Rohan, When I first raced the bike a few years ago, it was fitted with the standard box. It was revolting to ride, impossible to change down without a heap of revs, really difficult to be smooth. Then I bought a 4 speed CR box from the US. First gear was higher than a manx 1st. Getting the gearing right for most of the circuit meant that first was too high for the clutch starts. Most of my race starts were too slow. I've now bought a TTI 6 speed box with the first gear half way between standard commando and manx ratio. Because of my usual battle with the chains, sprockets and adjustment, I inadvertently used one tooth too high on the engine sprocket and the gearbox sprocket as well. The motor struggled to get the bike mobile, but as soon as it got rolling, it took off in a flash. I think the 6 speeder will be excellent, but I'm not looking forward to the next fight with the chains. I'm using single row chains, and I'm trying to avoid half links. It is the same old story, everything is either too big or too small. I end up with the wheel too far back or run out of adjustment on the gearbox. I can afford to end up with the overall gearing slighly higher than it has been, but even going up by one tooth presents problems. It's just one of those things with old British bikes - they are designed to drive you insane. But I can't really blame the British - I built the bike myself, when I was much younger and things were simpler.