Timing belts are typically only used on overhead camshaft engines. Timing belts are used to synchronize the crankshaft with the camshaft, similar to a timing chain on an overhead valve (pushrod) engine. Unlike a timing belt, a timing chain will normally last the life of the engine without needing service or replacement. Timing belts use raised teeth to mesh with sprockets to operate the valve train of an overhead camshaft engine.
Whenever a vehicle with an unknown service history comes into your repair facility or is recently purchased, here are some points that should be asked to help prevent costly engine damage:
Engines, chain- or belt-driven, can be classified as either free-running or interference, depending on what would happen if the piston-to-valve timing is disrupted. A free-running engine is designed with enough clearance between the pistons and valves to allow the crankshaft to rotate (pistons still moving) while the camshaft stays in one position (several valves fully open). If this condition occurs normally, no internal engine damage will result. In an interference engine, there is not enough clearance between the pistons and valves to allow the crankshaft to turn without the camshaft being in time.
An interference engine can suffer extensive internal damage if a timing belt fails. The piston design does not allow clearance for the valve to be fully open and the piston to be at the top of its stroke. If the belt fails, the piston will collide with the valve and will bend or break the valve, damage the piston, and/or bend a connecting rod. When this type of failure occurs, the engine will need to be replaced or disassembled for further internal inspection; either choice costing many times that of replacing the timing belt.
The average replacement interval for a timing belt is approximately 60,000 miles (96,000 km). If, however, the timing belt is inspected earlier or more frequently than suggested, and shows signs of wear or defects, the belt should be replaced at that time.
Inspect both sides of the timing belt. Replace the belt with a new one if any of the following conditions exist:
If none of these conditions exist, the belt does not need replacement unless it is at the recommended interval. The belt MUST be replaced at the recommended interval.
Removal & Installation
2.5L DOHC Engine
When servicing the timing belt, note the following:
- Remove all necessary components to gain access to the timing belt.
- If equipped with manual transmissions, loosen the 2 timing belt guide mounting bolts, then separate the guide from the engine block.
If the directional arrow and alignment marks on the timing belt are faded, and the belt is to be reused, remark the belt with white paint or a grease pencil as follows:
Using a Subaru tool No. ST-499987500 Crankshaft Socket, or equivalent, installed on the crankshaft sprocket, rotate the crankshaft until the crankshaft sprocket, left-hand exhaust camshaft sprocket, left-hand intake camshaft sprocket, right-hand intake camshaft sprocket and right-hand exhaust camshaft sprocket timing mark notches are aligned with the respective marks on the belt cover and engine block.
Make alignment and/or arrow marks on the timing belt in relation to the sprockets as indicated in the accompanying illustration.
Z1: 54.5 tooth length
- Using a Subaru tool No. ST-499987500 Crankshaft Socket, or equivalent, installed on the crankshaft sprocket, rotate the crankshaft until the crankshaft sprocket, left-hand exhaust camshaft sprocket, left-hand intake camshaft sprocket, right-hand intake camshaft sprocket and right-hand exhaust camshaft sprocket timing mark notches are aligned with the respective marks on the belt cover and engine block.
Loosen the center bolt from the timing belt idler pulley, then remove the idler pulley from the engine block.
WARNINGAfter removing the timing belt, DO NOT rotate the camshafts. Damage to the valves may occur.
Carefully remove the timing belt from all of the sprockets.
Remove the automatic belt tension adjuster assembly as follows:
- Remove the 2 timing belt idler pulleys, as indicated in the accompanying illustration.
- Loosen the automatic tension adjuster assembly mounting bolts, then separate the adjuster assembly from the engine block.
Inspect the camshaft and crankshaft sprocket teeth for abnormal or excessive wear or scratches. Ensure there is no free-play between the sprocket and the key. Inspect the crankshaft sprocket sensor notch for damage or contamination with debris or dirt.
NOTEWhen preparing the automatic tension adjuster assembly for installation, adhere to the following points:
Always use a vertical press, rather than a horizontal press or vise, to depress the adjuster assembly rod
Prepare the automatic timing belt tension adjuster assembly for installation as follows:
- Position the adjuster assembly in a vertical press.
- Slowly depress the adjuster rod with a force of 66 lbs. (30 kg) until the hole in the rod is aligned with the hole in the adjuster cylinder housing.
- Insert a 0.08 in. (2mm) diameter stopper pin or Allen wrench through the hole in the cylinder housing and rod, then slowly release the press force from the adjuster rod.
- Install the adjuster assembly onto the engine block.
- Install timing belt idler pulley No. 2 on the engine block.
- Install the timing belt idler pulley No. 1 on the engine block.
If the camshaft and crankshaft timing marks are no longer aligned, perform the following:
Position the crankshaft sprocket so that its mark is aligned with the mark on the oil pump cover on the engine block.
- Align the single line mark on the right-hand exhaust camshaft sprocket with the notch on the belt cover.
Rotate the right-hand intake camshaft so that the single line mark is aligned with the notch on the belt cover.
NOTEAt this point, the double line marks on both right-hand camshaft sprockets should be aligned.
- Turn the left-hand exhaust (lower) camshaft counterclockwise (as viewed from the front of the engine) until the single line mark is aligned with the notch on the belt co
- Position the crankshaft sprocket so that its mark is aligned with the mark on the oil pump cover on the engine block.