A wet timing belt is similar to a traditional timing belt, although in this instance, it actually sits inside the engine and is run in an oil bath. Much like a typical belt, wet timing belts are made out of rubber, but reinforced with various fibres and coated in fabric. While they are designed to be durable, problems can still occur, and when they do, the consequences can be dire.
One of the main headaches associated with wet belts is that, as they wear, fragments from the outer coating can break off, contaminating the engine oil in which it sits. This contamination spreads through the lubrication system, causing further mechanical damage as it goes. Furthermore, as a belt ages, the outer surface can begin to fray, reducing the belt's width and its ability to grip the pulleys on which it sits. The inside teeth, which are responsible for traction, can also smooth over time. If traction is lost, pulleys may slip, and the engine's timing will change. This misalignment can cause the pistons and valves to collide, resulting in bent valves, damaged pistons, or complete engine failure.
The reliance on correct oil levels also makes wet belts vulnerable. If the engine is low on oil or filled with the incorrect grade, wear on the belt could be accelerated, causing it to soften or crack, meaning your timing belt could snap. Ultimately, having a rubber belt operate within an oil bath, and that oil then being used to lubricate the engine's internals, is asking for trouble in the eyes of many, which is why wet belts have built themselves up a pretty dreadful reputation.
Beyond the risk of internal damage, wet timing belts have developed a bad reputation because of the cost and complexity of maintaining them. Unlike dry belts, wet belts sit within the engine's oil system, which makes access difficult and replacement labor-intensive. Nobody likes having to pay for a timing belt change at the best of times, and adding in more hours of labor is hardly going to win the wet belt design much in the way of support. It's common for such a job to easily run into four figures thanks to countless parts needing to be replaced, and multiple days' worth of labor in some instances.
Manufacturers also give inconsistent guidance on replacement intervals, as it all depends on the exact materials the belt is constructed from. Ford originally claimed its wet belts would last 150,000 miles or ten years, but later advised replacement at 100,000 miles. Despite this, many owners have reported failures well before these figures, even with full service histories.
Such instances have been highlighted in the UK by the BBC show "Watchdog". One couple's four-year-old Focus with 50,000 miles suffered engine failure, leading to a £3,000 bill, while another owner faced a similar cost after a belt failed at 52,000 miles. In the US, a recall affecting over 139,000 Focus and EcoSport models required a redesigned belt assembly to reduce the risk of teeth breaking off. Just these few examples demonstrate perfectly why many are right to be wary of picking up anything with a wet timing belt.