Noticed yesterday the oil in my '84 k-100 was half a litre low. I thought this was odd as it's never leaked and has never used any oil in the year or so I've ridden it. I did a full service about 6 weeks ago with new oil and coolant. So I topped it up and while riding today noticed my foot was wet - pulled over and an oil / coolant mixture was pouring out of my coolant overflow. At least I think it's oil - however when I changed the coolant the overflow jug was full of oily goop which I did my best to clean out but I know I didn't get it all. I changed the coolant because I noticed this despite the previous owner claiming he just changed it before the sale. When I drained the coolant it was as fresh as new. I poured it into a glass beside the new stuff and there was no difference at all - clearly he wasn't lying.

Anyway, I thought that if there was a leak it would go the other way anyway being that the coolant should have some pressure in the system. I'm not a great mechanic but very keen and capable.

Any insight as to what's happening and where I should go from here? Bike was still running perfect when I parked it.

The water/oil pump has most likely blown a seal on the oil pump and the drain hole is blocked so the oil gets pushed into the coolant.
Possibly a blown head gasket but I doubt it.

Thanks Rick, if this is the case will I be able to replace the seal or will the pump need to be replaced?

The seal can be replaced but you need a full kit and possibly a new shaft.  Motobins will have them  https://www.motobins.co.uk/
Pull the pump off first and see how bad things are.

Also, search this forum for information on replacing the pump seal. I'd go so far as to say you'll almost certainly break the first one (we all have!) so buy two....


... yes, I know they're expensive.

I pulled the oil / water pump apart and the seal around the coolant is in perfect shape.. doesn't look like it could have pulled oil back through it... what else could have gotten the oil into the coolant? Where do I go from here?

Thank you

.. and it gets better.. was just undoing the shaft and the shaft itself broke (with almost no pressure applied!) .. will have a look on motobins for this too.

Ah yes, inside of the unit is buggered.. sorry for all the rambling posts..

I see there are no gaskets on motobins for the pump cover itself.. do I make my own? also see the new shaft to replace mine which had the threaded part snap off is about $350! Ouch!!

The threaded part breaking away is par for the course with those water pumps. Motobins have them cheaper https://www.motobins.co.uk/bmw-parts.php?model=K%20Series&bikeref=K100 and you will need a kit at the same time. You might find a good shaft or complete pump but it's unlikely.
I got some parts from them a few days ago and it took 4 days.

yes kevin make your own seals with silicon sealant

have had good results from the recommended dreibond  from the bmw parts sellers  motobins is cheaper than buying local from bmw

but any good sealant like permatex ultra black and some others as well ...as always, use sparingly as over spread can leave silicon to escape into oilways and worse