QuestionI have a general question. I know someone with a 1982 CB900F. He is telling me that his oil pressure drops once the bike is warmed up and that someone told him he "spun a main bearing". Can you explain how this would be? How would a main bearing cause the oil pressure to drop and why when the bike warms up? I can see how low oil pressure might damage the crank bearings, but I don't see the other way around (cause and effect). I'm trying to help him and I'm a student of all things mechanical and bike related. Thanks!
AnswerI think the theory is that the "spun" bearing would have greater than normal clearance. As the engine warms up and everything expands the clearance will increase even more and the oil will become thinner. Thinner oil + greater clearance means that there will not be enough restriction to keep the pressure up as it pours out of the "spun" bearing.
However, while that might be the case, it isn't necessarily what's happening with your friend's engine:
- How low does the pressure go? A certain amount of decrease in pressure as the engine warms up (due to expansion of parts and thinning of the oil) is normal as long as it is still within spec when the engine is fully warmed up.
- Any failed bearing in the engine could cause the same symptom, not just a main bearing.
- It could also indicate an oil presure relief valve that is failing (or, depending on how high the pressure goes and how low it drops, simply doing it's job).
- It could even be something like an o-ring in the lubrication system that has become hardened and cracked.
And that's just what I can think of off the top of my mind. Someone with real experience with Honda fours could probably tell you better than me. I highly recommend that your friend join this forum http://www.cb750c.com/
If they can't help him figure out the exact cause of the problem, I'm sure they know who can.
