by zozman » Wed Jun 03, 2015 9:19 pm
You can mount a Honda CB 350 shock to a Honda CH 250 or Helix CN 250. I have done the modification on every CH 250 I have owned. It is a direct bolt-in application. It gives you a much steadier and firmer ride. Absolutely necessary for two up riding. I usually cut off the chrome covers on the top of the shock. I have found you need a larger bronze bushing to fit into the rubber grommet on the top of the shock. I found some at the hardware store. Minor grinding to the eyelet at the top of the shock. Here is the Tech Tip on the conversion.
Tech Tip #26 Adapting The Honda CB350 Shocks To The Honda CH250
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 carburetors, 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 provide 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.
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. The standard CH250 shock is 2" OD at the top and 1-7/8" OD at the bottom. The standard CB350 shock is 2" OD top to bottom. (NOTE: For this modification, any related vintage Honda rear shock from the Honda CB350-360; 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. It is unnecessary to remove the top eye mount rubber bushing on the shock. 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.
Existing Hardware:
This is the easiest method. Take the CH250 metal spacer which was removed from the shock, and wrap electrical tape around it so that it is over sized for the rubber CB350 bushing. Use some lithium grease inside the CB350 shock rubber cushion then push the metal spacer into the rubber bushing. Use the original M8 CH250 bolts for the top of the CB350 shock when re-mounting. Note: If you want to machine your own spacer then make it 18mm long, 16.5mm diameter, and have a 5/16" hole drilled through the center. These dimensions will give you a nice push-fit in to the rubber bushing without the use of electrical tape.
New Hardware:
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.
Question: I want to install OEM Honda Shocks on my Honda CH250 without any modifications. Which ones should I use?
Answer: These two part numbers, although for different Honda models, will mount up exactly:
SKU: 51400-KM1-672ZA (Honda CH 250)
FORK ASSY., FR. *NH1* (Honda Code 2142107) (BLACK) Use from Frame SN FK000268 Use from Frame SN FK000268 $68.59 ea.
SKU: 52500-KS4-931 Honda Helix CN250)
SHOCK ABSORBER ASSY., RR. (Honda Code 4552147) (SHOWA) $126.90 ea. (2)
NOTE: Photos of this shock modification can be found in the photo section, file CB350 SHOCK MODS.
Question: Will the Honda VHD shock used on the CB450-500 and 750cc bikes work on the Honda CH250?
Answer: The VHD shocks are too big (13.89 inches eye-to-eye)and will not fit the CH250 without rubbing against the splash guard and the air box. Their springs are 2-3/8" and must be bent to fit in between the air box and the tire. Additionally, the shocks are almost two inches longer than the stock CH 250 or CB 350 shocks. This pushes the engine down to its lowest level thus rendering the engine pivot bushings useless.
Randy Pozzi (2/2009) Dennis Flora contributed to this Tech Tip.
87 & 98 Helix