Pistons: What caused them to fail? How can I tell?

There are three basic reasons for piston failure: Defects, overheating, and detonation.

Defective pistons: A typical piston failure that is indicative of a design or manufacturing flaw almost always includes evidence of cracking. If it is a design issue, the crack will usually start at the top of one of the wrist pin bores where the crown is thinnest and propagate across the crown of the piston. The combustion gasses usually burn a hole through the crown before the crack gets to the other side, causing a loss of compression. When compression is lost, the piston loads decrease substantially, and as a result the crack stops propagating. See an example of this in the image below. This crack started at the right and by the time it got to the far side of the piston, the combustion gasses burned a hole through the top about 1/3rd of the way over from where the crack started:

When a crack is found that starts somewhere other than at the top of the wrist pin bore, it is usually a manufacturing defect. The crack will start wherever the defect is located, which could be random. One thing to note about either type of crack-related failure is that it is rarely isolated to just one piston.

That is because pistons are manufactured by many hundreds if not thousands at a time. The resulting problem usually affects many, many engines—not just one piston in one engine.

Overheating due to lack of cooling: This is when an engine gets too hot due to insufficient cooling. This could be due to a failure of a component and sudden loss of coolant, a clogged radiator, or a broken water pump belt for example. Overheating of this type usually causes the piston skirt to seize in the cylinder, causing uniform damage on the skirts– which is where the piston/cylinder clearance is tightest (.0015-.0035” diametrically on a GoWesty water boxer), and looks like this photo:

In the case of seizures due to overheating, the oil film breaks down as a result of excessively high temperatures. Initially, this causes mixed friction with individual friction marks. The material at the friction marks then heats up further and the piston loses all lubrication.

Overheating due to detonation: Detonation is also known as “pinging” or “knocking” or “pre-detonation.” Detonation causes a hammering load on the top of the piston, at a rate of around a thousand times per second. It also causes super high combustion chamber temperatures overheating the top portion of the piston. The top of a piston is fitted much more loosely than the skirt because, even under normal conditions, it is exposed to the combustion chamber and therefore gets way hotter than the bottom of the piston that is exposed to the crankcase oil. Because it gets hotter, the top of a piston needs more room to expand. When only the top gets too hot (detonation with good cooling system), it grows larger than the skirt and seizes to the cylinder, which makes it even hotter due to the added friction.  The resulting damage on the piston looks like this typical Google image search result:

Severe or prolonged detonation can cause damage to the pistons. In some extreme cases, the piston ring breaks away.

The cause of detonation is low octane fuel and/or too much ignition timing advance. On a water boxer, it is critical to set maximum advance to no more than factor specifications, which is 35 degrees at 3000 rpm:

This is especially important on larger displacement water boxer engines that have higher compression ratios. The original 1.9 and 2.1 water boxer engines sold in the USA had a compression ratio of around 8.8:1. GoWesty 2200, 2300, 2450 and 2700 engines have a compression ratio of 8.8, 9.0, 9.4, and 10.0 respectively and are therefore more susceptible to detonation. So much so, that we cannot warranty our 2700 unless it is also purchased with our GW-EFI system, which has a knock-detecting system built-in that automatically reduces timing if detonation is detected.

Yeah: GoWesty has your back!