by Jim Kerr

Knock, knock, knock, knock. No it’s not the neighbour at the door. It’s the disturbing sound of piston slap coming from your car or truck’s engine. Is it a problem? Maybe – it depends on whom you talk to. During the last couple of years, I have had hundreds of inquiries about piston slap. Let’s look at what causes the knock, and why it is much more common now than in the past.

In simple terms, piston slap occurs when the piston is forced rapidly against the side of the engine cylinder wall. The more clearance between the piston and cylinder wall, the louder the knock. Controlling piston slap is a complex process. Too little clearance between the piston and the cylinder wall and the parts will score and fail. Too much clearance and you get a knock. It doesn’t help that usually the piston and cylinder are made of different materials and have different expansion rates.

Several features are used in piston design to reduce slap. To keep the piston close to the cylinder yet allow room for expansion, the piston skirt (the part that slides against the cylinder) is tapered – it is bigger at the bottom than at the top. The top of the piston expands more, where the extra clearance is, because of higher heat at the top of the piston. The bottom always remains close to the cylinder.

Pistons are also made oval shaped. The large part of the piston is close to the cylinder, while there is clearance on the smaller sides. As the piston expands, heat is transferred into the smaller sides, so the piston becomes more round. Thus, the large sides of the piston always stay close to the cylinder and piston slap is avoided.

There are several other piston features to counter piston slap, such as offsetting the piston pin position, but I think you get the idea. Too much clearance between the piston and the cylinder and we hear that Knock, knock, knock.

In the past, the sound of piston slap meant trouble. Worn cylinders, damaged piston skirts, or cracked pistons were common causes, and all meant expensive repairs. Now things have changed.

Engine designs have changed to make them more compact, lighter, with less internal friction, and higher revving. To do all this, piston design had to change, and some of the major changes are shorter piston skirts and straight piston skirts. The short, straight skirts allow the piston to rock more in the cylinder, and we hear it as piston slap.

Closer manufacturing tolerances have helped reduce piston clearances and slap, but some engines need more piston clearance to allow for piston expansion. During the first few minutes of operation, the piston can expand several thousandths of an inch, yet clearances are typically in the one to two thousandths of an inch. Fortunately, the cylinder also expands, or we would find pistons seized.

On vehicles built in the last decade, piston slap that occurs for a few seconds on cold start is quite normal. My own vehicle, with only 30,000 km on it has piston slap for about 5 seconds when first started on a below freezing winter morning. Service information from General Motors states “A cold Piston knock which disappears in 1.5 minutes should be considered acceptable”. From experience, I have found that piston slap that occurs only during cold starts and lasts only for a minute or less causes no problems. Just don’t place a load on the engine until the pistons have expanded and the clearance has been reduced.

Speaking of clearance, we normally find piston to cylinder clearance in the .0005 inch to .002 inch range. A human hair is typically about .002 inches thick, so you can see the clearances are very small. Some manufacturers are using special coatings on piston skirts to reduce friction. This enables them to reduce clearances even less and prevent piston slap. Ford V8 overhead camshaft engines use coated pistons; so does the Corvette Z06 high performance engine, as well as other
manufacturers.

A good example to show the advantages of coated piston skirts is the Corvette. Clearance specifications for the coated pistons are from minus .001 inch to plus .001 inch. You read correctly; minus clearances! The piston is actually larger than the cylinder on the skirt sides of the piston. That coating has to be slippery! As the engine warms up, expansion in the cylinder block gives more clearance.

The correct engine oil can help reduce slap. Good oil takes up some clearance and is not easily scraped off the cylinder wall during cold starts. Sometimes, changing oil brands or viscosity can reduce a cold start piston slap.

If you suspect your piston slap is excessive, then there is an easy method to locate which cylinder has the problem. Before starting the engine, remove one spark plug wire and short it to the engine block. When the engine is started, that cylinder has less pressure pushing the piston sideways. If the knock changes, or is gone, then that is the cylinder with the problem. If the knock is still there, try another cylinder at the next cold start. It takes a little time, but it is much better to locate where the problem is before disassembling the engine.

So is piston slap a problem? Not for most engines built in the last decade and if the noise is there only for a few seconds during cold start. If the knock is in an engine of older design, the knock continues, or it is there during acceleration, then engine work is in the future. The worst part about piston slap is trying to explain that a cold start knock on newer vehicles can be normal, especially when you try to sell your vehicle!




About Paul Williams

Paul Williams is an Ottawa-based freelance automotive writer and senior writer for Autos. He is a member of the Automobile Journalists Association of Canada (AJAC).