This a really nice example of a good, solid bike, the old three-cylinder Triumph Thunderbird. The owner said he’d found a lot of scum in his coolant, and was worried it was oil. What we found was that where the bottle sits, a lot of of the rubbish from the engine’s internal water system could work its way into it, which it had. So we cleaned the bottle out and flushed then replaced the coolant. We also swapped his fork seals.
We’ve been looking after this bike for over 10 years, and it’s worth noting that as motorcycles age (and this one is now 16 years old…) things like this can happen.