There are a lot of misconceptions about the possibility of interchanging parts between the 250 and 305cc Scrambler models. I thought I would take a few moments to help clarify some of the interchangeable parts and those that are not directly able to be swapped out.
1. 305cc top ends on 250 engines.
The upper cases of the 250cc engines do not have sufficiently enlarged bores for the 305cc cylinders. To make a 250 into a 305 you generally have to bore the cases out to fit the 305cc cylinders and sleeves.
Adding the 305 top end will change the balance factor on the engine, as the 250 crankshafts have a different balance factor than that of the 305 engines. Bottom line: It will probably vibrate more than it would as a stock 250cc machine.
2. Cylinder heads
While the cams, valves, springs, and rocker arms are interchangeable, some of the cam sprockets might not be the same due to differences in the camshaft and sprocket spline configurations. The 305 cylinder heads have an enlarged squish band to allow the jump from 54mm to 60mm pistons to clear the combustion chamber edges. Note that the head gaskets have fire rings that are either 55mm for the 250s or 61mm for the 305 applications, to allow for use of up to 1.00mm oversized pistons and rings.
3. Carburetors
The switch from 250 22mm carburetors to 26mm 305 carburetors requires matching carburetor insulators and o-rings. The intake ports on the 250s seem to be a bit smaller than those of the 305 heads.
4. Throttle cables
The standard 273 code throttle cables are manufactured to match the height dimensions of the 22mm carburetor slides. If the 273 cables is used on the 305 CL77 carburetors, there is a chance that the slides will not be raised fully. Conversely, the later 305 throttle cables have split ends which have excessively long cables to the carburetors, so that you wind up with excessive cable slack that can’t be adjusted out with the cable adjusters on top of the carburetors.
5. Carburetor calibrations.
The stock main jet for the 305 CL77 carburetors is a #130 with a #38 idle jet. 250 carbs are going to see main jet sizes down to about #120-125 with #35 idle jets. The slide needles have different tapers from those on the CB carburetors, which will interchange with the CLs if the correct calibration parts are used. The round bowl carburetors can use the later square bowl floats, which have been superseded to the 286 parts from the 250-350 twins from 1968-73. Even the needle jets are of different sizes and part numbers.
6. Engine swaps
There is no physical difference between the 250 and 305 engines, so swapping out a 250 engine for a 305 is a straightforward proposition. However, the top covers on the 250 have solid bores for the engine mount bolt, whereas the 305s have a rubber cushion inserted into the cover
7. Engine changes
Honda revised the spline depth of the transmission and even the crankshafts in the later days of production. The later splines are shallower, which can be used on deep spline shafts, but not the other way around.
8. Cables
The switch from “small brakes” to “big brakes” necessitated changes in the brake cable dimensions as the reach from the backing plate to the brake arms is different. This applies to both front and rear brake cables, thus the change in the part numbers after about the 15k serial number series. The early cables with the big chrome adjustment knobs were superseded with standard CB72 lever brackets and adjusters and a CB450 brake cable, with a CB175 clutch cable.
9. Exhaust systems
Original 250 exhaust pipes were straight back with little baffles tucked in, flush to the ends of the pipes. In 1965, with the release of the CL77s, the exhaust notes were dampened down by the use of a slip-on rear muffler that joined both pipes together at the back. Each exhaust pipe still had individual baffles but the ends were extended past the pipe's terminations and the baffles were retained by headless bolts that were screwed into self-locking nut plates. As these were easily removed, Honda switched up to a muffler which was welded onto the upper pipe with the lower one sealed by a packing sleeve and a clamp. An additional baffle was installed inside the muffler body, as well.
10. Seats
CL72s and early CL77s used a double, metal-to-metal, fork mount on the front of the seat pan to connect it to the frame. To reduce the vibration being transmitted into the rider’s body, the seat mounts were rubber mounted with a U-shaped rubber mount up front and rubber cushions at the rear mount legs. Obviously, the frames had to be modified to conform to the front seat mount change.
11. Forks
Despite the use of the same fork bridge and stems for all models, the forks, themselves, were drastically different, and different versions of the fork boots were employed. Later model fork ears were also rubber-mounted and required an additional pigtail ground wire to allow for the full function of the headlight and instrument lights. The Type 2 alloy forks were employed on CL72s after 1008551, but the CL77s had the change just shy of the 15k serial number mark. Steel fork cases can be damaged if the original length fender mounting bolts are replaced with longer ones than specified.
12. Rubber mounts
Honda’s quest to reduce rider fatigue and component failures due to excessive vibration resulted in the rubber mounting of the seat, muffler, fork ears, and rear fender.
13. Ignition switches
CL72s used an ignition switch which was almost identical to the CB72-77 switches, except the threaded portion was extended so that the outer side cover latch to be attached. Later CL72s and all CL77s used the two-piece ignition switch mounting and switch assembly, thus the frame mounts are different between the two styles.
14. Wheels and hubs
The switch from small SLS brakes to the big 200mm DLS brakes required new hubs, spokes, and brake components, as well as the rims. All CL wheels are 19” but the spoke angle changes when the hub sizes are increased, so not only are the rims different widths between front and rear, but the spoke holes are angled differently so the spokes can reach the hubs properly.
15. Rear Suspension
The original early CL72s had double-eye shocks, which were replaced with eye-clevis by 1964 All CL77s had eye-clevis shocks. The early rear swing arms were machined to hold the sprocket carrier on one side, allowing the rear wheel to be removed without disturbing the drive chain. The swing arms, chain adjusters, and all of the mounting hardware for the early machines didn’t work for the later rear hubs, which were CB-based and used CB rear sprockets.
Bill Silver aka MrHonda
www.vintagehonda.com
www.mrhonda.guru blog site for more stories.
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