I have a packet of photographs of an early Honda RC-161 250 road racer, which is under restoration in the US. This is one of the hump-tanked models with the steeply downdraft carbs, which were withdrawn at half-season in 1960, I believe, after some bad problems with overheating. Yet this design is more remarkable for its similarity with the later RC165-6 six-cylinder 250 than for its differences.
Like the six, the four has its two cams each made in two pieces, but the pieces join by tongue-and-groove couplings machined in the cam ends, not with a separate piece held between cam halves as in the six. The cam drive comes up from the crank center but serves, as in the six, the dual purpose of primary and cam drive. A large gear on the crank drives an even bigger jackshaft gear above and behind it, from which a shaft extends to the right to drive the clutch pinion. More straight-cut gears carry power up from this jackshaft to drive the cams in the head.
As in the six, piston-type tappets are carried, in pairs, in cast bronze tappet guide blocks, each secured to the aluminum head by a pair of screws. Four valves are used in each combustion chamber, intake and exhaust set at the wide angle that permits cooling air to have access to the between-the-cams space.
The ignition generator, likewise driven from the jackshaft, is carried piggy-back above the conventional gearbox (six speeds in this four, with a wide blank space apparently available if a seventh should be required. The six had seven speeds).
The engine is built in three layers; an upper case/cylinder casting, which was Honda's contribution to leak-free rigidity (because separate cylinder-to-case joints are always a problem on high-revving engines), then a lower case that clamps crank and gearbox shafts to the upper, and finally a sump casting on the bottom.
Now the contrasts. The four-cylinder engine has full-circle flywheels, with weight removed for balance purposes by milling slots in the wheels' OD. The six lacks full-circle wheels, having throws that are just rectangular bars, only wide enough to accept the press-fitting of mainshaft or crankpin (integral pin construction in the six, at least). This, clearly, is done to reduce rotating weight.
The four has conventional, large-diameter rolling element main bearings that are clamped between the case halves. The six has the smallest possible needle main bearings. The OD of these small mains is small enough that it would probably fret against the case aluminum, so instead of the crank being clamped between case halves in the six, it is bolted to the upper half by means of "ears" on each of the main bearing outer races. These look like tiny pillow blocks, bolted to the underside of the upper case.
Conscious of the possibility of losing to friction what was to be gained by the six's higher revs, Honda engineers tried to reduce the size of the mains in this way.
Pistons in the four cylinder have long full skirts that make them look like pre-war car pistons. This may simply have been the dead hand of the past, or it may have been a reaction to the lube problems of this engine. Slipper skirt pistons increase the specific loading on cylinder wall oil film, so the very high operating temp of these fours may have required that the load be carried on a larger "sled" so it wouldn't sink through the "ice" (oil film). In the six, the pistons are shorter-skirted, but still full rather than slipper-skirt design. The first sixes (the so-called long-stroke models that were 39 X 34.5 mm) lacked the oil coolers that would finally make Honda's "air" cooled four-strokes completely viable no matter how hot the day, and so may hae needed some of the medicine fed to the previous fours. Or this may just be drivel that I am making up so I'll sound learned to myself. Who knows? I have a copy of Jacques Bussillet's new book on the six, but I haven't finished reading it. Maybe it will tell more.
There is also a contrast in the size of the drive gear located at crank center on these two designs. That of the four is of large diameter, perhaps to preserve crank torsional stiffness, or perhaps to serve as a press-fit coupler for the two halves at the center. The six has a much smaller gear, reducing the speed of tooth engagement and, perhaps, friction.
The four's pistons each carry three rings - two gas rings and a single one-piece, perforated oil scraper, with drain-back holes beneath it. The wristpin is set very high in these pistons and, if I were considering it in light of modern practice, I'd say they did it to permit use of the longest possible con-rod. The actual rod ratio is unknown for this engine, but is over 2.0 for the six. Pistons in the six use a single gas ring and a single plain (not perforated) scraper. Here in my collection of delicious and relevant junk is a piston from a Wright R3350 radial engine, and it, too, carries unperforated scraper rings (three of them, of very light tension). I'd speculate that perforated scrapers get gummed up or scuff in hot-running air-cooled engines like Honda's 1960s racers and aircraft radials.
The four's oil pump and oil intake screen hang below the gearbox, extending down into the slab-like sump, while the six's pump is driven from a skew shaft off the jackshaft, and runs diagonally downward and forward to pick up at the front of the sump.