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Piston slap is nothing new to piston driven internal combustion engines and compressors. It 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.

In the case of the famous GM piston slap engine defect, the piston design with hypereutectic (high silicon content aluminum alloy) pistons, reduced or eliminated piston skirts (to reduce reciprocating mass), and a higher ring pack to reduce unburned fuel mixture on the sides of the piston crown have made piston to cylinder bore fit much more critical. The amount of tolerance (variation or margin) in allowable clearance between the piston and cylinder bore to prevent audible piston slap has been reduced by a factor of at least 50%. Consistently hitting the narrower margin for piston to cylinder bore tolerance has not happened for GM during mass production. Thus, some engines have no audible piston slap and some have piston slap on only one or two cylinders. What might have looked really good in testing of hand built engines in the lab hasn't transferred to the production line of this corporate giant.
Make no mistake about it, while a lot of these engines don't appear to be driving rods through the blocks, the ones with louder and longer duration piston slap will wear out before the ones that are basically quiet. The perpendicular heavy impact of the piston against the cylinder wall over time will not come without a price. This is also why GM has released a recent TSB saying that opening 4 quarts of oil to add to your crankcase between a 7,500 mile recommended oil change interval (1 qt per 2K miles on an engine with 36K miles or less is "NORMAL". After 36K miles, all bets are off (there is no abnormal oil usage rate). This is why the now common offer of an engine component letter extending your warranty to 5 years or 100K miles is basically worthless. If the piston isn't laying in the oil pan in pieces, the engine will be operating "NORMAL" according to GM.

For further proof related to the damage audible piston slap can cause, you only need to look at the GMs own TSB # 01-06-01-005. GMs own illustrations will show you.

What Automotive Industry Experts say about Piston Slap


Dr. Victor Wong/MIT (Massachusetts Institute of Technology), is one expert who states otherwise:

Piston Slap. Few technologies have received more engineering attention than the internal combustion engine. Yet engine designers continue to be troubled by a phenomenon known as "piston slap." As a piston moves up and down inside its cylinder it also shifts from side to side, bumping first one side and then the other--a behavior that wastes fuel, wears out engines and makes an annoying bang. A computer model developed by MIT researchers can disentangle the factors that lead to piston slap, helping engineers make design decisions that will reduce its intensity. Given a description of the operating conditions and design of an engine, the model can describe the pathway the piston follows inside the cylinder, the force with which it hits the wall, and even how its shape changes due to the impact. In parallel work, the researchers have validated the model using an operating experimental engine. The team was led by Dr. Victor Wong, a principal research scientist at MIT lecturer in the Department of Mechanical Engineering. The work was funded by Nissan Motor Company. reference:

Bob Hagin/Syndicated Columnist/ Seattle Times disagrees with GMs rational on piston slap:

 The noise called piston slap is caused by one or more pistons having too much clearance between its side skirt and the cylinder walls. In effect, the pistons become too small and wobble in the cylinder bores. It can be cause by an engine simply wearing out (not common any more), a piston seizing because of a lack of lubrication (it runs out of oil) or it's put together wrong. This is easy to check and usually it doesn't happen to all the pistons. But there could be other causes, none of which could be caused by a "wrong" oil filter. Find out what brand oil filter your shop uses and call its service reps and tell them your story.

NWclassifieds/Autos- Research It: Auto Q&A

James E. Harris, proprietor of Engine Restorations in Portland, Maine also disagrees with GMs assertions regarding piston slap:

One way to check for piston slap: Remove three spark plugs, leaving number one in place. Crank the engine over until you feel the resistance of number one piston coming up on compression. Crank against compression until the piston is about half way up the cylinder. Now using the fan, rock the crankshaft back and forth and listen for a metallic knocking sound. If you hear a knock, you have piston slap and the only way out is to rebuild the engine.

Cosworth Performance

Jaguar World Magazine

Western Filter Company

Australian Energy News


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