Big Block rear main seal question - Chevelle Tech
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post #1 of 14 (permalink) Old Aug 11th, 19, 8:49 AM Thread Starter
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Big Block rear main seal question

Good morning
I've been chasing various oil leaks on my 496 since day one. The last one appears to be the rear main. I put the dye in the oil and traced it to the rear main, oil pan gasket or oil leaking in between the main caps (no sealer on the edges) it looks like the rear main. I removed the flex plate and full of oil I put the engine on a pretty good angle so the oil level is higher than the seals. I left it for a while and you can clearly see oil leaking past the seal.

So with that said, it's time to fix this once and for all. The engine is a Mark IV 445 block. Two bolt block with four bolt caps installed. Block was aligned honed for the caps and studs. Does this mean I need a rear main seal that is made for an line honed/bored block? I don't really want to remove the crank but can if I have to. The crank is/was new. I've seen a special off set rear main seal on a 400 small block but not the big block. I've also read that some have taken a thin strip of metal under the new seal to take up the oversize ?

I'm looking for tips and advice as it's not going back in until it's fixed.

Thanks Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #2 of 14 (permalink) Old Aug 11th, 19, 7:15 PM
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Re: Big Block rear main seal question

Quote:
Originally Posted by NW68SSBBC View Post
I've seen a special off set rear main seal on a 400 small block

That is because of the way the 400 rear main cap is made. It actually uses the same seal as all of the other small blocks but has an extra thick lip that holds the seal. When you align hone the 400 you have to remove some of that lip and then it needs the special seal. There is not an issue like that on Big Blocks


Which rear main seal are you using ? I cant say I have ever had a problem with a big block leaking. I use the standard blue Fel Pro seal. I dont offset them and I dont put any silicon on them. If I can find them I can show you some pictures of silicon on a rear main seal actually causing leaks. Put them in dry the way the factory does. Maybe a real light coat on the cap mating surface but I prefer Aviation Form A Gasket there.
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post #3 of 14 (permalink) Old Aug 11th, 19, 9:38 PM Thread Starter
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Re: Big Block rear main seal question

Quote:
Originally Posted by BillK View Post
That is because of the way the 400 rear main cap is made. It actually uses the same seal as all of the other small blocks but has an extra thick lip that holds the seal. When you align hone the 400 you have to remove some of that lip and then it needs the special seal. There is not an issue like that on Big Blocks


Which rear main seal are you using ? I cant say I have ever had a problem with a big block leaking. I use the standard blue Fel Pro seal. I dont offset them and I dont put any silicon on them. If I can find them I can show you some pictures of silicon on a rear main seal actually causing leaks. Put them in dry the way the factory does. Maybe a real light coat on the cap mating surface but I prefer Aviation Form A Gasket there.

Thanks for the information
I'd be glad to see the pictures! It may very well be that cap mating surface is the leak culprit. Its hard to see. But with the dye in the oil there was definitely oil on the bottom of the main seal but it could have been the mating surfaces dripping down to the seal? It also looked like there may have been some weeping through the last two oil pan bolts.
How do you feel about loctite gel seal 518.? That's what I used for years on outboard cases.

Thanks again
Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #4 of 14 (permalink) Old Aug 12th, 19, 9:11 AM
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Re: Big Block rear main seal question

Joe,
I guess an anaerobic sealer would be ok on the cap also. You just have to be VERY careful not to use too much because you can affect bearing clearances. Personally I cannot imagine oil leaking through those mating surfaces. GM never used a sealer there and to be honest with you there is rarely a problem with them leaking.


Here is a picture of a seal that silicon got onto and caused a leak. It was actually someone here on Team Chevelle.
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Bill Koustenis
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post #5 of 14 (permalink) Old Aug 12th, 19, 5:51 PM Thread Starter
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Re: Big Block rear main seal question

Quote:
Originally Posted by BillK View Post
Joe,
I guess an anaerobic sealer would be ok on the cap also. You just have to be VERY careful not to use too much because you can affect bearing clearances. Personally I cannot imagine oil leaking through those mating surfaces. GM never used a sealer there and to be honest with you there is rarely a problem with them leaking.


Here is a pwicture of a seal that silicon got onto and caused a leak. It was actually someone here on Team Chevelle.
Yup. I can definitely see where that silicone would cause a leak. I learned to be very sparing when using the gel seal. But it wis made to not change clearances.

I haven't pulled the pan yet. Maybe tomorrow but I'm just wondering what the cause of the leak is. I rebuilt this engine once before and it never had a leak. Four piece oil pan gasket too. ! This go around with the stroker, I wanted a "pro" build! I got quite less than that in my opinion but enough on that, it's going to be right when I'm done.

I'll update with some pictures when I pull the pan
Thanks
Joe

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #6 of 14 (permalink) Old Aug 13th, 19, 9:46 PM Thread Starter
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Re: Big Block rear main seal question

I pulled the pan and it looks pretty obvious the pan gasket was leaking as well as the cap mating surfaces

Do I have to pull the crank to replace the seal ?

There was no sealer on the cap surfaces

What do you guys think

I'll send more pictures, they load slow from the tablet
Thanks Joe
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"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #7 of 14 (permalink) Old Aug 13th, 19, 9:56 PM Thread Starter
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Re: Big Block rear main seal question

More pictures
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"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #8 of 14 (permalink) Old Aug 13th, 19, 10:10 PM Thread Starter
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Re: Big Block rear main seal question

Some more
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"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #9 of 14 (permalink) Old Aug 14th, 19, 2:53 PM
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Re: Big Block rear main seal question

Subscribed.

I too and fighting a rear main in a big block. This fall the engine comes out for more of an in depth inspection, but rear main is on the list to fix while it's out.

Never stop learning!
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post #10 of 14 (permalink) Old Aug 14th, 19, 7:09 PM
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Re: Big Block rear main seal question

I used a one piece oil pan gasket on my SBC in the Wifes Nova. I have a stock oil pan and stock timing cover. The one piece pan gasket has spacers inside the gasket preventing the front of the pan to seal on the timing cover. Do you think that this may be an issue for you on your pan leaks, but on the rear of the motor? I believe those one piece pan gaskets take a special pan in order to seal correctly. All I know is, when I have to pull the wifes motor for what ever reason, the pan gasket is getting replaced. Not a fan of those gaskets.

Chris
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post #11 of 14 (permalink) Old Aug 15th, 19, 6:36 PM Thread Starter
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Re: Big Block rear main seal question

Quote:
Originally Posted by Jayhawk500 View Post
I used a one piece oil pan gasket on my SBC in the Wifes Nova. I have a stock oil pan and stock timing cover. The one piece pan gasket has spacers inside the gasket preventing the front of the pan to seal on the timing cover. Do you think that this may be an issue for you on your pan leaks, but on the rear of the motor? I believe those one piece pan gaskets take a special pan in order to seal correctly. All I know is, when I have to pull the wifes motor for what ever reason, the pan gasket is getting replaced. Not a fan of those gaskets.

Thanksfor the reply
I don't think mine was leaking in that area. Looked to be the radius around the rear main cap and pan. That and there was no sealer on the cap mating surfaces. I don't think the actual seal around the crank was leaking. I don't know but maybe some Hi tack would help there.

I wonder if anyone makes a one piece seal for the Mark IV engine? It sounds funny at first because everyone knows this engine uses the two piece rear main seal. But if you look at them and where they fit it seems like you could slide it over the crank first, then carefully lower the crank in aligning the seal. Add a lite bit to the cap mating surfaces and drop the rear main cap on ! Done. I have to take the crank out anyway because th e assembler put silicone on the seal groove so I can't loosen the caps and spin the seal out/in.

That leads me to my next question. The rods I have are Callies Compstar H beams. Callies says I have to use their extreme pressure lube #3 on the bolts and threads to get an accurate torque reading. Anything else and you need a stretch gauge which I don't have. Is that the case ? What about the ARP extreme torque paste. Still use that under the bolt head ?
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"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #12 of 14 (permalink) Old Aug 15th, 19, 8:49 PM Thread Starter
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Re: Big Block rear main seal question

Scratch that one piece seal idea. Pretty stupid considering the flywheel/flex plate flange on the end of the crank. Whishfull thinking!

Sorry for the stupidity!

"... the very powerful and the very stupid have one thing in common. They don't alter their view to fit the facts, they alter the facts to fit their view." Dr. Who, 1977
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post #13 of 14 (permalink) Old Aug 15th, 19, 9:23 PM
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Re: Big Block rear main seal question

Joe,
I initially installed the rear main seal during a rebuild on the original 396 for my 69. That seal leaked so several years later I pulled the engine and among other things I replaced the rear main seal. From what I recall I just had to remove the oil pump and the rear cap. I was able to remove the old and slip in the new.
Couple things I do remember is several people recommended a " vitron" seal which I think was a Fel Pro brand . Also was told to put some assembly lube on the face of the seal after installing it to protect it during initial start up until it got some oil on it. Sadly mine still leaked even after the new seal so I suspect the real culprit was the block or the crank.
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post #14 of 14 (permalink) Old Aug 16th, 19, 10:20 PM Thread Starter
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Re: Big Block rear main seal question

Got the seal out of the block side. It was silicone'd in like the cap side. I managed to scrape the silicone off bu I nera found the broken part of the seal. Not sure if that would cause a leak but it will be fixed. I have the Fel-Pro Viton seal.

I'll have it running by next week and hopefully leak free !
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