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Engine Building Fundamentals: Boring/Honing & Torque Plate Honing

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Boring/Honing & Torque Plate Honing

08.41

00:00 - While this course is not designed to teach you how to machine an engine block or engine components, and it's expected that you'll be relying on a machinist to perform this work for you, a big part of engine building will be liaising with your engine machinist and specifying the work that you require completed.
00:19 For this reason, it's important to understand what actually goes into machining an engine prior to assembly so that you can speak the same language as the engine machinist and get the sort of results that you need in order to ensure the finished product performs as expected.
00:37 This will also help you select an engine machinist that has the quality of equipment that you require in order to get perfect results.
00:46 In this section, we'll discuss the main aspects of engine machining and the importance of these processes in achieving a reliable and powerful finished engine.
00:57 We're going to start by discussing boring and honing.
01:00 One of the most critical operations in the engine machining process is the finish of the cylinder bores.
01:07 This may involve boring the cylinders, which is a process used to quickly remove material from the bore walls to suit a larger diameter or oversized piston, as well as honing the bores, which is used to make finer adjustments to the bore diameter to set the piston to bore clearance as well as providing the final finished surface for the piston rings to run on.
01:30 Typically, when we're boring and honing an engine block, we'll be doing so to fit a new set of pistons that are purposefully selected to be larger than the stock board diameter.
01:41 In use, the engine's bores will tend to wear and they may also become scratched or damaged by debris.
01:49 If we were to put a new set of pistons that were the original stock's size, this would mean that we couldn't correct any damage or wear in the bores as the piston to bore clearance would becomes excessive.
02:01 An oversized piston allows the machinist to remove a modest amount of material from the bores and achieve a perfect bore size.
02:11 With this in mind, there's some terminology that's commonly used to describe the new oversized piston.
02:18 A piston that's described as first-over, which is short for first oversize, is typically 20 thou or half a millimetre larger than the stock dimension.
02:28 While a piston that's second over is a piston that's 40 thou or one millimetre larger than stock.
02:36 While it's not impossible, it's rare to go much larger than one millimetre oversized when we're boring the engine.
02:42 It needs to have a specific reason to do so.
02:46 Every time material is removed from the bores, the cylinder wall thickness is reduced and this can weaken the engine block.
02:55 Beyond the specific clearance, the key to boring and honing the engine block is achieving a finished bore that will allow the best possible ring seal.
03:05 The better the rings can seal against the cylinder walls, the more power the engine will make, but at the same time, we'll also be reducing oil consumption and blow by.
03:17 With this in mind, forgetting for a moment about the actual surface finish, we ideally want to start with a perfectly round cylinder bore, but we also want the cylinder walls to be perfectly parallel.
03:30 What I mean by this is we want the bore diameter to be exactly the same regardless whether we measure it at the top, the bottom, or the middle of the bore.
03:39 I'd hope that this should all make sense and seem pretty self-explanatory.
03:44 Essentially, if we can achieve this finish, the rings can do the best job possible of sealing combustion pressure from above as well as minimising oil leakage.
03:55 Of course achieving this aim isn't as simple as it might sound.
03:59 And there are a few considerations here.
04:02 Firstly, what we're really interested in is that the cylinder bore is perfectly round and parallel during engine operation.
04:10 It's all well and good to achieve this with the bare block sitting in the boring machine.
04:15 However, we'll often find that the stresses involved with installing and tightening the cylinder head for example may actually distort the engine block and result in a cylinder that's no longer perfectly round.
04:28 This is one of the reasons why torque plate honing is a common technique used on many performance engine builds.
04:35 Torque plate honing uses a thick, solid steel plate that can be torqued down on the engine block to replicate the stresses and distortion that will be present when the cylinder head is installed.
04:48 To allow the cylinders to still be machined, the torque plate has holes machined into it above each cylinder, to allow the boring bar or hone to pass through.
04:59 For the best results with a torque plate, it's important to replicate the exact installation you'll be using on the completed engine.
05:08 This means the torque plate should be installed with the head studs or bolts that you're actually going to use, as well as the head gasket too.
05:16 This will ensure that the distortion in the engine block is exactly the same as when the engine is assembled and running.
05:24 Continuing this theme, it's not uncommon in some high-end race engine builds to run heated fluid through the cylinder block's water jacket to ensure that the cylinder bores are machined at the normal temperature the engine will be operated at.
05:39 This is known as hot honing.
05:42 These procedures simply aim to ensure the most accurate possible bore dimensions under real engine operating conditions.
05:51 When specifying the bore dimensions with an engine machinist, the key measurement that's critical is the piston to bore clearance.
05:59 As its name would suggest, this is simply the clearance between the skirt of the piston and the bore wall and will be specified by the piston manufacturer if you're using aftermarket pistons, or the O.E. specifications if you're using stock components.
06:15 An important consideration here is that forge pistons will usually require a larger piston to bore clearance than a cast piston, since they tend to expand more as the piston heats up.
06:28 We'll cover this specification in detail further in the course.
06:33 Before we move on, I'll briefly discuss the actual bore finish too.
06:38 Again, this is a task performed by the engine machinist and hence, we don't have a lot of control over it.
06:44 However, it's still worth having a basic understanding of what it is.
06:48 The bore finish or hone pattern is critical to achieving a good seal with the piston rings, as well as creating minimum wear.
06:57 The common technique used to hone cylinders is called plateau honing and is a multi-step process.
07:04 The cylinder is initially honed with a coarse stone, which provides a coarse crosshatch pattern with sharp peaks and valleys.
07:13 Following this initial hone, a finer stone is then used to remove only the rough peaks off the hone pattern.
07:21 The result is a smooth surface that allows the rings to bed quickly and achieve a good seal.
07:27 However, the remaining valleys of the hone pattern still provide sufficient oil retention for proper lubrication of the rings.
07:36 The hone pattern needs to be a careful balance between a smooth surface that provides good ring seal and a coarse enough pattern that can still retain lubricating oil and correctly lubricate the rings as they pass.
07:52 During the honing process, care needs to be exercised by the machinist to ensure that the bores remain parallel.
08:00 It's possible to focus the hone for too long in one part of the bore, and produce a taper or bowing in the bore that will affect the ring seal.
08:10 Of course, skilled machinists with quality, modern machinery will be able to provide you with the best possible finish.
08:18 So, by the end of this module, you should understand what the boring and honing processes do and what we're trying to achieve with these two tasks.