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Practical Engine Building: Measuring Piston Deck Clearance

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Measuring Piston Deck Clearance


00:00 - The piston deck clearance is the measurement between the crown of the piston at top dead centre, and the deck surface of the cylinder block.
00:08 This measurement or clearance has a large impact on the clearance between the piston crown and the underside of the cylinder head, as well as the piston and the valves.
00:17 And it also affects our compression ratio.
00:21 This clearance can be measured by taking into account aspects of the engine design such as the deck height of the block, the stroke of the crankshaft, length of the connecting rod, and also the compression height of the piston.
00:35 However I always find that it's much more accurate to directly measure this, as minor changes to aspects such as the deck height of the block that could occur during engine machining where the block has been skimmed or surfaced multiple times, can add up to affect our final clearance.
00:54 In order to measure the piston deck clearance, I'm using magnetic deck plate, which we can see here fitted to a Toyota 2JZ engine block.
01:03 The deck plate holds a dial indicator, and what we can do is locate the dial indicator over the actual engine block which we can see we've done here.
01:13 And then we can zero the dial indicator.
01:15 What this means is that any time the dial indicator is reading zero, this is still the same height as the deck surface of the block.
01:23 Once we've done this and we've zeroed the dial indicator, we can move it over our piston.
01:28 Once we've done this we can now move our piston up towards top dead centre slowly, and what we want to do is watch for the point where the dial indicator stops, reaches its maximum, and starts moving back down.
01:41 So you can see we're just coming though zero now.
01:44 And this actually has a small amount of protrusion where the piston is actually sticking out of the block at top dead centre slightly, and we can see that where we peaked there was just below 5000ths of an inch.
01:55 So what this means is that in this particular case, the piston is actually protruding from the block by 5000ths of an inch or almost 5000ths of an inch at top dead centre.
02:08 Now there are a few considerations as to how we can take this, the deck clearance measurement, and we discuss these in more detail in the Engine Building Fundamentals Course.
02:18 My personal preference is to take these measurements in line with the wrist pin of the conrod.
02:24 Now this prevents or eliminates the rocking that we can see that can influence the deck clearance reading.
02:31 We do need to be a little careful though because obviously while our 2JZ is a flat top piston making this very easy, if we have a domed or dished piston, this will affect where abouts we can locate the dial indicator in order to get a true reading.

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