You might be thinking, shouldn’t I be rounding up, not down? Typically recommendations from manufacturers are quite conservative to avoid rings butting in use and damaging their customer’s engines. With a figure like this, we’d be safe to round the result to 15 thou, as it’s very difficult to work to more accuracy than 1/1000th of an inch. As you can see, for a high performance street/strip application, a minimum end gap of 4.5 thou per inch of bore is recommended, so that means if we have a 3.386 inch bore, we multiply that by 0.0045 to arrive at our recommended end gap of 15.2 thou. Luckily you won’t be flying completely solo when it comes to choosing your ring end gaps - the ring manufacturer will provide you with some recommendations like those shown above from JE for our Subaru FA20 piston rings. Of course, the amount of time you spend at wide-open throttle will also be a factor here. A high-boost turbocharged engine, for example, will produce more combustion heat than a naturally aspirated engine, while the combustion of pump gasoline will create more heat than an alcohol-based fuel such as E85 or methanol. The amount of heat the rings will be exposed to will depend on your engine and how you’ll use it. When choosing your ring end gap at room temperature, you need to consider the amount of heat the ring will be exposed to and also the material it’s made of or, in other words, the coefficient of thermal expansion. In this article: How To Choose The Right Ring End Gap | How To Measure Piston Ring End Gap | Adjusting Ring End Gaps | Optimising Your Ring Gaps | Running In | Conclusion A minimal gap also means a minimal path for combustion gases to escape through into the crankcase which can result in a small but measurable increase in power, and also decrease oil consumption. The idea with file-fit rings is that you adjust the gap when the rings are at room temperature ( 20✬) so that when the rings heat up and expand under operating conditions, the gap will be minimal but won’t risk the ends butting together. Let’s start with the obvious question - why don’t aftermarket manufacturers simply supply piston rings that fit, as you’d expect with a factory piston? This is to allow you, the engine builder, to select ring end gaps that suit your application. In this article, we’re going to explore how to select, measure, and adjust your ring end gaps to help achieve a reliable ring seal. When fitting an aftermarket forged piston, however, you will most likely be working with ‘file-fit’ piston rings. This means that once installed, they’re good to go. Whether you’re rebuilding a factory engine with a stock bore size or you are going to an oversize bore and fitting factory oversize pistons and rings, in most cases, the rings come pre-gapped, meaning they’re already set for the bore diameter.
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