in situ with the shaft sticking out remove the external compression spring from the seal and find the deep narrow socket that best fits just over the lip (17mm iirc) you have removed the spring from and use this instead of the special tool to get it seated as best you you can - ease the seal over the splined end of the shaft and down and then use the socket to tap it home, you won't get it perfect but get it parallel and in so just protrudes from the recess in the case and then pop the spring back on - oh yes a bit of moly grease will massively ease the installation, apply to all surfaces
