VCCA Home Forums TECHNICAL - 1945 AND NEWER 1946-1948 1948 rear main seal

I am currently trying to install a rear main seal in my 48 Chevy Sport Coupe without removing the engine and I`m not having much luck. It is the rope type and I have the KD tool. Of course the lowed half of the rear bearing is not a problem. I am aware that the manual says to remove engine for upper half but it sure seems to me that years ago we accomplished that without removing the engine. I have also heard that a 55 rubber seal can be substituted if you back it up with a .045 zip tie.

hoppy

Hi Hoppy,

We have had this discussion (replacing the rear seal) many times, the most recent in the 1937-42 Forum. It is the 7th post under the topic 1940-Oil Leak. The most sensible response is Gene's which is to leave the top seal in. In his method you compress both ends of the top seal in by taping on them lightly with a punch, then you fill the void with pieces you cut from the old bottom seal. You then take a new seal and install it in the bottom bearing cap groove.

The most important part of this whole process is to not disfigure any of the metal surfaces, which could cause potential leaks. I have had good results with fashioning the cut off end of a tooth brush to the shape I need and tapping on the other end of it to remove dents in stainless. I think you could make a good punch out of a tooth brush, or even a piece of wood, which would protect the metal surfaces.

With the engine installed it is possible to loosen the crankshaft bearing caps enough to give you more working room, say about 1/4 to 1/2 inches, but it is also possible to damage the largest timing gear, which is made of some kind of compressed plastic, in this process.

I have always pulled my engine to replace the seal. Others have tried doing this with the engine installed. It will be interesting to hear from them, and also hear if they solved their leak using this method?

You can still get original gaskets which have a better record of not leaking. They are for sale occasionally on ebay. I would not use the look a like seals from Best Gaskets or from FelPro or any rubber seals.

This is one project you do not want to hurry through without getting a lot of good advice. Checking through old Forum posts is also a must.

Good luck, Mike

rear main rope seals

Some thoughts on fixing a leaking rear main.
First the bearing should be adjusted as tight as possible (by removing shims). When engine is under power the crankshaft is being forced down so 90% of the oil is passing through the top half of the bearing.
Pulling a new top half of the seal thru with a device stretches the seal so a repack job is necessary.
Packing the top half of the old seal as described can help dependng on the condition of the old rope. (note this method is given in the Oldsmobile shop manuals as this is as late as 1990 because the Olds 307 engine still used a rope seal)
Using a rubber seal (which is for the 1955 and up 235 engine only due to them having the deeper seal graove) will work using the wire tie behind (underneath) the seal. Also requires taking some material off the ends of the seal because not going in as deep makes it too long.
Using Doug Bell's suggestion in his book "Cast Iron Wonder" (he was VCCA #6) ...CUT A SHALLOW GROVE IN THE FRONT HALF OF THE BEARING SURFACE FROM THE OIL GROVE FORWARD TO REDUCE THE OIL PRESSURE BUILD-UP IN THE BEARING. This is done by placing two hack saw blades together and going about 015" deep. I did this on one engine and it worked and after many hard miles caused no problem. Made the grove off to one side and not on the very bottom where the most pressure is.

I just did an Advanced Search on this topic. I found the best results using the Search Terms ( +oil +leak +rear +seal ). There is a space between each search term. I searched only the 1937-42 Forum, and used the Date Range from newer than 4 years and older than 1 week.

Of course you could change the search Forum from 1937-42 to 1946-48.

I appreciate Gene's comments on putting a groove in the bearing. I thought he had mentioned this several years ago. I will try to see if I have an old bearing around that I could take a picture of what he is suggesting. Saturday, I took a bunch of parts to the recycler and as luck may have it some were old bearings!!!

Mike, If you have an old engine front mounting plate could you take a picture of the rear of it (side that faces engine) .I would like a picture of the oil grove that sends oil down to the timing gear nozzle.

Hoppy,

Mike and Gene have given you good sound advice. They are both much more knowledgeable on treating leaks than I. Listen to them and not to me. However,...

I doubt any fix short of Wuwzat's sucking on the oil pan, and Gene's deep hacksaw blade groove method will last very long. Merely replacing the rope with the best new one available will last a day or 10 miles, whichever comes first.

The best advice I can give is to button the engine back up the best you can, get a wide flat pan to go under the engine when it's parked and don't ever park on a nice concrete driveway. All you have to do then is replenish the oil in the engine that drips out. That will be about a half pint a year.

The quest for total oil sealing the engine is a long, arduous one and seldom achieved without impractical devotion to time and effort. Just my opinion.

Best,
Charlie

Gene, I sent you the pictures you asked for.

Hoppy, Here is a picture of the rear crankshaft cap. Maybe, someone can photoshop it to show the exact position of the groove Gene is recommending in the bearing. I'm assuming by "front groove" Gene means the right side of the cap in the picture not the rope side!!!

Thanks, Mike

A picture of the cap with the bearing insert in place would be better.
First there is a grove that goes around the (almost) center of the insert. The sawed grove would go from the the large grove towards the front of the engine. Some oil would be returned to the sump.
The large grove intersects with the oil passage that pumps the oil to the bearing. There is a second smaller opening in the grove that intesects with the oil passage to the rear cam bearing...then if it is an engine with hydraulc lifters such as the 1950 -1952 Power Glide another passage leads from the cam bearing to the hyd. lifter passage that runs from lifter bore to lifter bore to keep the lifters surounned in oil.
Note also on the original main bearing insert (as well as correct replacements) the ends wheer the inserts but against each other are chamfered. This allows oil to spread over almost the entire bearing sureace. he chamfer does not run all the was to the edge so as to allow oil to build up.

Hi Gene,

As soon as I remove a cap with a bearing I will post it.

Thanks, Mike

My Dad's trick for rear main rope seals was to push the ends of the upper seal up past the block creating the pocket that Gene has mentioned. Next he would take the lower rope seal and create points on each end by rolling it between his fingers. The points would stick up past the main cap on each side by approx. 1/4 - 5/16 of an inch. He would then add a coating of RTV sealer to each end and then carefully insert the main cap.

I personally have not had a leak after performing this repair.

Ijust recently used it on my Brother in-Laws' '59 Corvette that he was restoring.

Chev Nut--This is the latest report on my 48 rear main seal. I went back to an old "Ask the Expert" article on pg. 20, Oct. 2010 and followed those instructions exactly and glad to say that it has seemed to have solved the problem.

Thank You Gene
hoppy

What method did I have in the article?

Hi Gene,

Great advice.

Thanks, Mike

Hi Gene,

I finally took the time to update the pictures of a rear bearing so you could identify where you wanted the groove cut. The first picture shows the bearing, cap, rear rope seal, and the .005 shims. The second picture shows where you recommend cutting in a groove in the bearing surface (between the two black marks). The last picture shows the depth that you want the groove to be. Following your directions that groove would be cut the width of 2 hacksaw blades at a depth of .015. The groove can be cut either side of the round indentation?

This should be a lot of help for us all in the future. Gene please add any comments to further clarify this repair.

Thanks, Mike




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Yes, yes, and yes.
Your location is about right as it is just past the area that carries the most of the load. (insert/babbitt is less polished)