The Trident engine already has an anti-drain valve in the crankcase but on some bikes this doesn't seem to work and it can be difficult to rectify. So we developed a purpose made valve which is incorporated into the gauze filter in the oil tank, so it is barely noticeable on the bike. There are 2 sizes, for 5/16" for most T150's and 3/8" oil pipe for T160's, so we need to know which size pipe you have. We also recommend removing the existing valve if this one is fitted. It should keep the oil in the tank for a few weeks, and avoids the problems of having all the oil in the bottom of the engine every time you want to ride the bike. It can also mean the bike is easier to start as you don't have the crank submerged in cold sticky oil. The valve operates on a "fail safe" principle, in the worst case it will just stop being a valve and let the oil through. Order part number LEG-0016.Please note, this will not fit R3 oil tanks as they are a different design.