Honda's 174cc overhead cam twin is a great engine but can suffer from camshaft bearing wear.
In 1968, Honda introduced a new series of engines to replace its earlier forward-canted overhead cam twins. First seen in the U.S. in 174cc and 325cc versions, the new twins promised efficient power and long-term durability. Yet as good as they were — and still are — they did have a few problems, chief among them a propensity for wearing out camshaft bearing blocks. Some people will tell you the fault lies in poor materials, but our experience suggests a combination of factors, chief among them owners failing to follow regular oil change intervals.
What happens is the camshaft bearings — or journals — wear from insufficient lubrication, allowing the cam to move. The steel cam runs directly in the aluminum cam journals, and if the oil pressure fails, the steel will wear on the aluminum.
The top end of the 174cc and 325cc engines gets oil from a feed in the right engine cover. Oil under pressure feeds into a channel in the cover, where it's directed to the right side of the crankshaft and also to the top end. The cover has two O-rings, one sealing it to the case and another sealing it internally. A failure in either O-ring will result in a leak and pressure loss, but when the internal seal fails there's no visual cue as the pressure loss is internal.
|Old, worn bearing journal is at left, used replacement at right. The old journal was shot, showing more than 0.040in (1mm) of wear.|
Compounding the issue, these engines don't use an oil filter. Instead, they use a sludge trap on the right side of the crankshaft under the aforementioned cover: It's basically an internal centrifuge that separates and traps major particulates from the oil. Further, a spring-loaded slip coupling (Honda calls it an oil guide) in the cover lines up with the sludge trap to direct oil. If for any reason the coupling sticks it can cause further loss of oil pressure. If the oil isn't changed regularly there's a risk of filling the trap with sludge (rare), but the bigger risk is dirty oil plugging the small internal feed line to the cylinder head.
Technically, this isn't a particularly complicated job. You have to remove the timing plate and the ignition advance unit, so if those tasks are beyond your ability, we'd suggest finding a vintage bike mechanic to do the work.
An experienced Honda mechanic can replace a pair of cam journals in a few hours, but if you've never done the job treat it as a weekend project. New cam journals are unobtanium, but used journals in good shape can be found. Expect to pay around $30-$40 per journal, with sets slightly cheaper at $45-$50. You'll also want a new seal for the left journal, about $4-$6.50, and it's a good idea to have a new journal gasket on hand, about $10. Run the engine to full operating temperature when you're done, followed by a hot drain and a refill with clean oil. As usual, have a good shop manual on hand to aid in parts identification and proper torque specs.
The first thing we did was remove and inspect the sludge trap cover on the right side of the engine. Although not visible, the inner O-ring seal was shot, allowing oil pressure to bleed off.
With the cover removed we discovered that the internal slip coupling, which is spring-loaded on a fixed snout to ensure oil feeds to the crankshaft, was stuck fast to its bore in the cover. It's shown here partially removed, ready to clean.
The slip coupling cleaned and reinstalled: Note the O-ring to seal the cover to the case. A second O-ring goes inside the engine; the raised cover extension seals against it. Note, too, the two oil galleys at roughly 6:30 and 3 o'clock; one is the feed to the cover, the other is the feed to the cylinder head.
We only needed to replace the left journal, which is typical as it's farthest from the oil feed and thus receives the least amount of lubrication. The right side is easier, as you don't have to disturb the ignition. Remove the ignition cover and both spark plugs.
Next, remove the two screws securing the ignition points plate to the cam cover. To aid reassembly, you can scribe a line on the plate centered on the cover. The replacement cover won't have the scribe, of course, but the line on the plate gives you a ballpark orientation.
Move the points plate out of the way and use a 10mm wrench to remove the bolt securing the ignition advance unit to the camshaft. It might need a light shock to break free but should release relatively easily.
Next, remove the ignition advance assembly from the cover, followed by the intake and exhaust valve inspection covers. The exhaust inspection cover is visible here just above the exhaust header. The intake cover is identical, but is located on the intake or carburetor side of the engine.
Remove the left engine cover. Using a 14mm wrench, rotate the engine and line up the timing mark on the rotor with the mark on the stator for TDC. Both valves should be loose. If not, rotate the engine to confirm TDC on compression. Next, fully loosen the valves to release pressure on the cam.
Remove the four screws securing the journal block and replace it with the "new" one, with the new seal installed. The mounting gasket can often be reused. If it's bad, do not use silicone as it can block oil passages. All that's left now is to put everything back the way you found it, followed by adjusting the valves, running the engine and changing the oil. Check the valves after the first cool-down and adjust if necessary. Happy riding!