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This is a well documented problem with all of these engines, C, CA, CB and CL, not all engines suffer from the problem but quite a few of them do. American Honda even issues a Service Bulletin about the problem so it was accepted as an issue by them.
I suggest you look back through old postings for more information and hopefully a cure. Was the problem apparent before you rebuilt the engine?
SL #65 6/1/66 HONDA MOTORCYCLE SERVICE BULLETINS OIL SEEPAGE NEAR SPARK PLUGS IN ENGINES WITH ALUMINUM CYLINDER HEADS
American Honda receives occasional complaints of the tendency for certain models with aluminum cylinder heads to seep a small quantity of oil near the spark plugs. In some cases, dealers have replaced cylinder heads in an effort to eliminate this seepage. This bulletin is intended to clarify our policy in relation to this problem, and to suggest some countermeasures that we have found to be effective. In cases where it can be confirmed that the spark plug sealing washers are not leaking, accumulation of a stain, or oil residue, near the spark plugs can be traced to oil seepage from the joint between the combustion chamber "skull" and the aluminum cylinder head casting. Since the spark plugs are threaded into the iron skull, rather than into the soft aluminum casting, this joint must "come to the surface" near the spark plugs. Although it is no mystery how oil reaches the outside of the engine, it is difficult to determine the source of the oil. Apparently, oil reaches the joint from the oil-bearing chambers through internal porosity in the aluminum casting; such porosity is extremely difficult to avoid. Once oil enters the joint, it has an almost unimpeded leak path to the outside because the skull is not bonded to the head casting. Our studies have shown that machines experiencing this problem can be graded into three broad categories, based on the severity; each category should be dealt with in a different manner: /. A stain or oily residue collects near the spark plugs over a period of several days or weeks. Seepage of this magnitude should be considered a normal, inescapable consequence of the cylinder head design, and no repair should be attempted. Customers complaining of such seepage should be assured that no defect exists; suggest more frequent cleaning of the engine.
2. More severe seepage causes definite accumulation of liquid near the plugs; following a hard
run, oil droplets or streaks can be found on the air cleaner covers, etc. Although a "defect" is not
necessarily indicated by this seepage, countermeasures are often necessary to satisfy customers.
We have found that seepage can be slowed or stopped in the following manner: a) Remove spark
plugs and completely clean the region around the plug holes so that the joint between the skull
and the casting can be seen, b) Using a dull punch, punch a ring of depressions, tangent to each
other, in the aluminum immediately outside of the joint.
3. Liquid oil "bubbles" from the skull/head casting joint, puffs of vapor can be seen when the
engine is suddenly accelerated. In these cases, a definite defect is indicated, i.e., actual
separation of the skull and head casting. In most cases, however, the cylinder, rather than the
cylinder head , is the faulty part. We have found this problem to be most commonly related to
"sinkage" of the cylinder sleeve in the cylinder casting, such that the upper surface of the cylinder
sleeve is below the upper surface of the cylinder casting.