John,
I checked and I used the 40mm ID size. It's been several years since I installed the hose, but I don't recall any issue installing it. I probably lubed it up with dishwashing soap to help it slip on. I also double clamped it with the hose clamps with "rolled" edges. The flexibilty is great. As you know, with our M35B engine, the exhaust hose makes a close to 90 degree bend to the muffler. This Vetus hose makes the turn with no problem. I think anyone with the orginal OEM hose, should switch it out with this hose because of it's flexibility, and in my mind, decreases any force imparted to the fiberglass input on the muffler.