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Observations upon the Honda 250-305cc Engine group Part 1.

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Having offered engine repair guides, both in print and on CD (PDF format) for some 20 years now, it seemed like it was time for some updated notations about what it takes to put one together and make it live a long and healthy life. The comments here pertain to the 1960-67 wet-sump production engines used in Dreams, Super Hawks and Scramblers, NOT the 1957-60 dry-sump Dreams. As the story was being written, it expanded and expanded until it was about nine pages, so it will be divided into three parts for brevity.

Interchangeability: What will fit what and what doesn’t…

Dimensionally, the engine cases, cylinder heads and cylinder blocks are all the same. A 305cc model of any of the three versions (Scrambler, Dream and Super Hawk) is just an over-bored 250cc base model. The single-carbureted Dream engines can be easily shoehorned into a dual-carb Scrambler or Super Hawk chassis, although there are few reasons to do such a swap. The reverse cannot be done easily, however, in wedging a dual-carb CB/CL engine into a CA72-77 Dream chassis because of the pressed steel frame’s restrictions around the cylinder head area.

The 1960-62 lower engine cases did not have a primary chain tensioner; however you can retrofit a later bottom case to an earlier upper case. The “rear breather” 1960-61 upper engine cases could be swapped out with a “normal” top breather top case, coupled with installation of the breather plate and matching top engine cover. The 1960-61 engine cases and cylinder blocks are a pair, however, due to the placement of the oil feed holes, which changed in 1962.

With a few exceptions, in the very early years, the crankshafts can all be interchanged between any of the engine cases. There is a difference in the balance factor between the 250 and 305 engines, however. Unlike a plain-bearing engine, these cases are not “matched” nor do they need line-boring in order to be used in another application. All rotating shafts are on bushings and element bearings, whether needle, ball or roller type. There were some changes in main bearing dimensions that prevent a one hundred percent interchange, however. With the correct, matching, top-end components, a Type 2 (360 firing engine) can be created using CB/CL72-77 engine cases and a Dream crankshaft. DO NOT try to use a 180 degree crankshaft from a CB/CL engine in a single-carb Dream application. The uneven firing impulses confuse the intake tract and one side starves for fuel. Early model connecting rods had slotted ends for lubrication, which were changed to a chamfered, round hole after 1964.

Other than the “rotary gearbox” transmissions found in some domestic Dreams and CYP77 Police bikes, ALL of the other shifting mechanisms are interchangeable between ALL 250-305 engines. The shift selector parts, except the shift shaft, are all the same, unless you have a rotary gearbox installation. Rotary gearboxes have a completely different shift drum and forks, plus a specific shift drum guide, stopper and gearshift arm. There are two different length shift shaft units, mating to the appropriate clutch covers used, so the wrong matchup will cause the shift shaft to bind up against the inside of the clutch cover.

While early 250-305 Dreams had lighter-duty primary chains/part numbers, eventually all were superseded to the 268 code primary chain, which is dimensionally the same for all engine configurations. There was a special chain and sprocket setup for the CYB77 racing engines, but those are rarely encountered without the matching close-ratio 4 or 5 speed racing transmissions. The primary chains are a 328 pitch design, which no one else in the universe makes available now.

The unique feature of the 250-305 transmissions is that you can “X” (criss-cross) the center four gears on the transmission shafts, which will change the gear ratios and gaps between the 1-2 and 3-4 gearshifts.

All CB72-77 and CL72-77 transmissions are basically the same, while the Dreams have different gear ratios. You can install an entire transmission from one engine into another, but you can’t mix/match CB and CA transmission parts. To improve shift selection motion, there are offset cotters which relocate the gears on the main and countershafts. This was a Honda engineering upgrade and the cotters were made in .020” and .040” offsets. ALL production engines came with “flat” gear cotters from the factory.

Generally, most engines will need the .040”cotters; however they are in short supply now. When the shift dog engagement is less than 40% (with all slack removed), there is a good chance that the transmission can jump out of gear under load. Repeated “jumping” episodes will eventually damage the edges of the gear dogs, ruining the expensive transmission gear sets.

A final note on the transmissions is that they changed the spline depths in 1966-67, going to a “shallow spline” design. The change was also made on the front crankshaft primary drive sprocket. You can put the shallow splined parts on deep spline shafts, but they won’t interchange in the opposite configuration.

The kickstarter shaft, low gear bushing, kickstarter pawl/spring/plunger, end bushing and rollers are all “universal” parts. There are two different part numbers for the bronze low gear bushing, but there are no apparent differences in the dimensions that anyone can find. The low gear bushing will probably need replacing in any motor you open up and the kickstarter pawl is the other commonly worn part inside any of the transmissions. Worn low-gear bushings can cause the large low gear to tip over slightly, aggravating the kickstarter pawl interface where it contacts the inside of the gear. Once the little ridge on the bushing wears down/comes off, then the gear has lateral movement which then makes the gear dog engagement problematical.

Likewise, the easiest and best clutch swap is to get one from a 1965-later CB or CL and put it into any of the other models. For Dream applications, use the lighter 425 code springs, though. For most CB77 and CL77 models, the 323 code (CB500F) clutch springs are sufficient for street use to prevent slipping and not give your clutch hand cramps from excessive pulling resistance. Late 5-plate clutch setups have “cush drive” outer clutch baskets for some shock-loading relief during on-off throttle loads. There are numerous configurations/part numbers for the inner clutch hubs, because of the several types of both friction plates and steel plate combinations which Honda tried in order to quell clutch slippage. It’s best to just go with the late 5-plate setup and be done with it.

The cylinder heads and valves of the CB and CL series bikes are all the same castings. The 250cc combustion chambers are true hemi-head shaped, while the 305cc heads have a machined chamfer around the combustion chamber edges to match up with the increased bore size (54mm vs. 60mm). Both the CB and CL engines use the same camshaft timing and lift profiles. The Dream engines use the same lift, but shorter duration cam timing to increase low-speed torque and airflow through the single port head. Note that the 1960-61 cylinder heads used a 10mm spark plug, which was increased to a 12mm plug size in 1962. The other change was at the front of the cylinder head where the fins from each exhaust port side converge in the center. Pre-1965 heads blend this transition with a gentle U-shape. After 1965, the fin pattern was a more defined V-shape. Those two little fittings, located beneath the intake ports, allow venting of any residual oil solids from the cross-drilled intake valve guides, as well as supply an air source, but it is not a vacuum connection. The air vent system was deleted for 1967 models, however, without explanation.

Dream intake valve heads are smaller in OD and the springs have less tension, plus the valve retainers are slightly different in shape. Valve sizes for Dreams are the same between the two engine versions. The ignition point cams, which run inside the right hand camshaft bores, come in two different shoulder diameters and two different point cam profiles, depending upon whether the engine is a Type-1 (180 – single lobe) or the Type-2 (360 – double lobe) configuration.

The camshafts are splined and fit into matching splines on the camshaft sprocket/spark advancer unit. There is a wide “master spline” which indexes the cams to the camsprocket. Grinding off one of the splines 90 degrees from the master will allow you to convert a CB profile cam, normally firing 180 degrees, into a Type-2 360 degree engine. Using the CL72-77 camshafts with the splines re-indexed can perk up a short-duration Dream engine by increasing the cam timing specifications. If you want to create a Type-2 Super Hawk or Scrambler, changing the master spline, installing a Dream points cam with Dream points plate and changing to a Type 2 Dream crankshaft will create a Type 2 CB or CL model. You will need to install an appropriate ignition coil and condenser to complete the conversion.

Speaking of camshafts and cam sprocket splines, there are at least 4 different spline configuration shapes and none of them will interchange with the others. You MUST match the cam sprocket with the camshaft sets, in order to mate them together successfully. The camsprocket for the Dreams have longer weights and lighter return springs to enhance a faster spark advance curve. The CL engines are the same as the CBs, so use the same cam sprocket/advancer units, which have smaller, lighter weights and stronger return springs. The cam sprockets are riveted together and the rivets will loosen up after awhile, causing the valve and ignition timing to cycle back and forth rapidly, making a fair amount of top end noise in the process. The Dream cam sprockets have longer, larger weights and lighter return springs, so the spark advance curve is faster than the CB/CL cam sprockets, whose weights are shorter and held back with stiffer springs.

My 1961 CB77 has a Dream cam sprocket installed, which happened to match up to the camshafts that were in the engine from the factory. With a quickened advance curve and even shorter overall advancer timing allowed the engine to be timed well before TDC at idle and still not exceed the (II) advance markings on the rotor at full engine speeds. The bike starts effortlessly and seldom needs much choke, even on a cold day or after sitting overnight.

End of Part 1.

Bill "MrHonda" Silver



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