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Rear Main Seal Replacement and Oil Pumps


Well-Known Member
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7:22 AM
Apr 18, 2016
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This summer has spelled the end for the rear main seal in my stock 340. I am now getting puddles of engine oil under the bell housing, and I am down to 500 miles/qt, so I guess its time to tackle the seal replacement. The engine is stock, and to my knowledge the rear main is original. I read the threads on the B/RB engine seal replacements, but havent found much on the LA engines. So a few questions for the small block crowd.

1) Any tricks / tweaks to the seal installation on the small block engines? The YouTube videos make it look easy, which always worries me... :)
2) Any recommendations on seals to use? I have a Mahle/Victor Reinz JV606 ready to go, but would be looking to do this job once, so right seal is key.
2) Since I have to remove the rear main bearing as well, are they any tricks to making sure I get this back into position correctly?
3) Since I have to remove the oil pump anyways, it seems like it would make sense to replace the pump. Mine has 85,000 miles on it, although the pressure gauge normally reads mid range or so at driving RPM. The question is what type of pump to use - stock replacement or a high volume, stock pressure pump? I was considering a high volume pump, as the clearances in the engine are going to be significantly larger than new - its never been rebuilt, but there seems to be lots online warning not to use a high volume pump in stock applications. What is the downside to more oil flow? Thoughts?

Thanks for the help folks...

A stock pump is fine. Don’t use a high volume. There is no such thing as high volume stock pressure oil pump.
This is for big blocks, but shows it as being a high volume/standard pressure.

Well look at that, thanks for posting.
I was looking at the Melling pumps as my choice for this job. The M72 and M72HV are the small block pumps. Some of the pumps I looked at say they are "standard pressure", but when you compare the pressure specs, they are actually significantly higher than the stock pump (60-70 psi). If memory serves, the stock pumps produce about 50 psi (?). I have not been able to find the Melling pressure specs yet.

However I did notice that the high volume pump is higher by 1.5" (to accomodate the longer rotors used to create the higher volume), which I expect would mean a change to the suction tube is required if a high pressure pump is used.
Also, got a question on seal orientation. My seals came with no instructions. The pictures aren't great, but hopefully enough to see the orientation. Do I have the seal oriented correctly with the lip wiping on the crank pointed toward the front of the engine?


I contacted Melling a few years ago, and they told me that the standard pump was made to have a standard pressure of about 45-50 psi. Luckily with the big blocks that can easily be changed with the external spring replacement or shimming.
Well spring is here and I am knee deep into the rear main oil seal job. Having a horrible time getting the oil pan off (see other thread), but that aside, I am wondering if the seal is the culprit or not. When I get drips under the car, it is dripping at the inspection plate on the front of the bell housing. It certainly appears to be coming from within the housing, as opposed to running down the outside. Hence why I was convinced the oil seal was toast.

However now that I have the entire dust shield off, I can see that the front of the ring gear is absolutely dry. There is oil and dirt buildup on the inside of the housing around the ring gear but the gear itself is perfectly dry.

So the question i have is with respect to what would I expect to see if the rear main is leaking? I kind of expected that you would see oil all over the front of the ring gear. Is that a wrong assumption?

The dust shield was dry on the internal side when I removed it.

So from your experience with bad rear seals, what would I expect to see - or how would I more firmly diagnose if its the problem?

Given the nightmare I am having getting the oil pan off, I am tempted to fit the new pan seal in place an button things up again



Decided to button up the pan with the new seals. Got that done and ran into a YouTube video suggesting one should check the oil pressure sender for leaks that could disguise as a rear main. Once the car was off the jack stands, I checked the oil pressure sender, and yep it was loaded with oil on the underside. Can't tell for sure whether its leaking at the rolled seal on the body of the sender, or at the joint of the mounting fitting at the bottom shell of the sender. However its covered in oil.

Replaced the sender with a new one and cleaned up the area around and below the sender.

Drips from the bottom of the bell housing at the inspection plate appear to be gone. Looks like a leaky oil pressure sender was the culprit all along...
Worked out nicely - good job. Gravity makes finding the source of oil leaks a sometimes difficult job.
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