Crankshaft rear bearing circlip DIN 471 - 40 x 1.75

This circlip goes on the rear end of the crankshaft, on the outside (rear) of the rear man bearing's inner race.

If you haven't damaged or distorted the old one, you can reuse it. If it doesn't seem to fit properly (especially if it fits loosely), you'll want to replace it.

Regarding the sizing, 40 x 1.75 means it's 1.75 mm thick and is designed to fit a groove that is 40 mm in diameter. That's why no measurement of the snap ring itself will give you 40 mm.