Wednesday, February 6, 2019

The 250-305 Engine overview – Part 1 of 2

The famous SOHC, 2-valve, 250-305cc engine has its design roots in the original 1957 C70- series 250cc Dreams. While the originals were dry-sump (separate oil tank) designs, many basic features were carried over into the 1960-later wet-sump engines, which power all the Dreams, Super Hawks and Scramblers, in various configurations. The engine’s rotating components spin on ball, roller and needle bearings in almost all cases. Low-friction element bearings only require a constant flow of lubrication, instead of high-pressure oil, which is necessary to maintain lubrication for plain-bearing engine designs.

Original Dream-series engines used 360-degree crankshafts, with a single carburetor. With the release of the high-performance CB72-77 Super Hawks in 1961, the crankshaft firing was changed to a 180-degree configuration and dual carburetors were adapted for high rpm work. The Scrambler engine is basically a modified Super Hawk design, lacking the electric starter and tachometer drive components.

The crankshaft is built up with a combination of ball bearings for the clutch end, and roller bearings for the middle two of the four main bearings. A heavy-duty roller bearing supports the crankshaft on the left side end for the alternator rotor. The connecting rods run on needle bearings for the big ends, but the piston pins run directly on machined small ends. The small ends of the rods tend to get oval-shaped, especially when they have a lot of miles on the engine or if the pistons have seized during operation. Oversized wrist pins of .004” are available to clean up minor wear issues.

Pistons are CB72 or CB77-based for Scramblers. Pre-65 engines have 9.5:1 compression, reduced to 8.5:1 after 1964. Cast pistons are used with chrome-plated cast iron rings. Early ring sets used a single chrome-plated compression ring, with the other two rings un-plated. Those ring sets were superseded to all-chrome plated rings for increased durability. Dream pistons originally had thick 2mm rings, but were superseded with a later design using 1.5mm CB ring sets. Some very early version CB72 pistons had 10:1 compression pistons.

The 305 and 250 crankshafts differ only in the size of the balance holes bored into the crankshaft counterweight assemblies. Honda used relatively long rod lengths to prevent piston rocking and encourage good rod leverage on the crankshafts. Heavy crankshaft weights helped to keep the momentum of the small displacement engines going between gear shifts, so that mid-range torque was favorable, despite the high horsepower peaking speeds. The use of a short 54mm stroke, allowed the engines to turn high rpms without creating excessive piston speeds (measured in feet per minute). Scrambler crankshafts lack an extra oiling hole on the rotor end, which is used to feed lubrication to the starter clutches on the Super Hawk applications.

Camshaft timing and valve sizes are the same for both 250cc and 305cc CB/CL engines. Cylinder head castings are machined differently between the two types, using a chamfered edge on the 305 cylinder head to add piston clearance for the 6mm larger bore size. The combustion chambers, spark plug threads and valve seats are a single cast-iron insert, surrounded by alloy castings. 1960-61 cylinder heads used 10mm spark plugs, which were replaced with 12mm plugs in 1962.

The camshaft assembly is composed of four major separate pieces. The camshaft sprocket contains the spark advancer weights and springs and is splined on both ends to accept the left and right side camshafts. Camshafts are retained by an expanding internal shaft on the left and a compressing nut on the right cam. The ignition point cam runs through the inside of the right side cam. An oil seal on the right side cylinder head cover contains engine oil that lubricates the camshaft and point cam combination. The camshaft is supported by four ball bearings.

The 94 link, 219 pitch camchain is guided by a lower roller, located between the cylinders mounted on the top of the upper crankcase, while a spring-loaded, mechanically-adjusted tensioner roller is mounted on the rear of the cylinder block assembly. There are “wide” and “narrow” camchain tensioners used on the cylinder blocks. The change to narrow versions came out in 1966. First version tensioners had the adjustment bolt on the left side.

The 26mm carburetors are almost identical between the Scrambler and Super Hawks, save for the calibration of the main jets and jet needles. On the 250 sizes, the carburetor types differ because of the use of the additional “power-jet” enrichening system on the CB72, versus a standard main jet design on the CL72s. On 250-305 Dream engines, they shared the same sized 22mm carburetor, but there were slight jetting adjustments made for each version

Transmission ratios are the same between CB and CL series engines, but they can be altered by “X-ing” the center gears on the main and countershafts. This interchanging of gears closes up the steps between 1st and 2nd gear, while increasing the jump to 4th from 3rd gear, which is beneficial for off-road riding. Dream transmission ratios are completely different than the CB/CL versions, so only a few parts interchange between the models.

The clutch assemblies are all interchangeable as a complete unit, between any of the engines, but there was a change to a “cush-drive” clutch outer for the 1965-on engines, which takes a bit of the torque loading off of the primary drive chain. There were 4, 5 and 6 plate clutch configurations, depending upon the year and application. Whatever you choose, the total stack height must be the same as what Honda installed originally. The inner clutch hubs have machined grooves for fine wire retainers to help trap a few of the plates in place when the clutch is disengaged. This minimizes clutch drag and the attendant problems of not being able to find neutral when at a stop light with the engine running. 

Each of the 4-5 clutch hubs are machined for the matching clutch pack setup. The friction and steel plates have different thicknesses, depending upon the total number of plates used in the assembly. The best solutions are to find a complete used 5 plate clutch assembly from a CB or CL77 and install the whole thing into your engine, no matter which model you have. Current clutch springs with a 323 code part number are equal to the original CB77 street bike springs. DO NOT use the 275-810 springs for street use.

The chain count in the Scrambler applications is four: Primary chain, oil filter drive chain and camchain and final drive chain. For a Super Hawk and Dream, the count is five, which includes the starter motor drive chain.  See notes below for replacements.

Ignition for the Type 1 (180 degree) engines is battery-powered, by use of dual points, condensers and coils. Ignition point sets are mounted on a single, adjustable plate, mounted on the right end of the camshaft. The dual-condenser set mounts on the ignition coil bracket, beneath the fuel tank. All Dream (360 degree) ignitions have a single twin lead outlet coil and a single condenser and set of ignition points.

Charging system components include a permanent-magnet rotor, air-cooled stator assembly, which feeds a Selenium or silicon-diode rectifier, to charge the 12v battery.


Common period modifications included 350cc big bore kits from WEBCO, Harmon-Collins racing camshaft profiles, including a roller rocker kit. Many other camshaft manufacturers offer various cam grinds. Cylinders can be bored to 3mm oversize (337cc), using stock cylinder liners. Due to the hemi-spherical combustion chambers, the valve angles prevent use of valves more than about 1 to 1.5mm oversize.

“Pops” Yoshimura created modified 250cc engines that were capable of beating CR72s at some racing events. His CB72s won an 18-hour endurance race, with sustained engine speeds over 10,000 rpms for the entire event. His CB72 engines were churning out 34 horsepower at 11,500 rpm, while the 305s were adding an additional 5 more horses, at the same rev limits. His bikes were built using modified factory production parts, which attests to Honda’s engineering prowess in creating durable and powerful engine components.

It is difficult now to grasp the full impact of these machines on the motorcycle market in the early 1960s. British, American and European-made machines dominated the marketplace, but their overall build quality and design expertise were sorely lacking, in comparison to the new wave of high-performance Hondas, which swept the world’s marketplaces. Advanced designs, coupled with superior engineering and materials enabled Honda to surpass the products of companies who had been in business for decades, in a matter of just a few years.  With the strength of the design of the Honda Scrambler engine, the task of riding from Tijuana to La Paz, Mexico in less than 40 hours, using production machines, was well within the design parameters of Honda’s CL72 dual-purpose machine, back in 1962.

Honda CL72 - CL77 engine specifications.
Total cylinder capacity:         247cc                                     305cc
Bore x Stroke:                         54mm x 54mm                       60mm x 54mm
Comp. ratio:                9.5:1 (’62-’64)                                     8.5:1 (’65-on)
Normal measured compression: approx. 150-175psi
Max. Horsepower:         24 hp                                28.5 hp @ 9,000 rpm
Max. Torque:              2.06 kg-m@7,500rpm /2.5 kg-m @ 7,500 rpm
Ignition: 12 volt battery and coil (dual points-Type 1, single points-Type 2)
Spark plug: NGK D8HA, except for 1960-61 models)
Carburetor:     CA/CB/CL72 PW22 (22mm) 26.5 float level
CB/CL77 PW26 (26mm) 22.5mm float level
Lubrication: Wet sump w/ gear oil pump
Clutch: Wet-type, Multi-plate
Transmission: 4-speed constant-mesh, foot shift
Ratios: CB/CL72-77: 2.788 (1), 1.661 (2), 1.171 (3), 1.00 (4)
Crankcase capacity: 1.6L (2.8 Imp pt/3.4 US pt) 30W, 10W-30/40

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