The tool is definitely something you'll want to get. I won't say it's "impossible" to install the seal without it, but definitely difficult. The other thing I've done is to apply a "small" amount of an anaerobic gasket sealer to the shaft before setting the seal - I think it's Loctite #518 if I'm remembering right. If there are any small grooves or dings in the shaft, it will help seal things up. Its only a "metal to metal" seal on the shaft, no gasket or factory sealer like there is on the seal O.D.