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Practical Engine Building: Mechanical Failure

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Mechanical Failure

04.57

00:00 - Mechanical engine failures are something nobody wants to experience, however they are a very real risk.
00:06 Particularly if you're beginning to push an engine to produce more power than the factory ever intended.
00:12 In most instances, a mechanical failure is going to make itself known pretty quickly, often in a fairly spectacular fashion.
00:19 So this area of failure analysis doesn't need a lot of explanation.
00:24 There are however situations where it may be less than obvious what has caused a failure to occur.
00:30 For example on face value a broken conrod may seem like a pretty straightforward component failure event, however the failure may be the result of several different causes.
00:41 Firstly it may be an assembly error where the rod bolts were not correctly tightened, leading to fastener failure.
00:49 Another common reason for a conrod to break is due to a lack of lubrication and then the conrod big end heat seizing to the crankshaft journal.
00:57 This has the effect of snapping the conrod in half and then punching a hole in the side of the block.
01:04 A third option here might be that the rings have butted together and the conrod has then subsequently pulled the crown off the piston and punched the remaining parts of the broken piston and conrod out through the side of the block.
01:18 As you can now see, what might at first seem obvious, may require further inspection to deduce exactly what the reason for the failure was.
01:28 While nobody likes an engine failure, it's futile if we can't learn from the failure, and prevent it from happening again in the future.
01:36 The key here when investigating a component failure, is to get a full understanding of the failure, while viewing each component involved, and how they've interacted, so that we don't jump to any conclusions.
01:50 There are two main reasons for mechanical failures, and these include operating engine components beyond their design limits, and fatigue related failure.
02:00 Each component in the engine has a strength limit whereby the component will fail if we exceed that limit even one time.
02:09 In this case the failure is instantaneous.
02:11 The material that the engine components are manufactured from also fatigues over time, as it's exposed to stress however, and this can have the effect of reducing the amount of stress that the component can withstand.
02:26 This is why we may see a component that's been in an engine for three seasons of racing, suddenly fail under the same operating conditions that it's happily put up with for its entire life so far.
02:39 Of course if you're operating a long way away from a component's ultimate strength limit, then you may find that the component will never fail, regardless how much use the engine is subjected to.
02:51 This may be the case for example, if you selected a set of conrods that were rated to perhaps 300 horsepower per conrod, and a maximum RPM sealing of 9000 RPM, yet your engine was only making 200 horsepower per cylinder while revving to 7000 RPM.
03:09 In this case there's just so much head room in terms of component strength that you're likely to never experience a fatigue related issue.
03:18 The fatigue limit will also depend on the particular material and as an example, aluminium fairs much worse than steel.
03:27 This is why aluminium conrods are generally only used for applications such as drag racing, where they can be replaced before suffering a fatigue failure.
03:37 One of the common areas where fatigue related failures can originate, is in the fillet radius of the crankshaft journals.
03:45 The crankshaft is a highly stressed component of the engine and the stresses tend to be concentrated in these areas.
03:53 Aluminium pistons can also be prone to stress cracks forming around the pin bosses, which can lead to failure if they aren't picked up in time.
04:02 Avoiding component failures starts by ensuring that the components you're fitting to the engine, are designed to support the sort of power, and RPM you're expecting from them.
04:12 With aftermarket components this is normally reasonably easy as the manufacturer will be able to guide you on the suitability of a part for your application.
04:21 If you're dealing with factory parts, this can get a little trickier, as you may need to rely on your own experience with what a stock conrod or piston may support.
04:31 Or if you've got no previous experience with a particular engine, your only option may be to research and see what others are achieving on that platform.
04:42 Beyond this it's also advisable to ensure that all components that are being reused, undergo crack testing.