340 rear main seal leak/gush

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70SwingerGuy

70SwingerGuy

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So my buddy recently refreshed his motor due to a cam change which included removing the crank assembly(67 Cuda 340 auto), upon reassembly he replaced the rear main seal of course. He also installed a higher stall torque converter, I only mention it because it was a change and probably unrelated to the leak. So he fires it up and drives it to my house a few blocks away(lets not talk cam break in right now :) ) and while bringing the revs up and holding them, a river of oil starts running out of the back of the motor, and not the stereotypical rear main seal drip, I mean this dumped enough oil onto the street that i actually got out the floor dry. His pcv system includes a valve cover vent running to his air cleaner, and also a vented oil filler cap.
Has anyone ever run into this before?? I cant figure out what could have caused that substantial of a leak.
 
I have seen/experienced leaks like that with new seals.
On more than one occasion, the seal ended up with a piece torn off the lip, which just created a direct path for the oil to escape.

IMO, that’s almost the best case scenario.
You pull the old seal out, find a defect, and replace the part.
 
Seen that be a cam plug. I layed on my back and replaced two rear main seals without a lift, and learned a valuable lesson.
Pulled the tranny on a hoist and saw the plug leaking right away
 
PRH said:
I had a 440 on the dyno that gushed so bad I was sure I must have put half of it in backwards.

Nope…….that wasn’t it.
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Well ........I'm waitin. lol
 
I never really figured out what the primary problem was…….. but it did take 3 tries to dry it up.
 
I changed my rear main once with engine in car put in backwards even though I knew correct way. Was dry until I drove it then poured oil out all over the place. Luckily I didn't run it dry. So got to do the job all over again engine in car.

I could never get mine to be 100 percent dry until I put thin film of RTV between block and rear main cap. Otherwise pressurized oil from main oil feed would squeeze out and look like a rear main leak.
 
i'm guessing you've investigated the crime scene enough to eliminate the oil pressure sending unit and oil filter/adapter plate?

if so... time to go deeper.
 
I Would check the rear pan seal sometimes when the pan is tightened it will slide in or out of the pan and cause a major wet spot . and more so when silicone is used .
 
If he drove to your place directly after "refresh" with no cam breakin, it could be a multitude of things. No RTV on pan seal? Rear seal offset?
 
340sFastback said:
I changed my rear main once with engine in car put in backwards even though I knew correct way. Was dry until I drove it then poured oil out all over the place. Luckily I didn't run it dry. So got to do the job all over again engine in car.

I could never get mine to be 100 percent dry until I put thin film of RTV between block and rear main cap. Otherwise pressurized oil from main oil feed would squeeze out and look like a rear main leak.
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I know that you guys are not going to believe me, but I've never put any sealer between the rear main cap and the block and I've never had any issues with leaks. I use real fine sandpaper to clean out the groove in the main cap and the block and then I throughly clean both grooves out. Cleanliness is the key and to not get in a hurry. I also check the crankshaft for burrs where the seal will ride against. Before I install the seal I check it out for any nicks as I've had a couple that where bad from the factory. I offset the ends of the seal and I put just a dab of sealer on the ends of the seal. By me doing it this way I can honestly say that I've never had a rear main leak, luck, maybe so.
 
Dan the man said:
I know that you guys are not going to believe me, but I've never put any sealer between the rear main cap and the block and I've never had any issues with leaks. I use real fine sandpaper to clean out the groove in the main cap and the block and then I throughly clean both grooves out. Cleanliness is the key and to not get in a hurry. I also check the crankshaft for burrs where the seal will ride against. Before I install the seal I check it out for any nicks as I've had a couple that where bad from the factory. I offset the ends of the seal and I put just a dab of sealer on the ends of the seal. By me doing it this way I can honestly say that I've never had a rear main leak, luck, maybe so.
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Dan, I’m one of those guys who’d tell you that not all of us clock the seal offset like that.

You need to learn to fit the rear seal rather than offset it.

Think of that seal like a bearing and then tell me if you’d ever offset a main bearing like that and why you’d do it.
 
Newbomb Turk said:
Dan, I’m one of those guys who’d tell you that not all of us clock the seal offset like that.

You need to learn to fit the rear seal rather than offset it.

Think of that seal like a bearing and then tell me if you’d ever offset a main bearing like that and why you’d do it.
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Actually, I've have done it both ways.
 
Newbomb Turk said:
Ok. Then tell me why offsetting the seal is the proper installation.

Trying to learn.
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It eliminates the parting line at the block surface. It's been done by engine builders for years. It must make a difference, that and the other preparation I do before installing the seal I've never had a leak
 
Dan the man said:
It eliminates the parting line at the block surface. It's been done by engine builders for years. It must make a difference, that and the other preparation I do before installing the seal I've never had a leak
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I fought a small leak till I put sealer between the main and block. Had pan off 5 times new 408. I just put a dab of black RTV at the seal ends. Seems to work for me, but I have considered off setting the seal ends...
 
Mopar Tim said:
I fought a small leak till I put sealer between the main and block. Had pan off 5 times new 408. I just put a dab of black RTV at the seal ends. Seems to work for me, but I have considered off setting the seal ends...
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Yeah, the way you fixed it is best.

Offsetting the seal effectively keeps the seal from getting the proper crush.

If you have a seal leak, and you offset it and it seals you reduced the crush. In other words, the seal is getting too much loading on its ends because those ends are too proud of the cap and block.

Correctly fitting the seal (both halves) for proper crush and the seal won’t need to be offset to not leak.

I tried to get into a conversation with DTM but he is convinced that offsetting the seal is the proper fix so I didn’t waste my time trying to show him that just because people are doing something doesn’t make it right.
 
Mopar Tim said:
I fought a small leak till I put sealer between the main and block. Had pan off 5 times new 408. I just put a dab of black RTV at the seal ends. Seems to work for me, but I have considered off setting the seal ends...
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Glad you got it to stop. Was the pan gasket re useable? I've known about off setting of the rear main seal for a long time
 
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