Hey engine gurus. I had my engine rebuilt and it sat for three years before I installed it and broke it in. It has a bad rear main seal leak. Is it common for this to happen and will it eventually swell up and stop leaking as oil lubricates it or has it dried out due to inactivity and need to be replaced?

You need to determine exactly where the oil is leaking out of, the main seal itself or the side seals or maybe even somewhere else
If it is leaking out of the seal between the crankshaft and the seal it will be slung out away from the crankshaft flange on the engine side of the flex plate or the flywheel
If it is not slung out on those then it is leaking from somewhere else
If that is the case you may be able to fix it a little easier
Check it out and let us know what you see and find out

as with any seal, it will not swell up on its own and stop leaking. if it sat for a bunch of years and then you started it, its not a big surprise that its leaking ultimately It was dry when you started it and either tore, or wore away, or it was installed wrong to begin with.

Good info. It's an A-727 car ands I can tell you that with the inspection plate off, motor oil is slinging all over the underside of my headers, more on the passenger side. With the plate on, it runs out the drainage hole. It's definitely not coming from the pan and it's not all over the torque converter. I had the pan off last week and the lower end of the main seal where it meets the pan looks well sealed but it's hard to see anything else.

I thought rear main seals were made of rope. They rip?

Is this enough info or do I need to get the car up in the air and run her until she gushes?

You should get it up in the air and clean it real good with brake cleaner.. while its in the air have someone
run it so you can find the leak.. if its leaking that bad it wont take long.. if needed clean it a few times

All the OEM Mopar V8 up to or maybe even a little later than 1985 had rope seals, they are not included in over haul gasket sets since around 1992 or so. They where replaced with a rubber like seal back then and the original paper type side seals went away around 10 yrs ago also
On your deal make sure you know exactly where the oil is leaking from before starting to fix it, don't be surprised if it has multiple leaks like from both of the side seals or one side seal and the oil pan gasket around one oil pan bolt hole or even a rear oil galley plug leaking or the cam rear cam plug leaking
I've had all of them leak in different motors in the past, not so much anymore with better seals and using a good adhesive sealant to completely seal up the sides of the rear main seal holder to the blocks Keeps in the loop on your progress, you can fix this

Thanks, guys. It’s absolutely not the pan. I fixed that one. I’ll get her nose in the air this weekend and see what I can find. I dread the thought of having to pull the transmission to fix something like the cam plug seal.

It's probably a long shot, but check the oil sending unit for leaks before getting too deep into rear seals. 2¢

Robert

Great idea, Robert. I'm going to clean it up and go over it with a fine tooth comb. Question. Will brake clean eat the paint off my trans or engine?

Brake Clean could be iffy, test a place first.
Carb Cleaner is harsher, in my experience.
I usually use Simple Green, or Wax & Grease Remover on a rag, follow up with a dry rag.
Least invasive is glass cleaner or kitchen-use "surface cleaner", but they might smear as much as clean...

I once had a Diplomat that poured oil out of its oil pressure sender, hope you're "lucky" that way.
I have a BB with a similar but less-leakage symptom to yours, so I'm curious what you find.

If you have decent paint on the engine break clean wont hurt the paint.. also if you start on the
top of the engine and work downward it tends to work better(less mess)but look at everything on the
top and work downward