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Old 10-06-2019, 12:56 PM   #1
john charlton
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Default Rear main cap question

I have an engine with a hefty rear main leak so much so it is not useable anymore . I have checked the rear main cap and find it has one shim 3.5 thou. on one side only . I got to thinking that would not allow the cap to sit square and maybe part of the problem . The end float is 27 thou but my CCPU has 35 thou.and no rear main leak .The centre and front cap both have shims in which I can see so I will check and see what I have .What think thee learned gentlemen of Ford Barn .

John in very heavy rain local floods Suffolk County England .
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Old 10-06-2019, 03:40 PM   #2
Purdy Swoft
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Default Re: Rear main cap question

I would remove the rear main cap . If the babbit looked OK I would check the clearance . I would tape a sheet of fine sandpaper , maybe 220 grit or whatever works best for you , too a glass pane or some such for a perfectly smooth surface . I then proceed to move the mating surface of the bearing cap back and forth over the sand paper to level the surface . usually you can look at the surface and tell where the sandpaper cut and when it is level . . I then remove the front plug in the main bearing cap for access to the oil passage that runs through the bottom of the cap . Snyders and Brattons both offer the plug . If the passage is obstructed with crud it will prevent the cap from draining out the drain tube . If the drain tube doesn't work properly, the overload will cause the oil to bypass the rear oil slinger and leak out the rear main cap on the ground . I would look at the aluminum , so called seal that fits in the block with a groove for the rear oil slinger to fit . This aluminum seal needs to be level with the block on both sides . If the aluminum seal is pushed to deeply in the block on one side or the other it will cause a gap where the oil can leak or pour . I then proceed to either shim or sand as needed and shim the rear main for a final fit . I've never seen a too tight engine run very long or go very fast . Some prefer a clearance of 1.5 . I feel drag at 1.5 and prefer a clearance of 2.0 thousands gap . Some have said that the rear main bearing is longer than the front and center mains and prefer to use three thousands clearance . . I clean the surface at the sides of the cap where it mates to the block . I snooth a thin coat of silicone sealer with my finger over the edge of the mating surface . the small amount of silicone sealer is used on the outside and can't fall off and contaminate the engine oil .Care must be taken when the rear oil pan seal is installed. The cork allways must be cut to the proper length . In other words , the seal must fit the full length ot the groove in the cap . The rear pan seal should fit tightly to the oil pan gasket with a dab of sealer where the pan seal meets the oil pan gasket . This is about all I can do with the engine in the car I prefer to do this procedure with the engine on my engine stand . With the engine out you could easily install the new main bearing bolt kit . The rear main bearing bolts can be a source of a small leak . I've heard of some wraping the main bearing bolts with string and Indian head shellac for a seal . I never went that far . There would be a few drops at shut down but not a puddle unless the car was parked with the front end way higer than the rear . The last two of my engines that I overhauled I used the main bearing bolt kit and got no leaks at all .

Last edited by Purdy Swoft; 10-06-2019 at 03:50 PM.
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Old 10-06-2019, 03:52 PM   #3
john charlton
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Default Re: Rear main cap question

Thank you very much Purdy all duly noted. The engine is on a stand and I have removed the shim .The crank still turns freely and I will "plastigage" to see the clearance and adjust to suit using what you have advised .I will do the centre and front as well .I have blown through the cap tube and it is clear no resistance .

John in same place same weather .
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Old 10-06-2019, 04:05 PM   #4
Purdy Swoft
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Default Re: Rear main cap question

Sounds good , John . Best of luck !!!
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Old 10-06-2019, 04:27 PM   #5
Kohnke Rebabbitting
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Default Re: Rear main cap question

Quote:
Originally Posted by john charlton View Post
Thank you very much Purdy all duly noted. The engine is on a stand and I have removed the shim .The crank still turns freely and I will "plastigage" to see the clearance and adjust to suit using what you have advised .I will do the centre and front as well .I have blown through the cap tube and it is clear no resistance .

John in same place same weather .
A half plugged cap hole, with babbitt, pipe screwed in to far, ect., will take 175 pounds, air pressure very nicely. Pull the soft plug!

Herm.
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Old 10-07-2019, 09:44 AM   #6
AL in NY
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Default Re: Rear main cap question

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The easiest way to remove the plug on the rear main is to use a very blunt large center punch. Place the punch in the center of the plug and gently hit the punch until you make the plug concave enough so you can remove it. Don't hit the punch so hard that you put a hole in it. You can install the same plug by flipping it over so it is convex in the main cap. Now use a flat punch that just fits in the plug hole and gently hit the flat punch until the plug becomes flat and snug in the hole.
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Old 10-07-2019, 11:14 AM   #7
Jim Brierley
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Default Re: Rear main cap question

As Purdy and Kohnke say! I use a figure 8 pattern to insure flatness. I use a liquid sealer VERY SPARINGLY where the cap mates with the block, and a SMALL bead of silicone around the outside edge of the cap. Clean that area with lacquer thinner after torquing the bolts. You are right about a shim on one side could cause a problem. Good luck!
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