Sealing rear main on engine assembly

W

wrndln

Well-known member
I am starting to assemble the engine for my 29 phaeton. I want to try to reduce leaking from the rear main. It seems like most model A engines leak a little from the rear main, but I want to try to minimize it. I don't know if RTV is the best product to "seal" the shims and the rear main in general. I have the correct amount of shims to achieve .0015" crank clearance, so I don't want to change it by adding sealant between the shims, block and cap. Any helpful suggestions would be appreciated on how best to reduce leaking.
Rusty Nelson
 

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IMHO, you're never going to get a Model A engine as leak-free as a modern engine. There is no rear main seal. If you have the right rear main clearance, and the thrust clearance is correct, you will have minimal oil seepage past the bearing to the oil slinger, which should not be overwhelmed by the amout of oil, and should return the oil back to the pan with minimal leakage. Chasing oil leaks is like chasing your tail!
 
I use a thin coat of liquid sealer, top and bottom of the shims. When cap is final-torqued, I run a small bead of black silicone around the parting-line of cap and block. Don't know if this last step helps but it makes me feel good about it. Everything must be super clean and oil-free. I also put silicone on the mid-section of the bolts before final assembly.
 
I just can't imagine it would be possible for oil to seep out between the block, shims, and bearing cap with the cap bolts torqued down...especially with an unpressurized system.
 
Make sure the shims touch the crank at the end of the shims. Make sure the return oil tube is not screwed in to far in the cap and blocking the oil return. They also like to leak around the bolts some times. Make sure the cap is flat, They like to bend a little in the middle of the cap at the bolt holes. Also some engine have one of the 2 holes at rear of the cam shaft that are drilled through the back of the engine to the inside. They will leak around the bolts. Put sealer on them.
With all that said I would use a small amount of sealer on the rear cap.
 
The engine rebuilder I consulted when checking my bearing clearances said to be sure to recoat the shims with shellac if I needed to "peel" the shims. I assumed it was best to use a thin coating of shellac on reassembly even though the shims did not need adjustment.
In his book, Les Andrews recommends adding a touch of RTV where the cork around the bearing cap meets the bottom of the pan gasket when installing the oil pan. My rear main seems to be nearly leak-free after reassembly.
 
The fastest way to get a leak in a mod. A is to forget to put the little tube in the rear main cap. It will leak a prodigious amount in no time. Dinner plate size puddles in ten minutes at idle! Don't ask me how I know!
Terry
 
Don't forget if you are using the rear rope seal to put it in oil for at least 12 hours. Don't cut the extra off the ends ether.
 
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