The story so far: bought an '84 K100RT that had stopped running because of a faulty fuel pump. Fixed that. Ran it and the oil filled with water. So I overhauled the water/oil pump. Ran it again and the oil filled with water again. In a couple of minutes all of the water was in the oil. Drained the oil and found this:



It's slightly domed, made of thin pressed steel and is corroded. There was no coolant and a fair bit of rust in the cooling system when I cleaned it out.

Is this actually from a K100 and, if so, from where? And, other than a blown head gasket, where else can the water find it's way into the oil?

did you replace the o-rings between the oil/water pump and the engine ?
also , there is a similar o-ring ,well 2 , between the oil sump(absorber gear housing), and the engine , that may be faulty

k-rider

There are expansion plugs in the cylinder head that are between the oil and water. You need to remove the cam chain cover and the cam cover to get at them.

Rick, you deserve the MVP award. I presume that this is the hole in the head behind the timing chain cover you meant:


The welch plug was completely gone. The middle bit I found the other day and the outer bit was sitting here:


The two bits look like this:


A cheap and easy fix. I hope there's not too much collateral damage.

k-rider, thanks for the suggestion. I installed all of the o-rings that came in the Motobins pump kit. I know nothing about the absorber gear housing yet but I may be about to find out.

Thanks again.

Take the front cover off the water/oil pump and make sure to get any scale out. The cover has no gasket and can be sealed by using Permatex RTV Red or 3 Bond and put the sealant on a very clean surface, put the cover on and lightly do the bolts up so the sealant spreads evenly then let the sealant set (1-2 hours) and tighten to 8-10ft/lb

Those Welsh, they really take a hiding. Poor Taffies.

In Americaland they're called freeze plugs.

Ah Two-Wheels-Better, just like a lady's fun parts they go by many names. According to the infallible resource that is Wikipedia they are really a Core Plug. Or a Freeze Plug, or a Frost Plug or an Engine Block Expansion Plug. They're also known as Cup Plugs or Expansion Plugs.

But as they were invented by the Welch brothers of the Welch Motor Car Company I like to call them Welch plugs in honour of the Welch boys.

What you call a lady's fun parts (even in Welsh) is entirely up to you.