Topic: Is my oil pump toast?

[TinyHouseYugeGarage]

I recently picked up a running '76 CB750K and have been going through some of the engine components. Today I dove into the oil pump to replace some of the o-rings and to measure the tolerances between the oil pump rotors. Tolerances were within spec but upon further inspection, noticed some pretty sever scouring/scratching on one of the rotor sets.





I'm thinking this is no good, but wanted some other opinions before I toss this pump and look for another. Buying an oil pump off ebay seems like kind of a guessing game. Anyone got an oil pump (inspected and within spec) their willing to sell me?

BTW, the oil pump screen did have some junk on it, but it didn't seem all that bad. I'm wondering how something got in there and wreaked so much havoc?

[rotortiller]

I've seen worse. Check it with a feeler gauge like the manual states. There is a large tolerance like 14 thou allowed which gives an indication how much the thing can be damaged. If it makes specified pressure then I'd keep it especially since you can access it in the frame. Some guys think everything needs to look like new, well it likely would if it were a new bike. 

PS match the dots upon reassembly, it should have some.

[Kevin D]

The wide rotors are for the scavenge pump, which draws through the screen, lubes the trans and clutch, and fills the oil tank. The narrow rotors are for the supply pump, it draws from the tank, pumps through the filter and to the crank, rods, top end.

You have had some garbage pumped through your engine.
Do you know where it is from? Replacing the oil pump is addressing the symptom.
What did the pan and the screen look like?
Have you flushed the oil tank?
How are you going to ride this bike?

 My own oil pump is roughed up, along the lines of what you have, but not nearly as severe, and it has been no problem for years and K miles. The scavenge pump doesn't have to work very hard, IMHO. It only has to lift the oil about a foot to the top of the oil tank, and push some through the tranny.
 I'd be more concerned about what made it happen.

[BomberMann650]

Did someone perform an oil change on the beach front?

[PeWe]

CB750 need a lockable oil tank dipstick.
I have recently start to think about how easy it is to damage the engine by putting sand in oil tank. No filter will stop the sand that will go direct to oil pump.

My oil pump had also some scratches. I removed the  stick-out burrs with 1000 grit paper and pump rotated easier. Got oil pressure, no damages seen on cam.

[Bodi]

That's the scavenge pump and it will work fine like that. It doesn't produce much pressure, it pumps sump oil back to the tank with a connection to the transmission shafts to keep them oily - the output pressure is just enough to pop open the anti-drainback valve (not much at all). I have never found a used oil pump that looks "new" and many are as bad as yours or worse, with no apparent problems. I do not know what gouges the rotors like that... but it's not unusual according to the 20 or more used pumps I've looked at, anyway.
If the smaller rotor set looks like that it would probably be an issue, that one pumps to engine pressure (the pressure bypass limits it at about 60psi - system pressure should be close to that at riding RPM, lower with really hot oil though). End wear on the pump body (or the rotors, but the case alloy wears faster than the rotor steel) reduces system pressure more than rotor gouges in my experience - and that can be "adjusted".

[PeWe]

There was a thread recently with a guy that had in mind to make new rotors, like Honda OEM but better. That might be somthing if possible to produce for price people can afford.

[TinyHouseYugeGarage]

Thanks for the info guys.

The bike has about 20k on it, and the engine overall seems in pretty good health (compression 135,135,130,125 COLD). The oil pan did have about a 1mm layer of sludgy goo in it, but I didn't notice any metal shavings or anything out of the ordinary. The oil I drained looked pretty clean. It was most likely changed right before I bought it.

Maybe the next step will be to pull the oil tank off and make sure there isn't anything weird in that.

I ran some 2000 grit sandpaper through the rotors to try and clean up any burrs. I'm thinking I'll put this pump back together and run it. Clearances were great (.004in between the rotors and outside body). I bought a rebuild kit off ebay with new o-rings (springs too!).

[waldezy]

I just pulled my oil pump due to pressure issues and my rotors looked just like yours. Did you end up putting them back into the pump and did it work out for alright?

[Dunk]

That's pretty bad, but I've seen worse. If the worst of it is on teh thicker rotor set it's not as big a deal (scavenge side) as the thin rotors that pressurize oil to the bearings and top end. I went through 4 or 5 oil pumps to find the best one to use for my restoration. There have been a few threads on here about possible new rotors being made, repurposing parts form other bikes with minor modification. I recall there was some discussion that  the rotor side to side clearance in the housing made a more appreciable difference in output pressure than the internal clearance or damage to the faces. It would in theory be easier to resurface the sides and tighten up that clearance.

[tlbranth]

You have had some garbage pumped through your engine.
 

Not necessarily. The oil pump is unprotected but from there it goes through a filter. The engine, most likely, did not see the junk that assaulted the pump.