Replace Honda CB175 Camshaft Bearing Blocks


| 6/20/2014 3:57:00 PM


Tags: july/august 2014, classic honda motorcycles, classic japanese motorcycles, how-to,
Honda’s 174cc overhead cam twin is a great engine but can suffer from camshaft bearing wear.

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.

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The problem typically shows itself as a ticking valve you can't get quiet. Adjust the valves and the noise will quiet down, but then shortly reappear. A subsequent check will show the valve adjustment has changed yet again. When it gets extreme, you can see it in the ignition points cam not being centered in the timing plate.

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 of wear. 
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.




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