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Practical Engine Building: Dirt & Debris Related Failures

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Dirt & Debris Related Failures

06.05

00:00 - Given the tight clearances that the engine relies on, almost any dirt or debris remaining inside the engine at start up has the potential to do damage.
00:09 I separate this damage into two separate areas, dirt and debris, and foreign objects, and we'll discuss both separately.
00:19 Dirt and debris are most likely to result in damage to the bearings, crankshaft journals, or potentially the skirts of the piston and the cylinder walls depending where exactly the debris is located.
00:32 The most likely situation is where a small amount of dirt and debris remains inside the engine and is then distributed through the bearings by the oil pump and lubrication system.
00:44 The result of this damage will depend on the amount of debris and how long the engine is run before the problem is identified.
00:52 In minor situations the debris may become imbedded into the soft top layer of the engine bearings.
00:59 This will be noticeable by small particles being visible on the bearing surface.
01:05 You'll typically also see scratches in the bearing surface in the same direction as the crankshaft rotates and possibly matching scratches on the journal surface where the debris has passed through before becoming imbedded in the bearing surface.
01:21 Under more severe conditions where the dirt or debris is larger or alternatively there is more of it, this may result in severe damage to the bearings where the overlay on the bearing is heavily scored revealing the underlying copper layer.
01:37 Initially this'll be visible as deep scoring that reveals the copper coloured layer beneath.
01:43 However it can also rapidly result in the complete failure of the bearing if the engine continues to operate under high load.
01:51 Another potential failure mode is where dirt or debris becomes trapped behind the bearing shell where it locates in the conrod or in the engine block.
02:02 This has the affect of raising the bearing shell locally and reducing the oil clearance in this point.
02:09 This may show up as localised polishing of the bearing shell or localised wear through to the copper underlay.
02:16 However continued operation under these conditions can rapidly lead to complete bearing failure.
02:23 With this sort of failure there will typically be evidence on the back of the bearing shell of the trapped debris, and this will normally be a localised failure of just one bearing.
02:35 If debris becomes trapped between the piston skirt and the cylinder wall, this will be visible by vertical scoring on both the piston skirt as well as the cylinder wall.
02:45 Often the offending debris will remain lodged in the piston skirt too as evidence.
02:52 Again this is likely to be an isolated problem that only shows up on one cylinder, so it can be identified and separated from other potential causes.
03:02 With the common practice of applying anti friction coatings to the skirt of pistons, many novice engine builders become a little concerned when they notice that this coating becomes marked or even partially removed in operation.
03:17 In particular it's not uncommon to see the coating polished away in the centre of the skirt, particularly on the major thrust face of the piston.
03:27 Granted this will vary from one coating to another, but many of these coatings are designed to wear, and this can be quite normal, and hence nothing to worry about.
03:38 Likewise it's not uncommon to see some minor scratching of this coating as a result of dirt or debris.
03:45 Note here that I said minor scratching, and any time you're seeing scratches that can be felt when you run a fingernail across them, or you can also see debris imbedded in the piston skirt, this is a sign of poor cleanliness during the assembly process.
04:03 On the other hand it's normal for small metallic particles to be removed from the peaks of the home pattern during the initial engine break in process, and these particles can cause some visual scratching to these skirt coatings, and this is not something to worry about.
04:19 Now we'll move onto a potentially more fatal problem, and that's foreign objects either inside the engine, or inside the intake system of the engine.
04:28 It shouldn't need much imagination to work out the potential damage that can come from this, however this doesn't prevent mishaps happening, and avoidable damage from being done to brand new engines because of carelessness or oversights, either on the part of the engine builder, or alternatively on the part of the mechanic or enthusiast reassembling the long motor, and fitting the intake manifold with a nut or something similar trapped inside a port.
04:57 This sort of damage should be very apparent as there will be witness marks where the foreign object has bounced around inside the cylinder and been trapped between the piston and the cylinder head or the valves.
05:09 It's also possible for it to damage valve seats and bend valves depending on the size of the object and how unlucky you are.
05:18 To avoid these potential problems, takes nothing more than an eye for detail and a thorough cleaning regime, as outlined in this course.
05:28 Cleaning is one of the most critical, yet often overlooked tasks an engine builder is responsible for, and fortunately it's not difficult and doesn't require any expensive equipment, so there's really no excuses for doing it properly.
05:45 As with every aspect of engine building, never take anything for granted and ensure that every part being fitted to the engine is cleaned and thoroughly inspected.