If at room temperature there was no gap between the ends of the piston rings, what would happen when the rings’ temperature was elevated to 180☌? The ring would have no room to expand as it heated up and it would get tight to the cylinder bore, start to want to stick, scuff the cylinder bore, and wreak havoc on your entire engine! This example and explanation is the reason it is so critical to check end gap of the piston rings when assembling a new engine. It’s time to revisit what I said last week about thermal expansion and how parts expand at different rates. Running your engine with too little end gap is a recipe for disaster. Now what if the situation was opposite what I just described? What if the rings had no end gap? Well, the outcome would have been much worse. Had I decided to run with the rings that had that excessive end gap it probably would not have resulted in a failure. The right cylinder simply would have been down on power since most of the gas being compressed could escape and the cylinder would have low compression. Next, I found that the right hand piston came with the wrong rings! I tried assembling the rings onto the piston and noticed that the gap was almost four times as much as the spec’d value given by the piston and ring manufacturer. First, I noticed that my gearbox was improperly shimmed by the company that back cut my gears.

It was tempting to skip all the steps I would call critical to the success of a good engine build, but I’m sure glad I didn’t. The first time I assembled my Kawasaki H2 two-stroke triple engine I couldn’t wait to hear it run. Checking ring end gap when putting a new engine together or rebuilding an old one is critical because it will determine whether your engine runs flawlessly for countless hours or if you just wasted a whole bunch of money on a bunch of parts that are about to become scrap the first time you run the engine hard.

What I want to share with you is why it is important to check ring end gap. Most of you are probably familiar with the practice of checking piston ring end gap when assembling an engine. Often times a special oil control ring is used to perform this function and is located furthest from the piston crown.

Lastly, on four-stroke engines the rings act as a seal between the combustion chamber and the engine oil. Secondly, the rings provide a means for heat to transfer from the piston to the cylinder wall and then to the cooling system. The rings’ primary function is to seal the combustion chamber and keep the pressure that builds on the compression stroke from escaping.

Pre gapped piston rings