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Observations upon the Honda 250-305cc Engine group Part 2.

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On the subject of cylinder head components, there are two styles of camshaft followers/rocker arms. Dimensionally they are the same, however the oil feed to the rocker arm pad is different. Early models shot oil in at an angle, aimed at the interface of the camshaft lobe and the rocker arm pad. This proved to be insufficient in high-stress situations, so Honda created a oil feed path that put oil right in the center of the rocker arm pad, directly lubricating the rocker arm and cam lobe at the same time. The center feed hole rockers are preferred for obvious reasons. Remember, the rocker pins have a different OD on the tips between the intake and exhaust sides.

While there are two different camchain tensioner widths, to fit the various cylinder blocks (sand-cast vs. die-cast), the camchain guide rollers are all the same. All engines use a 94-link 219 pitch camchain with a separate master link. Sources for master-link chains continue to dwindle, however. Most of the later Honda engines used an endless camchain (CB350 and CB750 SOHC engines use the same 94 link chain) which are widely available, but make installation on the 250-305s much more difficult. You CAN assemble an engine with the endless chain, but it does create some extra work and added challenges in the process.

The 1960-61 cylinders and engine cases, as mentioned previously, did not have the “extra” oil feed hole seen on all later production parts. The early cases/cylinder blocks require a specific cylinder base gasket in order to seal up the mating parts successfully. After that, all the engine cases and cylinder blocks can be interchanged. The top cases of the 250s have a smaller cylinder block sleeve hole set, so swapping a set of 305 cylinders often requires re-boring the top cases to increase the diameter sufficiently. The cylinder blocks were made of die-cast alloy in 1966-onwards, using a narrowed camchain tensioner design. Center to center bolt holes are either 32mm or 41mm, so caution must be made when ordering either a tensioner or a cylinder block to ensure that they are a matched pairing. The cylinder sleeves can be removed with some low oven heat and light pressure in a press or even with a plastic mallet.

Pistons and ring interchanges can be puzzling for a novice. Honda made oversized pistons/rings in five sizes: STD, .25mm, .50mm, .75mm and 1.00mm. Early 1960-61Honda Dream pistons used a thicker ring set than the later editions. Later Dream pistons used the CB72 or CB77 ring sets, which were thinner in cross-section (1.8 vs. 1.5mm). ALWAYS check the ring widths before ordering ring sets for Dreams. CB/CL engines all shared the same ring sets, but there were different ring configurations, too. Some rings have just a shiny chromed top compression ring and dull-finished scraper and oil rings. Honda switched to an all-chromed ring set, often stamped ACR on the box and the part number differs at the end of the first five numbers.

Honda Dream pistons are machined differently at the piston crowns than the CB types, which appear to lower the compression ratio. However, some Dream pistons have been installed in CB/CL engines and when the combustion chambers were cc’d with fluid, the compression ratio was actually higher than with stock CB pistons. Basically, you can install Dream pistons in CB/CL engines and vice-versa with no ill-effects, these days.

With decreasing numbers of available OEM pistons/rings, sometimes an aftermarket brand of piston must be substituted. Note that pre-65 CB/CL engines had 9.5:1 compression ratios, which were reduced to 8.5:1 in 1965. You wouldn’t want to mix up a pair of low/high compression pistons in the same engine, naturally. The early high-compression pistons certainly give more horsepower and torque, but often at the risk of piston seizures, when the fuel mixture or ignition timing settings are not spot-on.

The CB72-77 Super Hawks had different versions of the tachometer cable drive, but the whole assembly can be shifted from one cylinder head to the next.

In the early days, Honda sourced their AC generator assemblies from Nippon-Denso and Kokusan. Most of the later model bikes from around 1965 will probably be found with the N-D (L) stamped rotors which denoted the “low output” types, which generate about a ONE-amp net output. Each manufacturer had some slightly different dimensions on the starter clutch hub and adjacent stopper. While the entire generator assemblies will interchange between all three models, the harness leads are longer for the CB models than the CA-Dreams.

Nippon-Denso and Kokusan also took turns supplying the ignition system contact sets and condensers. The N-D points have small adjustment screws to assist in moving the contact plate back and forth, whereas the Kokusan point plate/contact set has a slot and pivot points to aide in gap setting. These ignition parts do not interchange, whatsoever and most of the aftermarket copies (Daiichi brand) fail to replicate the originals successfully, so are not a preferred option.

The neutral switches are all the same for all models and variations. The small wire leads often break their solder joints and fail to make the electrical connection to complete the neutral light circuit. Some careful cleaning and re-soldering of the leads will restore their function.

The crankshafts are model-specific: CL72-77 crankshafts have no oil hole drilled at the base of the crankshaft snout, which feeds the starter clutch on the Dreams and Super Hawks. There were Type 2 CL72s in 1962 and 1964, however they are rarely found in the US. CB72-77 crankshafts do have the extra oiling hole, which lies outside the edge of the CL72-77 crankshaft seal lip if you were trying to do a “lightweight” or racing version Super Hawk sans the electric starter parts. All the Dream crankshafts are Type 2 and will interchange, however the difference between any 250 and 305cc crankshaft is the size of the balancing holes in the crankshaft throws. 250s have about a ¾” hole and the 305s are approximately 1” in diameter. Crankshaft main bearings were located with hollow pins for three of the four bearings and a stepped pin was used on the right side bearing behind the rotor end shaft. The small stepped pin was replaced with one additional hollow pin, in later years, which reduced the chances of mis-aligning the pins in the crankcase holes. Mis-aligned crankshaft main bearing pins will cause the crankcase material to break away, as well as block the oil-feed hole to the bearing race. This is a very common “rookie” mistake for first-time engine builders and can cause severe damage to the crankshaft, as well as an oil leak behind the rotor/starter clutch hub portion of the right crankcase.

End of Part 2.

Bill "MrHonda" Silver



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