Vincent pushrod tubes like everything on a Vincent........preparation is the key........how to stop this problem for the next time.........Ok.........Lets start with the size of the flange at the top of the tube.......make sure it slides easily into the head, sometimes they are a very snug fit.......this will cause problems, and so will the sealing washer immediately under the flange, this too needs to be a nice slip fit into the head. I place a lower seal washer on the tube hard up against the flange, then linish both down slightly so it slides easily into the head. i don't use the original lower seals anymore, they are too inconsistent in size, I use 3 mm "O" rings backed up with a 2.5 mm upper one to act as a packer, and it helps hold the gland nut where I want it. Make sure the top flange is flat and check them for cracks, the lower tube should be 0.750" in diameter or close to it........custom fabricated ones that are larger in ID than that are a pain, as they can jam down the crankcase hole. All the available parts from the VOC spares co work well (except the lower seals) all other types of seals, nylon type packers etc are a pain and cause grief as they generally melt from the heat, or the over tightening of the gland nut crushes them so much they become almost welded to the head, and then the tubes are difficult to remove. On final assembly use a small bead of sealant on the top of the flange, and a tiny smear around the bottom of the tube taper, this will help the tube slide into the "O" rings in the case. I have done many engines this way now with no real issues with leaks, and you should not need to overtighten the gland nuts, unless there is a problem you may have overlooked..........Always take the time to pre fit things before assembly, it makes the final assembly much less stressful and you tend to get good results. I use this method for many things on these bikes............