Where's this oil coming from?

I keep getting a black oil deposit on top of the water in my rad. 2.25
Series petrol. Very little water loss, but any water that is pumped
out after a long hard run leaves a black oily deposit behind.
Compressions fine (apart from the cylinder with a dodgy exhaust
valve), no mayonaise in either sump, rocker or rad.

1. Is it oil? It certainly looks like it, except when deposited by the
overflow it ends up looking like a fine black powder deposit, untill
you get in on your hands whence it behaves exactly like dried on oil

2. Where the hell is it coming from? and if it is oil leaking into the
water, why isn't there water in the oil as well?

Alex

"Alex" <[email protected]> wrote in message
news:[email protected]...
> I keep getting a black oil deposit on top of the water in my rad. 2.25
> Series petrol. Very little water loss, but any water that is pumped
> out after a long hard run leaves a black oily deposit behind.
> Compressions fine (apart from the cylinder with a dodgy exhaust
> valve), no mayonaise in either sump, rocker or rad.
>
> 1. Is it oil? It certainly looks like it, except when deposited by the
> overflow it ends up looking like a fine black powder deposit, untill
> you get in on your hands whence it behaves exactly like dried on oil
>
> 2. Where the hell is it coming from? and if it is oil leaking into the
> water, why isn't there water in the oil as well?
>
> Alex

Dried on oil? Hm. Sounds more like rad corrosion to me.
TonyB