First-time engine builders and those working on mass-produced powerplants commonly use off-the-shelf pregapped piston rings. However, high-performance engine builders seem to spend a lot of time fiddling with ring gaps. Why? Because the piston ring is charged with the critical task of sealing the cylinder, and the endgap of the ring is the biggest leak in that seal. While it doesn't take an engineering degree to recognize that the ring endgap represents a leak, the question is just how much horsepower is really blowing out though those little gaps? File-fitting the rings to a prescribed endgap specification has become standard practice with high-performance engines, and we have a hunch that these experts aren't just wasting their time.
Ring endgap in a running engine doesn't remain constant, because the heat of combustion and friction cause the ring to expand and contract. As the ring expands, the endgap narrows to something less than the installed measurement. The characteristic closing of the ring gaps during operation is generally a good thing unless the endgap closes entirely or the ring ends butt together, which can cause numerous problems, from scuffing of the piston or bore wall to loss of ring seal, destructive engine failure, or grenaded pistons. Standard pregapped piston rings come packaged for a specific bore size with gaps conservatively set on the wide side to minimize the potential for destructive endgap butting. For popular applications, oversized file-fit rings are available that allow the engine builder to custom-tailor the ring endgap.
File FitsFile-fitting rings is a fairly simple procedure. It's simply a matter of squarely inserting each ring into the engine's bore, measuring the endgap with a feeler gauge, and adjusting the gap by filing or grinding until the desired gap is achieved. There are a variety of tools that can be used to open the ring endgap, from simple hand files, to inexpensive hand-operated cutting wheels, to more elaborate precision electric ring grinders. Whatever the method used, the goal is to create a square and parallel gap with the desired clearance, which requires a reasonable amount of skill and care. Typically, each ring is sized and assigned to the specific bore in which it will be run, accounting for any minor variations in actual bore sizing, which can affect the endgap.
Ring manufacturers provide various endgap recommendations, which vary according to application. Generally, as the heat and load expectation becomes greater, such as with nitrous or supercharging, the ring endgap recommendation gets wider. This is to compensate for the increased heat and expansion the ring will be subjected to in these applications.
Conventional wisdom says that top rings should be set to the tightest endgap under the anticipated operating conditions that doesn't cause butting to produce the best power. Clearly, there is a balancing act involved in deciding what endgap will provide optimal power without the danger of going too tight. Piston design can also play a role in the amount of expansion the ring will see. Ring placement, land design, and the piston's material and thermal conductivity all affect the operating endgap. Unless you have the resources to test and try specific endgap settings for a particular combination-a luxury few can afford-it's best to stick to the ring maker's recommendations. As noted, piston design can play a role as well, and ultimately, if the piston maker has specific recommendations on endgap, these should take precedence when gapping rings.
By Steve Dulcich
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