Piston seizure refers to a situation where a piston gets locked up in the engine cylinder. The seizure then causes the engine to stop running. 

This situation arises when the oil film that acts as a lubricant between the piston and cylinder wall is burnt off, or in some cases, scraped off, and the piston becomes too large for the cylinder. 

Perhaps you're wondering what causes the piston enlargement. Over time, pistons expand due to the friction and heat generated by its movement within the cylinder. 

Several other factors can cause the piston to seize, and we will look at them in detail.

What Causes a Piston to Seize?

The oil film between the piston and cylinder has a high boiling point, meaning, it can withstand very high temperatures. However, if the oil film was too thin or the amount applied was not enough, the piston's skirt surface would begin to rub against the wall of the cylinder. Friction causes scoring marks to appear on the skirt's surface. Eventually, the piston will likely seize.

Here are two factors that can cause the piston to seize, in addition to the factor I stated in the introduction:

1. Not Enough Clearance on the Skirt of the Piston

Clearance is a measure of the piston's degree of freedom of movement. You can apply piston-to-wall clearance in several ways, and these ways have different specs. A misapplication of specifications can cause the piston to seize. 

The clearance may have been either too small as designed by the manufacturer or narrowed just when the engine began to operate. Unlike seizure due to lack of lubrication, piston seizing due to insufficient clearance occurs after the engine works for a while after reconditioning it.

2. Too Small Clearance at the Bottom Skirt End

Clearance issues are sometimes restricted to the lower ends of the cylinder liner. The reasons may be due to incorrect installation, mismatching rings, or even the presence of dirt or residue. A simple distortion at the rings' end will cause the lower ends to contact the cylinder.

How Much Piston Clearance Is Too Much?

Piston clearance is influenced by engine type, application, block material, piston alloy and size, lubrication, and cooling, and so there's no one-size-fits-all answer to this question. 

When you unbox a new piston, one of the things you find is a sheet containing all the piston's critical specifications and measurements. The core spec is the piston-to-wall clearance. If you give too much clearance, the piston will rock inside the cylinder, which can cause damage to the walls, rings, or skirt. If you provide too little piston clearance, there won't be enough room for the piston to expand, making piston seizure inevitable.

Getting the clearance measurement can be tricky. The accuracy of thin blades in measuring clearance is debatable. Therefore, using feeler gauges is a must. It is recommended that you measure the piston's largest diameter and fit it with a clearance as stated by the manufacturer.

The rule of thumb is to allow a clearance of 0.001 per inch of piston diameter. And if the piston was forged, 0.0015 and 0.002 per inch can work. Getting piston-to-wall clearance wrong may just be the beginning of the end for your engine. 

Remember, poor clearance leads to piston seizure, and if piston seizure is poorly managed, the engine may seize. Read here about engine seizure.

How to Fix a Seized Piston

Fixing a piston is quite a task as the repair process is a highly technical one. Speaking of technicalities, it's worth emphasizing that fixing a seized piston isn't rookie stuff. The repair exercise is best left to a specialist. Find an auto mechanic near you. If you're not an expert, you probably shouldn't attempt it. That said, I'll highlight the process and give you a broad overview of what the process entails. 

Steps Involved in Fixing a Piston

1. Reduction of air pressure within the cylinders        

• Remove the spark plugs; it reduces the pressure buildup within the confines of the cylinders.

2. Reduction of the force needed to break the piston(s) free

• Remove all accessory belts. Accessory belts run over pulleys like the power steering pump and the alternator.
• Open the cylinders via the valve covers; this allows you to access the rocker arms.
• Remove rocker arms & pushrods. The force needed to turn the engine is significantly reduced when you remove the rocker arms and pushrods.

When you have done the above steps, you can now turn the crankshaft to free up the pistons. Rotate the crankshaft with the bolt at the circular harmonic balancer. It is tuned in the clockwise direction to avoid loosening the bolt before turning the crankshaft.

Conclusion

A piston is an integral part of the working system of a vehicle. A slight variation in the needed clearance is all it needs for the engine to start working poorly. There is no cut-and-dried rule dictating the exact clearances to use, nor is it stated on the spec sheet. Engine type, experience, desired performance, and engine application will go a long way to help you determine what clearance is suitable. Proper caution should be taken when tinkering with clearance—in fact, only experienced mechanics should attempt to do it.