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Discussion Starter #1
I just put a new motor together. Its a 434 SBC with a Little M block with 400 mains. I'm chasing an oil leak at the back of the block and I think I found it. I was re-read the Dart literature and just realized that their 400 main blocks require a Fel Pro 2909 rear main seal and of course I used the 2900 that came in the R.A.C.E. set that I bought. I assumed that I could use a standard rear main like a stock 400 block. Before I pull the pan do you think this is where my leak is coming from? Its very hard to tell even with the motor running as the flexplate is throwing oil. I also am using a Moroso vacuum pump (3vane) and what to make sure a 2909 seal is what I should use. Thanks, KW
 

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If thats your only problem your doing well. don't sweat it. what else would you be doing on a friday night.
 

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Well the only bad part is if you put the rear main in like I do, you're not rolling it out. Mine get siliconed in. 2909 will fix it.
 

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I think i remember reading somewhere that the 350 main Little M's take a different seal also, if you are running a vaccum pump. Anyone know for sure, I'm about to assemble one and dont want to do it twice. Chris, do you stagger your seals or just silicon them in place straight across ?
 

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Discussion Starter #15
Its strange how we learn the most when we screw up. I'll get a new seal and pan gasket coming and deal with it. The bad thing is I need to lift the motor out to get the pan off.
 

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What he said.
The OEM 400 SBC main bore is larger than the main seal lip. Once the block is line honed the seal lip becomes the same bore as the main. On the aftermarket blocks the seal lip and main bore are already the same. The 2909 is for aftermarket 400 main blocks and line honed OEM 400 blocks.

If you were to run a straight edge down the main bore on the OEM 400 block the lip on the rear main seal would be in the way.
 
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