1985 K100 RS water pump impeller. Pressed steel vs old cast iron.

Just wondering if there is any point at all to upgrading the water pump impeller from the old style cast iron unit to the newer pressed steel type? It certainly looks like the new style would circulate coolant better.

I couldn't find anything specifically about the impeller, but did find this thread that hinted at the '85 pumps having an issue. It was more of the shaft as the problem. Will be nice to hear from owners as to what they've done.

https://forums.bmwmoa.org/showthread.php?26699-water-pump-life-span-k100

I think this is a "if it's not broken, don't mess with it" scenario. If it's not leaking, I'd be hesitate in pulling it apart as there are a lot of problems with new oil and coolant seals on old components. It seems that most people end up buying a whole new unit because replacing the seals on the worn/pitted shafts doesn't work.

Now, if you're having a problem then it's a different story...

I just went through the photos I took this past June when I installed my temp gauge and wiring. My impeller is attached with the external nut. As it isn't leaking I'm not going to pull it apart. If I do eventually have a coolant or oil leak (I've jinxed it now!) I'll decide then if I want to rebuild or replace. The only concern being that if that is a few years down the road will the parts still be available?

If memory serves, the original early models had the impeller held on with a nut on a threaded extension of the shaft. Subsequent shafts had a tapped hole and used a bolt to secure the impeller. I understand problems were occurring with the threaded end of the shaft breaking off. If it hasn't happened to 30+ years you are probably ok until you have to get in there to deal with the seals.

Frank

And what was the change in drive gear teeth all about?

I'm not aware of any changes to the drive gear teeth. Those gears are the oil pump. I've rebuilt mine at least a couple of times. What happens is the impeller shaft corrodes between the oil and coolant seals. There is probably a 10mm space between them that is actually exposed to the outside through the weep hole in the bottom of the oil sump. When the corrosion can reach the seals, either coolant or oil, a leak will commence. I've tried filling the corroded portion (weld and epoxy) with dismal results. Best in my opinion is to bite the bullet and get new parts during an overhaul. I can't say which would be cheaper rebuild or new replacement assembly but my feeling is that getting the parts would be cheaper if your basic housing is in good condition.

Frank

There are two different pump designs with different rotor diameters and different tooth counts. K75 and K11 are different, maybe? From there you can see which other K models get which.

All of which reminds me that I have a water pump article I've been meaning fo finish for years.