In 1965, Honda created its first “big bike”, the CB450. But, the highest  selling motorcycle in American history was the Honda CB350, made between 1972 and 1977. The CB350 had a twin cylinder, overhead cam, air cooled engine. It also had two carbs, five-speed gearbox, and electric starter.  With more than 300,000 units sold, 660,000 in various forms, it was a motorcycle that appealed to the masses.

To assist that 360 lb. bike in handling and cornering, Showa rear shocks were installed with either the Showa 455 chrome covers (removable) over the top of the head or Showa 369 exposed spring. Either will work in this modification. They provided excellent damping and firm control over most  highway riding conditions. The shocks were adjustable with five firmness settings. There was a huge 7-3/4” long single coil spring which had a diameter of 1-7/8" (50mm). Ride suspension varied from its low end street setting to its most firm setting for racing. 

Then, there is the mega-shock, the Showa V.H.D.--Variable Hydraulic Damping-- which was used on the rear of the heavier 1972-1977 Honda CB-Fours (Four cylinder, SOHC)--the CB350F, the CB400F and the CB500F. This shock features a two-stage compression, four-piston rebound damping and five-level adjustable settings. It has a whopping 2-3/8" diameter spring (61mm) that is load rated at 440 lbs! This shock (PN #447-733), which is similar in length to the Showa 369 and 455, will provide years of riding stability and improved ride for your CH250. The V.H.D. Showa shock that was used on the Honda CB650/750, is one inch longer and has a half-inch wider spring. Some modification of the splash pan cover and inner fender is required for its use.

Caution---The shock spring on the OEM Honda CH250 shock is 9-1/2 inches--most of the length of the shock. It is only 7-1/2 inches on the high-power V.H.D (Variable Hydraulic Damping) shock used on the CB350 and CB650 and 8" on the CB750. But, the  V.H.D. shock spring does not go all the way down to the horseshoe mount like the CH250 shock. There is 4" of shock stem rising from the horseshoe mount until the spring begins. This appears to be enough to possibly allow those shocks to also work on the CH250. If, however, you are running wider tires than the OEM standard 4.00x10--like 120/90 or 130/90 metric, they will not work.

The Honda CH250 rear shock, by comparison, is ineffective in almost every street application. It appears too bouncy with single riders, and replicates a pogo stick event with two-up riding. Adapting the Honda CB350 shock, which is only a quarter-inch longer in length compared to the OEM Honda CH250 shock (12.75 to 12.50 inches top eye mount to bottom clevis mount) and is a logical choice for improved handling. Both the CH250 and CB350 shocks have similar mounting positions.  (NOTE: For this modification, any related vintage Honda rear shock from the Honda CB350-360-400-450-500; CL-CJ-360 from 1965-1977 will work. The shocks from the CB-CJ250 will NOT work).

The top mount diameter of the Honda CB350 shock measures .22 mm wide while the CH250 shock measures .18 mm. You must first remove the top eye mount  rubber bushing with a screwdriver. You will have to grind off the excess .04 mm--2 mm from each side center-- from the CB350 shock eye so that the top of the shock fits into the CH250 mounting space.

You have two choices for the top shock eye mount--re-use the rubber bushing and spacer from the CH250 shock with tape or, if unusable, replace them with new hardware.

Existing Hardware:

This is the easiest method. With the CB350 rubber bushing removed, cut off excess from ends so that it measures 3/4”. Take the CH250 metal spacer which was removed from the shock, and wrap electrical tape around it so that it is oversized for the rubber CB350 bushing. Place contact cement inside the CB350 rubber bushing then vise the metal spacer it into the rubber bushing. Use the original M8 CH250 bolts for the top of the CB350 shock when re-mounting.

New Hardware:

Remove the OEM CB350 rubber bushing from the shock. Replace the OEM rubber 
bushing with a comparable hourglass-shaped polyurethane or cylindrical rubber strut bushing from an auto parts store. Rubber refrigeration compressor mounts also work. Any bushing measuring .18x.25 mm (3/4” x 1”) should work. Re-use the CH250 spacer or obtain a 3/8 IDx1/2 ODx 3/4” bronze sleeve bearing from your hardware store. The sleeve bearing acts as the spacer and will allow you to torque the mounting bolt without 
compressing the rubber shock bushing. Install the rubber bushing into the shock 
first, then vise the metal spacer into the rubber bushing. Use the original M8 CH250 bolts for the top of the CB350 shock when re-mounting.

No modification is necessary for the bottom of the shock as the distance between legs for the bolting area is exactly the same--0.19mm. However, the mounting bolts are different. The mounting bolts for the bottom of the CB350 shock are M10x1.25 pitch. The original mounting bolts for the CH250 shock are smaller diameter--M8x1.25 pitch. To adapt the bottom of the shock to the CH250 crankcase, you must still use the smaller diameter bolts in the larger CB350 holes. The reason for this is that the larger M10 bolts will not fit 
through the crankcase mount on the left side nor the swing arm mount on the muffler side of the scooter. For the bottom, use M8x1.25 pitch bolts that are 1-1/2 inches long (45mm) with a M8 stop nut.

Prior to mounting the shocks, the Honda CB350 shock preset should be set to #1--the softest ride setting. For a firmer ride, set the pre-load to #3. The stiffest ride is #5 which compresses the spring. You adjust these shocks by clamping the shock head in a vise and turning the bottom of the shock with a screwdriver inserted through the bottom shock mounting holes and rotating the bottom of the shock with the screwdriver to the desired setting.

Randy Pozzi (11/2005) Dennis Flora contributed to this Tech Tip.

NOTE: Photos of this shock modification can be found in the photo section, file CB350 SHOCK MODS.