Piston Slap (what is it?)

[Supracentral]

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.

 

[Junior]

Forged pistons are known for piston slap because the forged metal has a higher coefficient of thermal expansion, and as such "grows" more, therefor must start off smaller. This means that there's a higher piston clearance (a bigger difference in the outside diameter of the piston, and the inside diameter of the cylinder) and allows them to slap when cold.

Piston slap SHOULD clear up as the engine warms. Aluminum expands faster than iron. closing the tolerance.

If the piston clearance is to small however, you're susceptible to what's called a cold seizure. When the piston expands faster than the cylinder, and a clearance fit becomes an interference fit, this is when the whole assembly comes to a grinding halt, you score the crap out of your cylinder walls and pistons. Then you've gotta tear down and rebuild, and set the piston clearance properly this time around. You can also agitate this situation by flogging on a motor before it's warmed up. Particularly if you live in a cold climate.

 

[Adjuster]

I have noticed that along with clearance, the design of the piston is either going to add to, or subtract from how much slap you get when it's cold too.

Longer skirt design pistons tend to slap "less" or sound quieter when they do than the short skirt designs. (In my experiance.)

Trade off is friction. More surface area has more friction. (It should be oiled pretty well by the engine, so this friction is not a bad thing, but when people are trying to get as much power as possible, skirts get very short, or trimmed to the extreame.)

Another down side to minimal skirt design is thermal transfer. More surface area tends to keep the piston cooler as it can transfer more heat to the bore wall via the larger skirt...

Then we come to coatings (Sorry, but could not resist.)
Some say they do nothing, but I'm a believer that they work, and work well. I've also found the clearance specs for bare forged pistons to be on the loose side when your running those same pistons coated. (And there has not been any agreement on what clearance to run to avoid excessive slap.)

I found on the ROSS pistons, fully coated with a thermal barrier on the crown, molydisulfied on the skirts and thermal dispersant on the underside to need a clearance of 4k v/s the 6k reccomended by ROSS. (6K slaps even when warm on fully coated pistons in my experiance.)

Current motor is just a hair under 4k, somewhere between 3.5 and 4. It was bored out to 3.5, then honed for a crosscut, and that takes about half a thousandth off. Also this was done with a tourqe plate, and we found the bore was distorted as much as 4k with the plate tourqed down.... This is huge. It means that the first motor that was done without the plate could have resulted in clearance of as much as 10k in places when it was cut for 6k clearance, and then honed... so really more like 10.5... Slap city my friends.

4th time around was the charm. Tourqe plate, 3.5K cut, 4k honed with coatings and it only slaps when cold, then clears up when fully warmed up.

Remember, this is on a longer stroke motor too, so your results may vary on the stock engine.

 

[IJ.]

Higher the pin the more slap usually.

 

[Adjuster]

Thanks IJ, that helps explain why the stroker is such a cold slapper..

The pin is very high, up into the oil control ring actually.