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Engine Building Fundamentals: Piston Ring End Gaps

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Piston Ring End Gaps

08.35

00:00 - Another critical task when setting clearances in the engine is setting the end gaps for the piston rings.
00:07 Unlike production engines, most aftermarket ring packs, that you might likely use will be what's referred to as file-fit.
00:15 This means that the rings are purposefully supplied a little on the large side, which gives the engine builder the ability to file the end of the piston rings to achieve their preferred end gap.
00:27 This obviously begs the question though, what end ring gap should you run? Unfortunately, answering this question is difficult since so much will depend on the application and specifically how much heat will be transferred into the rings.
00:44 The rings obviously expand as they heat up and the aim, at least as far as the top compression ring goes, is to achieve a minimal end gap when the engine is at normal operating temperature.
00:56 When the end gap is minimal, this will result in reduced leakage of combustion gases past the rings, and this adds up to improved compression and reduced cylinder leakage, a reduction in blow-by gases past the rings and reduced oil consumption.
01:12 All of these are obviously desirable traits, however, if we go too far and run the engine with ring gaps that are too tight, this can result in the ring ends butting together as the ring expands, and this can result in catastrophic engine failure.
01:29 It's obviously sensible to proceed with caution and apply a margin of safety to the end gaps that we choose to run.
01:37 Now that we understand that the correct ring end gap will be dependent on the heat the rings are subjected to, we can consider what aspects will affect us.
01:47 In particular, the specific power level of the engine will have a big impact here as the more fuel and air we're combusting, the more heat will be released in the combustion chamber.
02:00 With this in mind, we're going to increase ring end gaps as the engine's power levels rise, and in particular, this means that we're going to need wider ring gaps on engines that use turbo chargers, super chargers, or nitrous oxide in order to increase power.
02:18 Likewise, we also need to give some consideration to the way the engine's going to be used.
02:23 If the engine is being built for street use, for example, then it's usually quite difficult to subject the engine to long periods of wide open throttle operation, and hence the amount of heat the rings are subjected to is somewhat limited.
02:38 For a competition car, however, that may spend extended periods of time at wide open throttle, understandably more heat may be generated, and we need to take this into consideration with our end gaps.
02:51 You'll find that the ring manufacturer will provide a data sheet with the ring set, that gives you their recommendations for ring end gaps for various applications, such as NA, forced induction, nitrous, etc.
03:05 This data is usually presented in terms of a specified end gap per inch of bore, such as, for example, five and a half thou per inch of bore.
03:16 In order to calculate the required end gap, we simply multiply this by the bore diameter in inches.
03:24 For example, let's say we have a bore diameter of 3.366 inches and the end gap recommendation for the top ring is five and a half thou per inch of bore.
03:36 The required end gap would be 3.366 multiplied by 0.0055, which equals 0.0185, which we can round up to 0.019, or 19 thou.
03:55 Once we have set the gap for the top ring, we need to consider the second ring, as well as the oil rails, which fit above and below the oil ring expander.
04:04 The second ring isn't going to see as much heat as the top ring, so on this basis it would be reasonable to expect that we wouldn't need the end gap to be as large.
04:15 What we need to also consider though, is that regardless how well the top ring seals, some combustion pressure will inevitably escape and if the second ring gap is too tight, this may cause pressure to build up between the top ring and the second ring.
04:32 In this case, it can cause the top ring to become destabilised in its ring groove and this is detrimental to the overall ring seal.
04:41 Again, the ring manufacturer will have a recommendation for the second ring end gap, but to prevent pressure building up between the rings, typically, you'll expect this end gap to be larger than the top ring by 2 thou, or more.
04:57 As far as the oil rails go, these are fitted much further down the piston, and hence are relatively immune from the main combustion heat the top ring faces.
05:07 Unlike the top and second ring, the oil control rings do nothing in terms of sealing combustion pressure, and their sole task is to scrape oil from the bore walls.
05:18 The end gap, therefore, is a little less critical than the other rings.
05:23 This doesn't mean the oil rails can be ignored, but typically, a minimum end gap will be specified, and provided the gap is larger than this specification, no further work is required from us.
05:36 If you're building an engine for the first time, and have no prior knowledge of the particular engine, or the rings that you're using, then you are going to want to follow the ring manufacturer's recommendations for end gaps.
05:50 As you could imagine, the ring manufacturer wants to be 100% certain that the rings will never butt together, and hence their recommended gaps are usually a little on the generous side.
06:02 The listed end gaps will be suitable for most applications.
06:05 However, if your specific usage differs dramatically from those listed, it'd be advisable to make a call to the manufacturer's technical department and they'll be able to offer advice specific to your application.
06:19 If you're interested in experimenting with end gaps, often a small, but worthwhile improvement can be had by adjusting the end gaps slightly tighter.
06:30 Understandably, this needs to be approached with some caution though, as getting it wrong can have dire consequences.
06:38 A good guide is to consider the ring end gaps used by the OE manufacturer if you're rebuilding a factory engine with aftermarket pistons and rings.
06:48 If you're taking a naturally aspirated engine and turbocharging it, then an allowance should be made for the additional combustion temperature you're likely to experience.
06:58 Another technique is to make small adjustments of perhaps, 1 to 2 thou from the ring manufacturer's recommendations, and then inspect the engine after it's been run under high load.
07:11 This needs to be done slowly over several engine freshen ups, and there's a technique that's ideal if you're building a lot of very similar engines, and then seeing them back after competition use for a freshen up or rebuild.
07:25 What you're looking for is any sign that there's polishing visible in the piston ring end gap.
07:32 This is indicative that the ring ends are beginning to butt together and rubbing against each other in operation leading to the polished appearance.
07:41 If you're noticing any polishing, then you'd want to increase the ring end gap by 1 to 2 thou to provide a safety margin.
07:49 If, on the other hand, there's no sign of polishing, then it is possible to go slightly tighter again with your ring end gaps, but of course this does need to be done with caution and understanding of the risks involved.