R75/5 oil pump rebuild question

Hey guys. I have the engine out of my bike replacing all of the gaskets (engine leaks)
When I tried to take apart the oil pump on my R75/5 to clean and inspect it i had a difficult time sliding out the outer ring (sorry don't know the name of it) It kept getting jammed on the way out. I thought this odd until I got it out and discovered that the outer surface of that ring had lines machined into it. Then I looked at the motor and the cavity that it set in had matching lines machined into it. I thought maybe this was how it was built so i looked up used ones on Ebay and found that they are supposed to be smooth. Bummer. I am guessing that at some point a piece of debris got stuck in there and did some machining.
My question- Do I need to do anything about this or will the oil pump still work fine the way it is? I asked two BMW buddies of mine. One said that I need to replace the ring part and take the engine block to a machine shop to have them press a metal ring into the cavity. The other buddy said it should be fine the way it is, just clean the surfaces up some with some emery cloth.
What do you guys think? I already put the oil pump cover back on with a new gasket but I have not put my flywheel back on yet so if I am going to do anything I need to do it now. I have limited funds so the idea of paying a machine shop to fabricate a ring ant installing it scares me. Also what if they do something wrong and screw things up even more?

Looking for advice from more "experts" on this sort of thing.

Are you talking about #10 here:

http://www.realoem.com/bmw/enUS/showparts?id=0260-EUR-01-1971-2476-BMW-R75_5&diagId=11_1973

I don't know that much about the pump as most people never deal with it...seems like they're pretty bullet proof. The way I understand it, the outer ring #10 does not move...the inner rotor is keyed to the camshaft and turns inside and creates the pressure by squeezing the oil as near as I can understand.

If that's the way it works, then the outer ring just needs to be properly placed and seated in the block. It probably should be interference fit. If yours came out or had been pulled out sometime in the past, the matching lines in your case was probably knurling which created a surface mismatch which allowed the ring to remain seated. If it was loose, one way to fix it is as you suggested, machining the cavity back and bushing it with something. If the fit with #10 is close enough, maybe some kind of Loctite would work. I suspect that there must be a critical alignment there.

Here are some pictures

I appreciate your trying to help but that ring definitely should not be locked in place! It needs to spin, that is how the pump works.

Looking for more expert opinions.
Anyone out there rebuild these motors?

Still not an expert ( ) but what causes the outer ring to turn? The inner rotor is keyed to the shaft.

Update: I read an old post by Snowbum on the Airmail list and he says they both rotate relative to each other. Was just curious how the outer one rotated. If that's the case, then those lines indicate some kind of scoring and can't be good.

Snowbum has a short section on the oil pump at the bottom of this page:

http://bmwmotorcycletech.info/flywheelremovalwarning.htm

Hopefully Brook will come along at some point...he rebuilt a '73 R75/5 and had issues with the oil pump...he documents that on this page:

https://brook.reams.me/bmw-motorcyl...l-rear-main-seal-oil-pump-cover-cover-o-ring/

On a bit of a tangent - You mentioned in your intro post that you "may" cafe it a bit... As long as the flywheel is out, consider having it lightened (& rebalanced) - makes it quicker to spin up the throttle, quicker to spin back down (faster braking!), and easier to shift. But less weight may increase vibration if everything else isn't perfect.

20774 said:

Still not an expert ( ) but what causes the outer ring to turn? The inner rotor is keyed to the shaft.

Click to expand...

The lobes on the inner rotor contact and drive the outer rotor. In so doing the spaces between the two rotors fill with oil and the oil is squeezed creating oil pressure.

Ok well I was hoping this forum was more active. I guess I am just going to go half way between both suggestions that I received.
I am going to replace the other rotor with a good used one but leave the housing as is. I have specs. on what everything should measure so I am going to measure everything before moving on. In the Haynes manual that I have in the oil pump section it says "Inspect for scoring" It doesn't however say what to do if you find scoring lol. It doesn't say how much is acceptable or anything, it just says to check for it.

Bill -

I think the Airheads area here is relatively active, but I can't remember anyone ever discussing this...that's why so few have come forward. Something like this I would be sending to someone else...but that's just me and my skills and/or confidence. There are other resources you could consider. First would be becoming a member of the Airheads Beemer Club and getting involved with local Airheads at local tech days...probably doesn't help with your ongoing rebuild. There is a mailing list that is loosely connected to the ABC but one doesn't have to join, just sign up for the emails. It's not a forum, but old school news group exchange via emails. There's Boxerworks also free to join. Of course, there are expert Airhead mechanics out there, Ted Porter, Tom Cutter, and Anton Largiader come to mind. They are in business so probably have little time to deal with questions. Not sure where you're located (nothing in your profile)...maybe there are some resources that are nearby for someone to put eyeballs on where you're at.

Given what Paul said, I can't imagine that any scoring is acceptable. If the inner rotor drives and moves the outer rotor, movement across the score marks doesn't sound like a good long term condition.

Ok so I ordered a used rotor from one of the best know suppliers of vintage BMW parts in the US and he tried to tell me that the outer rotor doesn't spin. Seriously this is discouraging. I know very little about these bikes, I'm not a mechanic, but seem to know more than most of the population of vintage BMW guys out there. Well it seems that way so far anyways. Ugh...

https://youtu.be/MWVyljh8N3c

Well you video seems to prove the outer rotor turns. Would suggest there needs to be a smooth surface on the case.

Who told you it didn't move?

20774 said:

Well you video seems to prove the outer rotor turns. Would suggest there needs to be a smooth surface on the case.

Who told you it didn't move?

Click to expand...

I don't want to say. He is well known supplier of BMW parts, etc. and I don't want to defame his name as he is a good guy and has given me good deals on parts. Also not the point that one person didn't know it,...the point is how few people that are hardcore into BMW bikes know how these things work.