Originally Posted By: 440Magnum
Originally Posted By: 1999nick
I see no mention in this discussion yet of the all-to-common Chevy V-8 fault of "piston-slap". This is not present in all Chevy V-8's, and sometimes only affects one or two cylinders. A main cause of this in Chevies can be traced to the fact that back around 1998, they quit the practice of matching piston size tolerances to cylinder bore tolerances as a manufacturing economy. As a result, tolerances sometimes stack up and result in a loose piston; there you have piston slap, especially in a cold engine before the piston warms and expands.
Chevy made engines without piston slap for years. Why some bean counter would force this change and result is symptomatic of the quality problems that have sullied GM's reputation.
Its more than just that. Its really the result of the fact that they went with Hypereutectic pistons about that time to reduce cylinder leakage and thermal loss and improve efficiency, and also moved to shorter piston skirts to reduce friction. HE pistons require a much tighter cylinder-to-piston fit than ordinary cast or forged pistons because they expand less as they heat up than regular pistons, and short skirts allow the piston to "rock" more and so aggravate the problem. There is much less room for error in fitting HE pistons. If you get an ordinary piston on the loose side, it'll warm up and expand very quickly. If you get an HE piston loose or even within spec but on the looser end of the range, it will either take FOR EVER to warm up and quit slapping, or it just won't ever fully quit.
But that's better than fitting one too tight because although they don't expand much, they DO expand. You want to get them loose enough so that if you have an inadvertent overheat, or get them hotter than normal towing a trailer up a long grade (for example) you don't get binding and galling. Most of the OEM engine builders and virtually all the aftermarket HE piston makers are now coating the skirts of HE pistons with a solid lubricant that can take the binding when hot, and allows quiet operation even when cold. My '66 440 is built with some older (non-coated) Keith Black short-skirt HE pistons and they were very carefully fitted right in the middle of the recommended clearance range. It sounds like a Navistar 7.3 diesel when its cold and has audible slap for about 15 minutes after its warm, but I've gotten used to it and at 20,000 miles so far it burns much less oil than any other big-bore v8 I've owned despite the noise.
Piston slap is an annoying noise, but it really is harmless. Virtually zero added wear results from it.
Ford began using hypereutectic pistons for the 1993 model year in the 5.0L Mustang. The cars with the (less desirable) hypereutectic pistons are apparently identifiable by a black oil pan, whilst the older, TRW forged slugged engines had grey pans.
They don't slap.
GM began "bulk fitting" pistons to bores at the same time the piston slap issue came about with the LSx engines. AFAIK, this is what is regarded as the cause. The pistons are no longer hand-fitted and so the clearances vary bore to bore, piston to piston.
They later went to coated skirts (like Ford used in the Modular engines) to help deal with the problem.
Other than being annoying, it apparently has no affect on engine life expectancy as far as I know.