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03X MT
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Discussion Starter #1
This subject pops up here from time to time and I think I hear my engine making this noise on cold start ups.

Could someone please explain what is going on inside the engine. Like what is "slapping what"
 

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2009 Forester X 4 A/T
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308 Posts
Here you go:

Piston slap is the secondary (sideways or perpendicular) movement of a piston against the side of a cylinder bore where the primary movement of a piston is intended to be parallel (up and down) to the cylinder bore. All piston driven internal combustion engines and compressors have a certain amount of piston slap.

Excessive piston slap occurs when the clearance between the piston and the cylinder bore is too great. The piston to cylinder bore clearance becomes too great either through wear, mismatched pistons and cylinder bores at manufacturing or, a combination of both. The audible noise associated with excessive piston slap is due to the perpendicular impact of the piston against the wall of the cylinder bore. Audible piston slap is typically loudest when the engine is first started up. The pistons then expand with heat reducing the piston to cylinder bore clearance thus, reducing the perpendicular impact of the piston against the cylinder wall and its resulting noise.
 

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03X MT
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847 Posts
Discussion Starter #4
Thanks for the explanation(s). I think I hear the folks at Subaru saying that Boxer engines are prone to this and not to worry "this is normal"
 

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2015 Forester X 6MT 6-Gear Manual
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Can be reduced by using higher Cst oil viscosity or HTHS. Subaru may be more prone due to the piston aspect ratio, where the bore is large compared to the crank throw. Also piston skirt length is being reduced causing more "cocking" of the piston coming off TDC on the power stroke. A Designer can somewhat counteract this slap tendency by offsetting the piston wrist pin from the crank cL. I use Rotella T 10w-30 CJ-4 a great oil for this engine IMO - its more passengercar friendly than most HDEO with its reduced SAPS CJ-4/SM formulation compared to previous CI-4 formulations. Cost about $12/gal. Note a sloppy TB can make a slap noise, as well as overclearanced rod big ends.
 
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