How would I go about
Doing this self-job?
There is a leak at the
Rear Main Transmission Seal

1973 Dodge Dart
225 Slant Six Engine

Do you have an automatic?

A rear main seal is the seal on the back end of the crank shaft controlling engine oil. Perhaps you meant the tail shaft housing seal where the drive shaft connects to the transmission.

You have the later NON trunion stile, (1966 and newer I believe) front U joint, the job is easy:

Jack up the car, and secure it with jack stands, and wheel chocks. Make sure it won't roll, and can't fall. Put the car in neutral.

Remove drive shaft by disconnecting rear U joint from the differential, four bolts hold it in place. Don't let the two baring cups fall off the U joint wrap them with tape to keep them in place. Gently pull out the drive shaft along with the front U joint from the tail shaft housing, and place it some where out of the way. The seal looks just like a wheel baring seal and is pressed into the housing. Carefully pry it out with a large screw driver. Gently and evenly tap in the new seal in place, and lube it with a little transmission fluid. Slide the drive shaft back in the tail shaft housing. Than reattach the rear U joint to the differential being careful not to lose the caps, and torque to specification.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC