Thanks for that Ron, and on the sandcast subject, I remember a BMW mechanic telling me about porous Moto Guzzi sandcast castings many years ago, have you heard of this?
He reckoned that the hot oil could actually seep through the casting? I wonder if that is a common problem with all sandcast castings, or was it just unique to moto Guzzi? Cheers, Terry. 
That is true. It shouldn't happen if the casting is made properly, but in a mass production scenario, its difficult to keep the quality control high enough on sand castings.
The CB77 had this problem with a certain run of their cylinder heads. Mysterious oil leaks. I can't remember what the fix was. Either they tightened up the quality control on the sand castings, or they switched to die cast, which never (never say never) have this problem.
PS: I just looked it up. It was a combination of porous castings allowing oil to reach a seam between the aluminum casting and the cast iron "skull" of the head including the spark plug hole. Reading this Bulletin from 1966 re: the CB72/77 models, is enlightening to how the factory thought (and thinks)
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:
1. 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. When this "sinkage" occurs, the
Honda 250-305 Super Hawk Restoration & Maintenance Guide
combustion chamber skull is not forced into intimate contact with the cylinder head casting and
severe oil or vapor leakage, near the sparkplugs can occur. The recommended repair is, of
course, to locate and replace the faulty part.
Source: SL #68 4/29/66 HONDA MOTORCYCLE SERVICE BULLETINS
The internet is freaking amazing!
