CB350 Cam Chain Tensioner Rebuild

(Brought to you by guest contributor Dan Falkenstrom)

Anyone familiar with early 70’s Honda engines is no doubt familiar with their rubber wheeled cam chain tensioners. Often they are the first component of an engine to fail, putting slack in cam chains and little bits of rubber in oil filters. This is exactly what happened to my little 1971 CB350.   

Due to the surge in popularity of these bikes, replacement OEM parts are all but impossible to find. Once in a while a NOS set can be found on eBay, but the quality of the rubber is suspect due to its age. Except for 100% historically accurate rebuilds, most would avoid putting these in their engines for fear of enduring a premature rebuild in the not-so-distant future.

A rare example of a new OEM replacement for the CB350 cam chain tensioner. The large wheel that attches at the other end is still available from some parts suppliers; this small wheel with steel support arm, however, not so much.

A comparison of small wheels: left is brand new, right is well-worn.

Two very worn large wheels. 

The popular option has been to upgrade to a tensioner system that does away with rubber altogether, using instead Nylon 6/6 (or similar polymer structures). Nylon is becoming more common throughout the automotive industry as polymer technology evolves, offering lighter weight and excellent resistance to wear. In this case, nylon will wear before the cam chain does (which is good), and will last much longer and fail less dramatically than rubber (which is also good). 

The only reliable commercial option has been the custom tensioner available from Bore Tech Engineering. This product has been a preferred upgrade for race engine builders for years, and it has more than proven itself on the track. But at $130, for those building a bike to cruise towards a buck-twenty cup of coffee instead of a buck-twenty speed at the drag strip, it can be an excessive upgrade. Every once in a while an entrepreneurial soul will manufacture a small batch of nylon roller wheels similar in size and design to the OEM ones, offering them for sale through message boards or eBay. Unfortunately, these folks are typically hobbyists with other full-time commitments, and they cannot cater to mass market desires for high-volume and fast shipping (at least, not for very long).

For the rest of us, we are left with no choice but to manufacture our own. My weekend jaunts between the hubs of New York’s Capital District demand little of my CB350’s performance curves, so I opted to rebuild my cam chain tensioner with wheels made of Nylon 6/6, retaining the original design dimensions and steel bushings. The nylon was purchased at $25/foot, and a replacement needle bearing for the small wheel (Torrington B-78-OH) at $15. A few afternoons on the lathe, and I had myself a small nylon wheel, an aluminum retaining pin for the small wheel bushing, and a nylon large wheel, all for a reasonable cost. We shall see how long they last. 

A side note on turning nylon; unlike metals, with high heat it begins to melt, making tolerance parts a challenge. Slow speeds, consistent fluid, and frequent breaks while turning help improve surface finish. 

A breakdown of the components in the CB350 cam chain tensioner system. The stamped-steel bracket and three steel bushings are original. The needle bearing was puchased new. The nylon wheels and aluminum retaining pin for the small wheel were made on the lathe.

A comparison between the OEM rubber wheel and the self-made Nylon wheel replacements.

Large wheel close up.

Small wheel close up, with aluminum retaining pin/rivet.

UPDATE 8/25/14:

After some tea and discussion, I decided to add a brass bushing between the large nylon wheel and original steel bushing. Since the steel bushing was some oddball size, I turned down the ID of a commonly available 0.75" OD/0.5" ID bushing. It press-fit nicely into the nylon wheel, and resulted in noticeably smoother rotation.