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MC How-To

Step by step: From general maintenance to complete restorations, we share tips and tricks for working on classic bikes.


Replace 1970 Honda CB350 Fork Seals

replace fork seals 

The forks shown here are off our 1970 Honda CB350 project bike, which, given the state of the rest of the bike, we knew needed new seals at the very least and more likely new fork tubes. Stripping them apart we learned two things: First, early series 1968-1971 CB350 forks are amongst the easiest we’ve ever worked on; and second, yes, we need new fork tubes.

To be fair, our original fork tubes are perfectly serviceable. They might not be pretty, but they’re straight and the critical wear surface area where the seal makes contact is fine; it’s just the top third, hidden under the upper fork tube covers, that’s rusty. Although we’ll be swapping in a new set of fork tubes from Forking by Frank, what you see here is pretty representative of what you’ll see on most old survivors; fork tubes that are a little battle scarred and ugly, but still perfectly usable.

BikeMasterRemoving the forks is straightforward. First, remove the fork drain screws and drain the old oil. With the bike suitably supported, remove the front wheel and the front fender. The upper end of each fork tube is choked down and fits hard up inside the upper triple clamp. Remove the chrome bolts (these are also the fork oil caps) securing the tubes to the upper triple clamp. Next, loosen the clamp bolts on the triple clamps, two on each side, and pull the fork legs out.

As always, we suggest having a shop manual on hand for parts identification and placement, and to confirm proper torque specs.

replace fork seals 

1. Early series CB350s used external fork springs. The springs aren’t noticeable installed as they’re covered by the upper fork tube sheet metal. They fall loose from the fork as soon as you remove the fork tube from the triple clamp assembly. Note the position of the plastic bumper inside the spring; it should be installed at the top or upper end of the spring. Once the spring is removed the chrome fork leg cover around the top of the fork leg will pull straight off.

replace fork seals 

2. Getting back to the task at hand, the next step is to remove the snap ring securing the oil seal and fork bushing — and hence the fork tube — to the fork leg. If the snap ring is rusty, spray it with penetrating solvent and let it soak for a few hours before trying to remove it.

replace fork seals 

3. With the snap ring removed, hold the fork leg, compress the fork tube then give the tube a sharp pull out. It may take several tries before the seal releases and the fork tube, complete with seal and fork leg bushing, pulls free from the fork leg as shown above.

replace fork seals 

4. With the fork tube separated inspect all parts. As noted in the text, our fork tubes were rusty at their upper ends but critical wear surfaces were still OK. This photo shows all the parts of the fork, with the exception of the orifice tube inside the fork leg, which we left in place during cleaning. It’s a very simple design.

replace fork seals 

5. During cleaning, carefully inspect the fork tube bushing for excess wear. Both bushings on our forks showed wear, but fortunately not enough to warrant replacement as new bushings are expensive and getting harder to source.

replace fork seals 

6. Thoroughly clean all parts in a parts washer or with a suitable solvent. Spray brake and electric parts cleaner works fine. Dry with compressed air. We took a few extra minutes to clean and polish the fork legs and the chrome fork leg covers. The fork tubes cleaned up well, although we’ll ultimately replace them. Here are our parts ready to go back together.

replace fork seals 

7. Loosely assemble the fork tube in the fork leg. Slide the fork bushing over the tube and push it down into the fork leg. Smear a light coat of grease or fork oil on the lip of the new seal and around the top of the fork tube and carefully push the seal over the tube and down to the fork leg as shown.

replace fork seals 

8. Next, making sure it’s straight, push the fork seal into the fork leg as far as you can using finger pressure. To drive the new seal home, take an old seal and slide it over the fork tube and on top of the new seal. You’ll use this as a driver.

replace fork seals 

9. Place a suitable piece of 1.75-inch PVC pipe on top of the old seal and drive the new seal home with several blows from a rubber hammer.

replace fork seals 

10. Confirm the seal is fully seated by inspecting the groove for the snap ring, which should be fully visible. Next, reinstall the securing snap ring. 

replace fork seals 

11. Exposed to the air, the fork springs can and do rust over time. Looking for a quick and easy way to remove the rust, we soaked our springs in a small bucket of Evapo-Rust rust remover. The spring at top is untreated while the lower one shows the results after a 24-hour soak. All we had to do was rinse it off and we were good to go.

replace fork seals 

12. With the seal installed, push the chrome fork leg cover back on, short side down. Next, slide the spring with plastic bumper over the fork tube. Install the forks in the triple clamps. Tighten the top chrome bolts, then the upper triple clamps. Remove the top bolts and fill each fork with 200cc of fork oil. Honda originally specified 10w30 engine oil but we prefer a straight-weight like Spectro 15w fork oil. Install the fender and wheel. Push the front end down several times, then tighten the lower triple clamps. MC