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Engine Building Fundamentals: Measuring Piston to Head Clearance

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Measuring Piston to Head Clearance


00:00 - Measuring the piston to head clearance, as we've discussed in the last module, is simply a case of adding the deck clearance and the compressed thickness of the head gasket together.
00:10 The thickness of the head gasket should be provided by the gasket manufacturer, so this specification is relatively easy to source.
00:19 There is, however, a lot of debate as to the correct way to measure the deck height and we'll discuss this now.
00:26 The deck height is simply the distance between the deck surface of the engine block and the crown of the piston when it's at TDC.
00:35 The deck height may be a positive value if the piston sits below the deck surface, or negative if it protrudes out of the block.
00:44 The issue is that as we've already discussed, the piston can and will rock in the bore and as it rocks from side to side, this also affects the deck height measurement.
00:56 For example, let's assume that we have an engine with zero deck height where the piston is flush with the deck surface at TDC.
01:04 Now if we push down on the piston on the intake side of the crown to cause it to rock over, we'd find that the piston crown on the exhaust side would now protrude out of the block.
01:16 At the same time, the piston crown on the intake side where we've just pushed down will now be lower than the deck surface.
01:24 There are a variety of techniques employed to measure deck height including rocking the piston in both directions and averaging the results, or only measuring the maximum protrusion when the piston is rocked over.
01:38 I tend to simplify this by measuring the piston height directly above the wrist pin where the deck height isn't affected by this piston rock.
01:48 This means we're only making one measurement and there is nothing to average and no error to creep in.
01:54 The reason I take this approach is because at a proper operating temperature, the piston will have expanded and hence the piston to bore clearance will be tighter than when we're checking at room temperature.
02:07 In turn, this means that the amount of rock we witness when the engine is at room temperature on the engine stand will be reduced significantly in operation.
02:18 Secondly, we're basing the piston to head clearance that we require on the way we're making this deck height measurement.
02:26 In other words, my suggested clearance takes into account that there may be a slight reduction in clearance when the piston rocks over.
02:34 If you want to try measuring the maximum protrusion when the piston's rocked over and use this as your deck height, my recommendations won't be relevant as they'll be excessive.
02:46 So now we know what we're going to measure, we need to know how to actually make this measurement.
02:51 The most accurate way of making this measurement is with a tool known as a deck bridge.
02:57 As it's name implies, this tool is shaped like a bridge that spans across the bore.
03:03 It has a provision to locate a dial gauge and this gauge can accurately measure the difference in height between the deck surface of the block and the piston.
03:13 To use the deck bridge, we need to first zero the dial gauge on the deck surface.
03:19 Once the dial gauge is zeroed, we can then span the bore and locate the dial gauge on the crown of the piston.
03:26 We want to centre the dial gauge in the middle of the piston above the wrist pin as I've already explained.
03:33 However, most pistons will have some amount of dome or dish in the centre of the piston, so we want to offset the dial gauge towards the edge of the crown.
03:43 The reading on the dial gauge will indicate your deck height.
03:47 Remember that this may be positive or negative depending whether the piston crown is sitting above or below the deck surface.

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