Topic: Oil pump locks up while tightening screws

[Toby84]

Cb750 k7.

I rebuilt the oil pump early in the restoration process, and everything seemed to work fine. I primed the pump, and the bubbles went away, the rotor turned perfectly.

About a year later now, and I started to doubt myself, if I checked the clearances properly, and so on. So I decided to take the pump apart again - to check all this.

Clearances were good, and I did not forget to put new o-rings in - all good.

But when I’m about to close the rotor house once again, and tighten the three screw I suddenly notice how the whole rotor locks up on me! I tried everything for hours now, and every time I get to the very last part of tightening it gets stuck.

All three screws are evenly tightened, and I can feel exactly when it starts to lock up if I spin the rotor while tightening.

It seems to be the rotors closest to the drive wheel (engine right) that is the problem. And if I only mount the inner rotor it will spin as it should. But as soon as I add the outer again it will lock up.

I’ve inspected the parts closely and they don’t show any odd things. I placed them on a mirror and they seem perfectly flat. The rotor house has some minor scraping, but that was there the last time I assembled the pump as well - nothing new has happened.

I feel I’m missing something obvious here, why is this happening?

[Korven]

Did you check the orientation of the rotors? The inner and outer rotors should have a small mark, both should face the same way (and maybe "outwards", dont take my word on this), not sure if this could cause this issue or not. easy thing that could be worth to check.

[Toby84]

Did you check the orientation of the rotors? The inner and outer rotors should have a small mark, both should face the same way (and maybe "outwards", dont take my word on this), not sure if this could cause this issue or not. easy thing that could be worth to check.

Yeah, they both face the same way (outwards), but I’ve even tried every other combination possible with the same result.

[Nurse Julie]

Did you replace the centre seal and if so, is it seated all the way in?

[Toby84]

Did you replace the centre seal and if so, is it seated all the way in?

No I did not, since the existing one seemed to be allright and I've read here on the forum what a pita it is to replace it.

I really did not do anything more than opening up the rotor to double check the clearances, and then it just wouldn't go back to previous working condition.

As I tighten one of the screws the last part I can feel how something gets to tight, and start scratching (I think..), until it's totally stuck. I tried to advance every screw only a minimum and then go to next screw and so on to place even load on all of them, but to no success.

[PeWe]

Rotors not swapped?
Remove the shaft with rotors to ensure everything sit where it should, the small pin included.

[Toby84]

Rotors not swapped?
Remove the shaft with rotors to ensure everything sit where it should, the small pin included.

How do you mean swapped? From one side to the other? Since they are different width I'm pretty sure they're are on the correct side - or do you mean swapped in any other way?

Yes, I've had the shaft out, and everything seem to be in the right place.
At this point I've even tried to assemble it wrong - just to double check! Obviously that didn't work 

[Flyin900]

I just looked at the CB750 K7 parts fiche and item #4 shows a thin paper gasket that seals the upper and lower bodies together. Did you use an aftermarket gasket here, or reuse the old gasket?
I just went through this with a CL77 305cc motor oil pump. The aftermarket paper gasket from a gasket kit is slightly thinner than the OEM Honda one that was on there. I'm talking a really small difference, yet I experienced the same issue as your getting now.

The pump was fine until you tightened everything down then it was binding somewhat compared to prior to taking it apart.

I used a very thin layer of Hondabond sealant making sure to stay away from the outer edges of the gears/rotor and oil passage areas internally. I coated both sides of the oil pump aluminum body and this was enough to compensate when using the thinner aftermarket gasket. It gave enough clearance to allow proper rotation of the gears/rotor in my pump without any serious drag.

[Toby84]

Solved!!

For some weird reason (I truly don't know why) there had been a damage in the rotor cover (gear side), with a very very small piece of aluminum protruding against the rotor. I did look for such things earlier, but it was so small and I could almost not even feel it with my finger tips. I sanded it down with 2000 grit paper, and assembled the pump again - and voilá!

Thank you for all your input!

[MauiK3]

Good to hear

[Don R]

 Thanks for the report. We all learn something when we hear the solution.

[ZingZing]

Hmm interesting. I’m having the exact same issue and everywhere online people just slap it together and it’s good to go. I need to take a closer look to check for any damage.

Also how easy should the pump rotate? In Hondaman’s book he says it should be really free, able to push with one finger. Un-tightened it still takes a tiny bit of effort to spin though that might be due to some undiscovered damage.

[PeWe]

Not about how free pump rotate....
I have read about too thick gaskets that affect the function.

I oiled the thin paper gasket before assembly.

[HondaMan]

This 'lockup' often happens just from slight misalignment on reassembly.
First, check for the "dots" on the sides of the rotors, and make sure both are on the same side (it really doesn't matter which side, just both on the SAME side).
Then, when tightening the screws, go slowly and evenly on all 3 screws a little at a time and turn the rotor by hand after each 3-screw turn, and when it smoothes out, a little tighter. At the end it will slightly drag, so get some oil into the rotors (turn pump upside down, squirt some in, turn rotors).

I've often found that the little pin isn't [yet] sliding back-and-forth as easily as it should, which causes the bind. Sometimes it was just flipping the pin over the other (original?) way that fixed it up.

[ZingZing]

This 'lockup' often happens just from slight misalignment on reassembly.
First, check for the "dots" on the sides of the rotors, and make sure both are on the same side (it really doesn't matter which side, just both on the SAME side).
Then, when tightening the screws, go slowly and evenly on all 3 screws a little at a time and turn the rotor by hand after each 3-screw turn, and when it smoothes out, a little tighter. At the end it will slightly drag, so get some oil into the rotors (turn pump upside down, squirt some in, turn rotors).

I've often found that the little pin isn't [yet] sliding back-and-forth as easily as it should, which causes the bind. Sometimes it was just flipping the pin over the other (original?) way that fixed it up.

Does it matter the alignment of dots on one set of rotors to the other side? I've tried all sorts of combinations with the rotors and still have the same issue. I'm out of town so I can't take a look at the pump right now but I feel like I'm overlooking something simple.

Also any tips regarding the installation of the shaft seal? I just drove it in but I'm thinking maybe I did not drive it in far enough while trying to be gentle on the rubber.

[rotortiller]

If it were me and it was a gasket thickness issue I would cut a thin piece of paper and laminate that to the gasket or lap the pump elements carefully. It's not rocket science, it's a CB750 clunker being fine tuned lol.

[bryanj]

Shoulnt be a gasket in there, if you fit one rotor end clearance will be too great so droping oil pressure

[HondaMan]

The shaft seal must be slightly below the rotor's surface so it doesn't drag: usually I gently tap it home 'in quadrature' in 8 places around the seal, tapping only on the metal face of the seal from the back side (this is the pressure side, with the smaller rotors).

The punch marks: they should both be on the same side of the rotors in the set. In most pumps I've opened the punch marks were toward the seal on the pressure side and toward the cover on the scavenge side, BUT I have also opened virgin pumps where they were opposite this arrangement - always with the punch marks on the same sides of the rotors in pairs, though. So, I can't say for sure which way Honda meant them to be, or if they just let the technicians figure it out because these were all hand-made pumps. The rotors were NC cut and lapped in an automated fixture(s) in several production lines, in both factories. Honda actually made these pumps, unlike a lot of 750 parts!

[rotortiller]

Shoulnt be a gasket in there, if you fit one rotor end clearance will be too great so droping oil pressure

I think he was talking about the gasket under the pump body being too thin and causing distortion.

[brianc]

Gasket too thick = locked up oil pump.
Been there done that.
Standard gasket is only a few thou thick.