The SM 420 was discontinued in 67. If it is a 68-69 it will be a SM465. It is the same procedure for replaceing the rear seal. Like said previously, you do not have to remove the rear cover. To get to the front seal remove the front brg. retainer. Nothing will fall out but don't let the shaft & brg slide forward more then about 1/2 in. That will be a good time to check the front brg. Spray A little brake clean on the brg. to clean it & look in with a light as you slowly turn the shaft. If it has any pits on the balls or races it needs a new brg. It is common for this brg to go bad & it will cause whinning noises in the lower gears.
George
