Having battled leaking tachometer cables and blown seals on the 1961 CB77, it became apparent that the engine needed some freshening up, so out it came today. Actually, it was out, apart, back together and installed/running in about four hours.
Having assembled the engine with some somewhat worn rings and pistons, back in 2011, when the bike was acquired, I knew that the rings should be replaced and some had been purchased from an eBay seller recently. The cylinders were bored to first oversize, which is .25mm. The pistons and rings are all stamped as to their size, at least usually. In this case, the “.25” oversize rings came out of the opened box in an unusual mix. The second scraper and wide oil control rings were all stamped .25, as expected, however the two top compression rings were stamped .20(!?!). When the rings were placed in the cylinder bores to check the end gap, the two top ring ends butted together before the ring could be squared up in the bore. Obviously, the “.20” was a reference to a 2nd oversized ring, instead of a first over item. The rings came from Riken and marked RIK, in addition to the size mark. This is one of Honda’s OEM ring manufacturers, however special pistons were made off the original molds to create some S8 and S12 (2mm and 3mm oversize) pistons and rings were made to match some of these “big bore” kits. The odd thing is that these are not “big bore” rings and if they were 2nd over they should have been marked .50 instead. I was able to make them fit by grinding down the ends slightly with a Dremel tool, using a small cutoff wheel. This was a very odd occurrence, but you do learn something new every day, don’t you?
Well, that wasn’t all I learned today! With the cylinders off the engine cases, to allow the ring replacements, I noticed that the center guide roller for the camchain was REALLY, REALLY loose and wobbling around at the slightest touch. That was shocking, as the roller was replaced when the engine was built and only has about two thousand miles on it since the engine overhaul. I removed the guide roller wheel from the pin, which is held in place with a thrust washer and snap ring. With the parts in my hand, it was obvious that there were huge clearances between the pin and roller. The micrometer measured the difference between a good part and the used one at about 2mm (.080”). Looking at the pin, you can see where the guide roller, which has a steel insert was slowly grinding the pin down, changing the shape from a circular pin to a cam shape. Looking at the pin material, it was obviously made of aluminum or some kind of powdered metal that is non-metallic. Digging through the used parts bins, six used rollers on pins were brought to the bench for inspection. Half of them were made of the same soft material and the other half were steel! In checking the part numbers, it was determined that the original 14612-258-000 part number had been superseded to a 14612-250-300 part number. The 258 code refers to a CB71, of which no production machines were ever produced. It was a predecessor to the CB72/CB77 Super Hawk, but had a dry sump engine. Interestingly enough, the 250 code replacement comes from the first 250cc C70 Dream engine, which preceded the CB71. Sometimes, you have to revert back to the original designs when the modified parts fail, I guess.
So, today’s words to the wise are: (1) Check your piston ring markings carefully, especially of the box’s seal has been broken and the contents disturbed. (2) Always use the steel guide roller pins when rebuilding these 250-305cc engines.
Because of the engine pressurization issues inherent in the “rear breather” engine case designs, I decided to change the top cylinder head cover to one with a breather tube fitting. This required the addition of the thin alloy breather plate and the matching gasket. My concerns about the engine stack height when adding more “height” to the top of the engine were unfounded, as the measurements of the 1961 cover vs. the 1964 cover revealed a difference of about 2mm at the gasket surfaces, measured to the large front top engine mount bolt hole. The later cover height had been reduced sufficiently to allow the added plate and gasket to the cylinder head and not affect the engine’s mounting hole spacings.
The bike fired back up easily, as before. I allowed it to warm up for a few minutes before taking a test drive. The engine is somewhat quieter now, due to the firm location of the camchain guide roller. The new rings will take some time to seat into the cylinder bores, but there was no sign of blowby or smoking after a quick fifteen-minute run up the road. Everything seemed fine until I returned home and shut the bike down… the tachometer cable is still leaking oil! $#%$%!
Tomorrow the tach drive comes off the head for some close inspection. Honda changed the design several times (numerous part number suffixes, 040, 050, 060), but all I was aware of was some re-angling of the thread section to clear the cylinder head nuts more easily. Checking part numbers the change in the bushing was at number: CB77E-110200. This bike engine is CB77E-110177 so apparently needs an updated part!
Stay tuned for more “what I learned today” news!
Bill “MrHonda” Silver