What is a SUBARU interference engine?

June 10, 2011. Author:

So what does it mean if a SUBARU engine is 'interference'?  SUBARU engines are often designated as interference or non-interference. 

Interference means the valves and pistons can contact, or interfere, each other.

In a SUBARU interference engine, the possibility exists of the pistons and valves in a cylinder coming into contact with each other.  Suppose for example the timing belt or timing chain breaks or slips or jumps timing.  Then the piston rising in the cylinder may hit one or more valves that are not completely closed.  (Contrast with the SUBARU non-interference engine, where this type of contact cannot occur).

Also the intake and exhaust valves in a cylinder may be interference

On engines with separate camshafts for the intake valves and exhaust valves, such as many SUBARU DOHC (dual overhead cam), it may also possible for the intake and exhaust valves to hit each other.

How to repair bent SUBARU engine intake and exhaust valves

To repair bent SUBARU valves generally requires the cylinder head to be removed.  In some cases, mainly on earlier models, the cylinder heads may be removed without removing the entire engine from the vehicle.  This ability is due to SUBARU's continued use of a longitudinally mounted engine.  Many other vehicle makes, mainly on FWD (front wheel drive) passenger cars, the engine is mounted in a transverse fashion making access to the cylinder head nearest the firewall rather difficult.  If any of the pistons were dinged from hitting the valves, further engine disassembly such as splitting the crankcase is required to replace the damaged piston(s).

Cost of repairing bent valves

The cost of these repairs at a SUBARU dealer is often USD$1000 on up (emphasis on the 'on up' part).  Therefore, on interference engines with a timing belt, the recommended timing belt change interval needs to be given attention.  This information can be found in the SUBARU owner's manual.  The timing belt change intervals often include a time limit as well as a mileage limit.  Heat is a major enemy of the timing belt.

Which SUBARU vehicles typically have inteference engines

Interference engines are generally found on later model SUBARU vehicles.  Interference designs can have a higher compression ratio and other characteristics desireable to vehicle manufacturers.  SUBARU vehicles manufactured up to and including 1996 typically non-interference SUBARU engines.  The 1996 2.5L phase I DOHC engine is likely non-interference piston to valve, but interference between intake and exhaust valves.

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