The intake runner measures way closer to 37mm vs 38mm. I feel the 1.5" couple will cause vacuum leaks. Silicone may seem like the way to go, just need a direction what people are using and where to obtain them.
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You are correct, 1.8 do not like vacuum leaks.

I could never find a silicone tube that I liked the fit.

Not what you asked, but when I was DDing my 1.8L in college I replaced those boots with high temp, heavy wall heat shrink tubing from work (Spencer Turbine stuff, most typically used for plant vacuum tubing). Worked great and cleaner look! I'd grab a picture, but engine is disassembled.

Seems like a place like this would have something:
https://www.siliconehose.com

This has to be awful close:
https://www.siliconehose.com/1-380-id-4-ply...ej20-compliant/

Maybe this?

https://www.amazon.com/Silicone-Polyester-B...r/dp/B072MZ5T8P

All good options. I spent some time last night looking for a solution, and that 1.380" silicone hose seemed to be the closest. Thanks Rand, I was on that site the most last night. The intake runner is around 37-38mm, and I wasn't sure how stretchy the 1.38" (35mm) hose would be.
The food grade 37mm silicone hose seems like it will be perfect, not sure if I'm a fan of the transparency.

I thought I saw a picture of someones engine bay that already had these and made a bad assumption that some place was selling these as a kit.

Not like horse shoes
Those hose arent engineered to stretch deflect like rubber hose, needs to be pretty close to the correct ID & typically uses clamps
At 4 ply they are pretty firm

yeah, stretching silicone hoses is a loosing battle. They are usually nylon reinforced and that stuff does not stretch well.

Though the stock connectors don't use a clamp, you can put a clamp on silicone to make it squish down a little more. For turbo applications you always you a clamp to keep from blowing the boot off under boost.

Zach

Th OD of s 2.0L tube is approx 1.54", the plenum side is a bit less so the 1.5" stuff would be a better bet