I believe the correct original belt size for 2.3/4hp machines was 3/4". This size does not seem to have been made for many years. Optimat belt comes in two sizes for motorcycle use 17mm suitable for small veterans (it will work on the Douglas but rides a bit low in the pulleys) and 22mm which is a bit large and rides high in the pulleys (it is this size that is most commonly used). This belt, if genuine Optimat, seems to have quite a good life and does not stretch too much after initial fitting. It is joined by their own Optimat clip comprising two plates and two screws that go through the holes that are spaced at about 1cm distance along the whole length of belt. There are cheap and nasty versions of belt of identical appearance that stretch very badly and is soon affected by oil. The genuine Optimat in England is not cheap and is certainly very much more than the price quoted by Graeme. Chris.