1968 GMC 250 In-line 6 Rear Mail Seal

[68gmcdude]

Hi folks...
I'm starting a new thread as my previous thread was under a different title but has resulted in my current issue.

I replaced my rear main seal with a Fel Pro seal (part # BS40013). After replacing it, the rear main is leaking just as bad as before. I've recently discovered that there is another rear seal made by Fel Pro that has the lip offset so that it seals on a different area of the crankshaft. It is part # BS11829-01.Has anyone ever used this rear seal and if so have you had success?

Attached is a picture of my crank (as best as I could get when i had it apart).

Here is the link to my original post that has some rear main seal conversation in it. https://67-72chevytrucks.com/vboard/...d.php?t=837356

Thanks for your help and advice.

[RustyPile]

Did you polish the area of the crankshaft where the seal runs?? Did you clean the recess where the seal fits. Did you put a small amount of sealant on the area of the block between the gasket and seal?? This prevents oil from seeping between the block and main cap.. And lastly --- are you sure it's the seal that's leaking???

[68gmcdude]

Originally Posted by RustyPile

Did you polish the area of the crankshaft where the seal runs?? Did you clean the recess where the seal fits. Did you put a small amount of sealant on the area of the block between the gasket and seal?? This prevents oil from seeping between the block and main cap.. And lastly --- are you sure it's the seal that's leaking???

I cleaned the area of the crankshaft where the seal runs but I did not polish it. How would I go about polishing it?

I also cleaned the recess where the upper half of the seal inserts by feeding a small flexible brush and brushing back and forth.

The seals were put in with the end(s) offset 3/8" so they did not line up with the ends of the cap and block. The end of the seal halves had RTV sealant added. I also put it on the surface of the cap.

I'm about 100% sure it is the rear main seal that is leaking. The valve cover gasket and the side pushrod covers also have had the gaskets replaced. All is completely dry around those areas. When I replaced the rear seal, I obviously had to replace the oil pan gasket. All is dry around that as well. The only place there is oil is on the inside of the flywheel cover, which is then dripping on the ground from the hole in the bottom of the cover.

['68OrangeSunshine]

You don't want oil contaminating your clutch friction disc.

[68gmcdude]

Originally Posted by '68OrangeSunshine

You don't want oil contaminating your clutch friction disc.

The clutch disc and everything else up in there looks dry.

[RustyPile]

Originally Posted by 68gmcdude

I cleaned the area of the crankshaft where the seal runs but I did not polish it. How would I go about polishing it?

I also cleaned the recess where the upper half of the seal inserts by feeding a small flexible brush and brushing back and forth.

The seals were put in with the end(s) offset 3/8" so they did not line up with the ends of the cap and block. The end of the seal halves had RTV sealant added. I also put it on the surface of the cap.

I'm about 100% sure it is the rear main seal that is leaking. The valve cover gasket and the side pushrod covers also have had the gaskets replaced. All is completely dry around those areas. When I replaced the rear seal, I obviously had to replace the oil pan gasket. All is dry around that as well. The only place there is oil is on the inside of the flywheel cover, which is then dripping on the ground from the hole in the bottom of the cover.

Use a product called Crocus Cloth, about 220 - 320 grit.. Here's a link to help you find it. https://www.bing.com/search?q=crocus...ANAB01&PC=LCTS

Tear off a strip a foot or so long and narrow enough (maybe 1/4") to fit around the crankshaft.. Pass one end of the strip through seal groove and pull the ends tight and "row" the ends back and forth until the shaft area is shiny.. After polishing an area, you'll need to rotate the crankshaft a few degrees each time until the entire circumference is shiny. Be sure you clean off all the grit and debris after polishing.. Leave NOTHING behind..

No sealant is necessary on the seal, especially not on the ends of the seal.. Also, there is a front and back to the seal. Instructions with the seal should explain how to identify. Beginning with the introduction of PCV systems, the seals were manufactured with a double lip.

Too much sealant on the cap will keep it from fully seating against the block.. Remember there is only .002" clearance between the crank journal and main bearing.. You only need a very small thin layer in the area I mentioned in my post.. The volume of the sealant for this area is about the size of a BB - very small amount.. It will spread out when you tighten the main bearing cap.

I was an automobile mechanic for over 30 years. I also built race engines for 12 of those years.. In that time I probably replaced and/or installed hundreds of rear main seals.. I never "staggered" the ends of the seals in any engines. It's almost impossible to set the "stagger" equal amounts on each half.. Also, I can't remember having to redo one because it leaked using the method I just described..

[68gmcdude]

RustyPile...

Thanks for the tips. Crocus cloth is ordered. I'll probably be putting in the new seal this week sometime.

As far as the sealant, am I understanding that I only need on BB size dab on each end of the block/cap? I've attached a picture of the cap surface with the yellow dots being my understanding of where I need to put the sealant.

Thanks!

[RustyPile]

It's better served applying the sealant to the block, but yes, that's the area.. Be sure you clean all surfaces before installing the seal. This includes removing all old gasket, sludge, and any accumulated debris... Spread the RTV out forming a straight line between the seal and the "foot" of the rubber end piece. Install the cap before the RTV starts to cure.. It needs to spread out when the cap is tightened into place....

Check your PM inbox, I sent you a message...

[72SB]

OP

hard to see in your pic if there is a wear line from original seal which is why the offset type seal is made so it ideally rides on a fresh surface.

If it were me, I would use the offset version even after polishing as Rusty notes....especially given you tried the standard version

I believe the stagger the RMS thing was more done on the old rope types on those replacements. Not sure if it was more effective as rope types were...well rather wet seals regardless

My 250 also has a RMS leak. just a drop or two after long drives but so far I have held the position...is the juice going to be worth the squeeze....more based on your current outcome and same opinion of those who do it for a living under it's current minimal leak condition. I just use a pan under truck which shows a very small drip smaller than a dime.

Oil takes the path of least resistance to drip out which is why it appears it is staying on the engine side of FW......so far mine appears to not have affected the clutch also

[68gmcdude]

Originally Posted by 72SB

OP

hard to see in your pic if there is a wear line from original seal which is why the offset type seal is made so it ideally rides on a fresh surface.

If it were me, I would use the offset version even after polishing as Rusty notes....especially given you tried the standard version

I believe the stagger the RMS thing was more done on the old rope types on those replacements. Not sure if it was more effective as rope types were...well rather wet seals regardless

My 250 also has a RMS leak. just a drop or two after long drives but so far I have held the position...is the juice going to be worth the squeeze....more based on your current outcome and same opinion of those who do it for a living under it's current minimal leak condition. I just use a pan under truck which shows a very small drip smaller than a dime.

Oil takes the path of least resistance to drip out which is why it appears it is staying on the engine side of FW......so far mine appears to not have affected the clutch also

I'm definitely going with the offset version. If it was a drop here or there I could live with it. But last time I took it out it was dripping a drop in my driveway about every two feet. Then it really dripped a lot immediately after parking it.

[68gmcdude]

Got my offset rear main seal. The one on the left is original style. The one on the right has the offset lip. Going to install this weekend.

[68gmcdude]

Installed the new offset rear seal today. Cleaned and polished the crankshaft with crocus cloth first. Cleaned up real nice. Letting everything sit overnight before adding oil and starting up. Fingers crossed.