First some background… Most modern engines today are overhead cam engines, which require a long belt or sometimes a chain to drive the camshaft(s). Most Subaru engines today are DOHC, Dual Over Head Cam or SOHC, Single Over Head Cam. The DOHC has four camshafts (two per head) and sixteen valves (eight valves per head) while the SOHC has two camshafts (one cam per head) and sixteen valves. In both setups the camshafts are driven via one timing belt. The timing belt also runs the water pump. The timing belt is driven by the crankshaft.
Subaru Engines are horizontally opposed or boxer engines, meaning the pistons move horizontally within their cylinders. The camshafts are located above each of the two cylinder heads. In the picture above the two heads are located one on the left and one on the right side of the engine. The Timing belt serves the same function as timing chains on other cars, but are quieter, cheaper to manufacturer and do not require lubrication, so most new car manufacturers are using them. The draw back is that they do not last as long as a chain and often need to be changed every 60,000 – 80,000 miles. On newer Subarus, replacement is recommended every 100,000 miles.
Newer Subaru engines are Interference Engines… This means without a properly placed timing belt, internal moving parts will come in contact or Interfere with each other.
So, what does all this mean?
If your timing belt breaks, the engine will continue to turn, but the camshafts stop. If any of the valves are open, and your engine is an interference engine, the piston will slam into the open valve and cause expensive damage. Most modern engines are built with very close tolerances, and to squeeze every last bit of power into a small package, engine manufacturers sometimes create issues with clearances. Subaru is no exception. In addition to valves colliding with the pistons when the timing belt breaks or slips, the intake and exhaust valves often strike each other if opened at the same time. In short, any “valve timing disruption” will likely results in valve damage. The most common causes of valve timing disruption is broken or slipped timing belt do to age and neglect and/or failure of the water pump, tensioner or pulleys that the timing belt rides on..
You can see now why it is so critical to change your engine’s timing belt at the intervals recommended by the manufacturer. Newer Subaru(1998 – 2010 2.5L) timing belts need to be changed every 100,000 miles.
Hundreds now, or Thousands later
It is crucial, that all of the oil seals, timing belt pulleys, tensioners and water pump are meticulously inspected before the new timing belt is installed. Failure of any one of these parts will destroy the new timing belt and thus cause major engine damage costing thousands of dollars.