Motorcycle Repair: oil pressure, cb500 four, crankshaft bearings

Question
QUESTION: i have a 1972 cb500/4 i have rebuilt the motor , when the motor gets warm my oil light comes on,any suggestions. thanks mick

ANSWER: Michael, did you rebuild the WHOLE motor or just the top end?  

The possibilities are:

Defective oil pressure sender or intermittent short to ground on the sender wire.

Improperly fitted crankshaft bearings or damaged ones.

Actual real low oil pressure... see if you can measure it with a gauge tapped into the oil galley. There used to be aftermarket oil pressure gauges that had fittings setup to go into the existing oil galley passages.  56-85 psi is specified.

Air being drawn into the oil system from a loose oil screen connection.

Bypass valve in the oil pump propped partially open due to debris caught in the bypass valve.

http://www.cmsnl.com/honda-cb500-four-k0-general-export_model14652/partslist/E13

Bill Silver



---------- FOLLOW-UP ----------

QUESTION: Thanks Bill.yes just the top end the rest of the motor about 5000 km ago .

Answer
Michael, you didn't indicate how many total miles are on the lower end, so it could be tired out or there might be debris sucked up in the pump or any of the previous comments might apply. If I read your reply correctly, you rebuilt the lower end, as well within 5k km. Did you measure the crankshaft and fit new bearings to it, previously? If the engine has 100k+ km it may just be worn out and need new bearings and/or a crankshaft.

Check the easy stuff first, obviously. Drain the oil and pull the pan off. That will probably tell you a lot, if there is debris and worn out metal bits at the bottom. The oil pump comes off easily, so check that first, then the pan. Try another sending unit, but try to get a direct reading gauge on the motor to check for actual pump pressure before you go too far.

Bill Silver