To change the front transmission seal on a Massey Ferguson 40, you will need to gather the appropriate tools and supplies, such as a new seal and sealant. Start by draining the fluid from the transmission, then remove the housing screws and split the housing. Remove the input shaft, then remove the old seal. Clean the area with brake cleaner, then apply a thin layer of sealant to the area. Install the new seal and the input shaft, making sure to use the new seal. Reassemble the housing, and fill the transmission with the appropriate fluid. Test the seal for any leaks and monitor the transmission periodically.