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How to replace Rear Main Seal plate in car for LS1 swap?

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Old 09-20-2022, 04:15 PM
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Car: 1985 Pontiac TransAM
Engine: 400SBC 24x conversion
Transmission: T700r Stage2 Shiftkitted
Axle/Gears: 2.77 9 Bolt
How to replace Rear Main Seal plate in car for LS1 swap?

Hey Guys,

I took my car out for its first big run and did a 100km(65mile) stretch with no issues and the car ran beautifully with no issues.
Next day it went about 5 miles and then started pissing oil out between the engine and the trans.

I checked the car over and its dry as a bone on the top and sides of the motor, so the only place it can be coming from is the RMS.

My question is, has anyone been able to successfully change on of these on the car, i.e. get the trans off and out of the way to gain access to the plate, or do I have to pull the motor out to get to the top trans bolt?

If I have to pull the motor I'm going to chop out the radiator supports at the front of the car and get a bolt in piece welded up to make it easier to pull motor and trans together because lifting out over the top is a pain in the ***.

There's plenty of instructions for rear main seal replacement for an ls1 and plenty for thirdgens, just not the 2 together.

Old 09-20-2022, 06:40 PM
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Car: '89 Firebird Formula
Engine: TBI 305
Transmission: T5
Axle/Gears: 10-bolt, 3.42s, Torsen diff.
Re: How to replace Rear Main Seal plate in car for LS1 swap?

Step one: Remove transmission.

Step two: Replace seal and/ or seal housing gasket (just do them both at the same time).

AND IT'S JUST THAT EASY!!! (Read this in Billy Mays' voice. RIP)

It's not rocket science.

Also, here's a pro tip: Loosen all of the oil pan bolts slightly before you remove the rear plate; enough that you can slightly move the pan with your hands. This will take tension off of the plate.
Install the rear plate, BUT ONLY SNUG THE BOLTS DOWN. Have it to where you can just barely move the plate if you really put some effort in with your hands.

Tighten up a few of the oil pan bolts that go TO THE BLOCK temporarily.

Tighten the two 6MM bolts that hold the rear plate to the oil pan. Then torque the plate to the block.

Now slightly loosen the oil pan bolts your tightened (including the long ones going into the plate), and install the transmission.

Snug the transmission to oil pan bolts by hand.

Torque the oil pan bolts, then torque oil-pan to trans bolts.

You're done.

WHY do it this way? It squares the rear plate to the oil pan, and the oil pan to the trans.

"OIL PAN ALIGNMENT/ TORQUE CRITICAL" is cast into the oil pans from the factory for a reason.

I've seen too many times where a rear seal leak comes back a week later because it wasn't aligned properly.

Of course, all of this could have been avoided if GM had just added SOME F*CKING DOWEL PINS, but that's a little too much to ask, apparently.



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vorteciroc (09-21-2022)
Old 09-21-2022, 05:09 AM
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Car: 1985 Pontiac TransAM
Engine: 400SBC 24x conversion
Transmission: T700r Stage2 Shiftkitted
Axle/Gears: 2.77 9 Bolt
Re: How to replace Rear Main Seal plate in car for LS1 swap?

Thanks for that.
As I mentioned I had tutorials for doing the plate once I had access but it was more about getting access to it.

A shifty one person and getting to the top bolts of the bellowing was **** but I got it out.

Had to f&(@in remove the entire exhaust system including headers as well, so it was a pretty big job.

All back together minus 2 bolts at the top of the bellowing and the exhaust to go back on.
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