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Engine Building Fundamentals: Piston to Head Clearance

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Piston to Head Clearance


00:00 - The piston to cylinder head clearance, is simply the clearance between the piston at TDC, and the deck surface of the cylinder head.
00:08 It's really just a combination of the deck clearance, or how far above or below the deck surface of the block the piston sits, and the compressed thickness of the head gasket.
00:19 It might seem like any amount of clearance is sufficient provided the piston doesn't touch the head under static conditions, however as usual there's a little more to consider.
00:30 First of all we always end up with some slight amount of stretch in the conrod assembly at high RPM.
00:37 If we're using a steel rod then the stretch will be relatively minor, however in engines using alloy rods in particular, the rods will stretch more and also grow as they reach normal operating temperature.
00:50 This means that in engines running alloy rods, we'll need to provide more piston to head clearance.
00:56 Secondly we need to consider that the piston will rock in the bore as it moves up and down the cylinder.
01:02 The piston will rock about the wrist pin, and as it does this we'll find that one side of the piston tends to rise up a little and the opposite side will drop down a little.
01:13 You can see this effect by simply raising a piston to top dead centre and then pressing down on the inlet side and then the exhaust side of the piston.
01:22 The correct piston to head clearance will depend on many factors, including the conrod material, the weight of the piston, and the maximum RPM the engine will operate at.
01:33 In practice I like to see a minimum clearance of 40 thou or one millimetre unless you're dealing with factory components, and the factory specification is tighter than this.
01:45 If you're running with an alloy rod then the minimum clearance needs to be increased to 60 thou or 1.5 millimetres.
01:53 You'll find that many alloy rods are actually designed to provide a reduced deck height and give you the additional clearance you require, without any machining work on your part.
02:04 When you're inspecting an engine that's been running, it's always a good idea to inspect the piston and the deck surface of the head, to ensure there's no sign of contact.
02:14 Of course it won't take very much contact for you to end up with a bunch of broken parts.
02:19 However when you've got very minor contact, then the engine may survive, but there'd be witness marks to show you that you have insufficient clearance.
02:28 If you're suffering from light contact, this will also be very audible, giving you another hint that something is not right.

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