Try to get the 3.5mm on at first, and if it is too much effort, use the 4mm. Even though, 0.5mm does not seem like much of a difference, the appropriate size will soon become pretty obvious. It should take some effort to push on the Viton, but not a lot.

And remember, when removing the old hose: twist, don't pull.

 

No diagram. I suggest using the 3.5 everywhere under the rats nest where things are really tight.

IME, there is a bigger difference in feel/size if you compare the 3.5mm vs. 4.0mm silicone hose. The viton feels similarly beefy in both sizes.

Dave

 

The Viton vacuum hose I bought was only available as 1/8" (3.18 mm) and 1/4" (6.35 mm). As you might guess, the 1/8" stuff was quite a tight fit, and the 1/4" was loose enough to make me use nylon tie wraps. I did find that I had to hand-ream a few of the 1/8" lines due to somewhat larger hardline pipes in a couple of places. But the end result was very good, and a year later I found all the connections in good shape.

To handle 25" (Hg) vacuum, I found that the hose wall needed to be quite thick (1/8"), which was expensive: $530 total, including extra. You can get an idea of the wall thickness of the hose I bought from the attached photo. (See the hose pointing at you in the lower left corner of the pic). I'm hoping I will never be sorry I chose Viton over the alternatives.