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Rear main cap install question

Huicho417

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I asked this in a previous post but got no answers to this specific question, regarding a persistent rear main leak. Thanks to everyone help I have managed to minimize the leak greatly. Only a few drops when pushing past 1500 rpm.

The only thing I can think may be an issue is the following. When I torqued the rear main cap to specs, it tends to no line up with the front and back of block as seen in the pictures. I’d say the cap is pushed toward the front of the engine about 1/32” plus or minus. Could this be causing the rear main seal halves to not line up? I can tap It into place aligned with the block, but then the bolts will not go in.

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So many threads going on here.... I tried to read them all to catch all the information, but I think you may have done better to stick to the ONE... once you respond, everyone will jump back in again. So, a couple things I will add; not sure why this came apart to begin with?? Oil leak?? When you pull a pan, look carefully for deformation issues, and look very close at bolt holes. Hammer and dolly to straighten, and take care of any pooched-out bolt holes. When you need a sealer, use the utmost quality oil, etc. resistant. Ultra grey is my current favorite, as it is very close to the old Ford gasket-maker... GM had something similar and they used no gaskets at all. Dow makes very high tech rubber sealers that can handle some heat and are better than silicone. Don't apply sealer until you are ready to put final torque on the job. And finally, use a light thread lock such as blue Loc-Tite on your pan bolts.... you need a base built up so you won't have to torque down as tight for fears of backing out.... As others have said, i like to feel and watch as opposed to following the gospel of the service manual for pan bolts, valley covers, etc. in any application. Another thing you should sadly note, porosity is always a possibility....in the block itself for sure, and the rear main cap too. Seems you need to try the camera trick right after a re-sealing test drive, looking for that first drop so you know for certain just how high up on the engine this is coming from. Good luck, Lefty71
 
Thank you Lefty 71. I will follow your suggestion on sticking to one post.

The original reason for this work was a rear main seal leak. Before I did that I made sure the oil sending unit or valve covers were not leaking. Also, after multiple attempts at using and aftermarket pan I straightened and cleaned my stock one like you suggest. Lesson learned there, the aftermarket one was not stamped correctly.

The only question I have now is why the rear main cap does not site flush with the block edges along front and back. And if this cause the seals not to match up like they should.
 
That assembly should have been machined with that cap torqued. So don’t worry about it.

or take the crank out and torque the cap and see if the two sides of the rear main groove line up with each other.

Do you have a small block mopar there?
 
if you offset the seal halves slightly....then the sealing lips will be lined up because the groove forces them into line. that way the cap is automatically the correct distance into the block.
only offset aprox 1/8 or risk shaving material from o.d. of seal as you cant see the end enter the groove
as it goes in when pushing the cap down. torque to 25 ft./ibs. good luck. another thing....consider removing .005-.010 from the bottom of the cap [seal retainer] which will compress the seal a bit more.
 
Thanks everyone. This is a small block 318. The lower seal has some “wings” that fit into the lower cap, not really able to clock or offset the seals.

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why the rear main cap does not site flush with the block edges along front and back.
I think the reason is because that's not the place where precision machining needs to be done. They cut enough back out of the way..... the precision surface is where the block meets the cap for proper bearing crush (after line bore). That gap you are worried about could be a real issue if it were zero or less. The reality is that with proper sealer and seal install....the gap won't come into play. Unfortunately I don't have one apart here in front of me to look for a similar gap....... I'll bet someone over at FABO does tho, if no one here does.

Really wondering if you and Cranky didn't nail it here;

cranks surface prep / rms question

If you do indeed have a rough edge chewing up the new seal, then you will need to address it somehow.
 
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The only way to find an oil leak is to clean the area up with brake cleaner. Check the oil to make sure it's full. Then start the engine and lay under the car with a good light and watch where the oil is coming from. You are making this way harder than it is.
 
I asked this in a previous post but got no answers to this specific question, regarding a persistent rear main leak. Thanks to everyone help I have managed to minimize the leak greatly. Only a few drops when pushing past 1500 rpm.

The only thing I can think may be an issue is the following. When I torqued the rear main cap to specs, it tends to no line up with the front and back of block as seen in the pictures. I’d say the cap is pushed toward the front of the engine about 1/32” plus or minus. Could this be causing the rear main seal halves to not line up? I can tap It into place aligned with the block, but then the bolts will not go in.

View attachment 1468578

View attachment 1468579
Absolutely that could cause a leak. You could measure the distance from the rear of the block to the seal groove, then the rear of the cap to the groove and you will know for sure where to slide the cap, sometimes you need to make the holes in the cap bigger so it can slide a bit. Offsetting the seal is smart as the ends have to line up then, I do about 1/2" but aligning the cap is still smart. The smart money says the first step in any engine build is to install the rear main seal with no crank in the block and see what is going on with seal alignment, correct it, and mark the block.
 
Don’t alter the holes on the cap. How are you going to get the cap on straight? Your main bearing won’t like that Modification.
@4406bbl You may have missed the part about him having a small block. So the cap is the main bearing cap and the seal cap all in one.

The block and main cap are machined together. I’ll bet you your favorite beverage that the seal grooves line up just fine. But you have to take the crank out to find out.
 
The only way to find an oil leak is to clean the area up with brake cleaner. Check the oil to make sure it's full. Then start the engine and lay under the car with a good light and watch where the oil is coming from. You are making this way harder than it is.
That’s exactly what I did. Laid under the car for about 5 minutes, but no leak. Not until I rev the rpm’s past 2000.
 
Don’t alter the holes on the cap. How are you going to get the cap on straight? Your main bearing won’t like that Modification.
@4406bbl You may have missed the part about him having a small block. So the cap is the main bearing cap and the seal cap all in one.

The block and main cap are machined together. I’ll bet you your favorite beverage that the seal grooves line up just fine. But you have to take the crank out to find out.
The cap indexes in the register in the block to align side to side, not the bolt or stud, there is a little play in the assembly so the cap can slide front to rear a little. You do know a big block rear main cap can be slid back enough on stock bolts to cause the seal retainer hit it and sit too far back misaligning the seal right? On those you need to be sure the cap is slid forward enough so the rear cap machined relief lines up. He can measure a small block in car, I agree it should be fine, but if the cap is .020 to far back it could cause a leak. Pic is of 318-340 rear main bearings, sliding the cap .020 is not going to cause any issue as the oil groove is wide back there for a reason. Some caps are too tight when you install studs and have to be drilled a little.



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Everything ain't so perfect once you start lookin and measuring, it's a wonder more don't leak.
 
I really appreciate everyones feedback.

Earlier I began disassembling to give it another shot. I noticed one of the nipples from the rubber seal that sticks through the pan was angled towards the front as if the seal did not seat correctly. All others were pointing straight down. This is in the rear main cap end. Is it me or do the pictures show the seal pushed out a little?

I am still going to remove the cap to measure to make sure the seal halves are lining up properly. If they are not I will open the holes a little to shift the cap back slightly.

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That’s exactly what I did. Laid under the car for about 5 minutes, but no leak. Not until I rev the rpm’s past 2000.
get someone to throttle the engine up for you while you watch. Lay under there looking for as long as it takes.
 
get someone to throttle the engine up for you while you watch. Lay under there looking for as long as it takes.
Thank you for the suggestion. I did that as well. Which is how I know the leak happens between 1,500 and 2,000 rpm. I had my son press on the pedal until I noticed the leak. I could also feel how hot the headers get very quickly.
 
The cap indexes in the register in the block to align side to side, not the bolt or stud, there is a little play in the assembly so the cap can slide front to rear a little. You do know a big block rear main cap can be slid back enough on stock bolts to cause the seal retainer hit it and sit too far back misaligning the seal right? On those you need to be sure the cap is slid forward enough so the rear cap machined relief lines up. He can measure a small block in car, I agree it should be fine, but if the cap is .020 to far back it could cause a leak. Pic is of 318-340 rear main bearings, sliding the cap .020 is not going to cause any issue as the oil groove is wide back there for a reason. Some caps are too tight when you install studs and have to be drilled a little.
You were spot on. I took my feeler gauges and placed each on the small ledge created by the cap going past the block toward the front of the engine. As you can see from the pictures the .020 gauge matches exactly with the ledge on both sides of the cap. I need to now check if this causes the lip of the lower and upper seals to not line up. My gut feeling is that it will.

If I enlarge the holes a little the entire length, will the area circled in red in the picture be an issue? The bolts have a shoulder and maybe I just need to enlarge to clear that.

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Go talk to an engine machinist and tell him you are going to grind those cap holes larger. Then you will learn the problems involved.

you think the factory machined it wrong and your backyard grinding will fix it? LOL
 
Go talk to an engine machinist and tell him you are going to grind those cap holes larger. Then you will learn the problems involved.

you think the factory machined it wrong and your backyard grinding will fix it? LOL
Thank you for the suggestion on talking to my machine shop. I will let you know the results of this option.
 
I really appreciate everyones feedback.

Earlier I began disassembling to give it another shot. I noticed one of the nipples from the rubber seal that sticks through the pan was angled towards the front as if the seal did not seat correctly. All others were pointing straight down. This is in the rear main cap end. Is it me or do the pictures show the seal pushed out a little?

I am still going to remove the cap to measure to make sure the seal halves are lining up properly. If they are not I will open the holes a little to shift the cap back slightly.

View attachment 1468962

View attachment 1468963
Try tapping the cap back before drilling it, most will slide a bit with stock bolts without drilling
Go talk to an engine machinist and tell him you are going to grind those cap holes larger. Then you will learn the problems involved.

you think the factory machined it wrong and your backyard grinding will fix it? LOL
I hope you are not serious, they drill them all the time if the bolts or studs are fatter, it will not affect the bearing. What we need for both small blocks and big blocks is an aluminum or steel bushing machined to fit that seal groove perfect, then you will see how far off they can be. The last big block I did had to have the seal retainer moved .020 forward on the passenger side to align the seal lips. If you think the machining back there is perfect you are wrong. It is even off side to side on some big blocks, small blocks are better.
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