#1 A sealer that will setup without air #2 On the main cap outer edge where it contacts the block surface,they put a "chamfer" so its not a sharp edge,#3 yes put a thin coat of sealer on the block surface where the main caps goes,you do this because the oil can leak through the "chamfer" if you don't. I would really check out you main clearance,It sounds like you have the wrong size main bearings in there, If you have resealed the rear main with both types of seals and still leaking? sure its the rear main seal? Do you know that the crank bolts go through into the oil? did you seal them, maybe you had a bad bolt and the oil is leaking through the bolt threads.

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