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Engine Building Fundamentals: Piston To Valve Clearance

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Piston To Valve Clearance

06.28

00:00 - One of the quickest ways to destroy a freshly built engine is to have the valves crash into the pistons the first time the engine is started up.
00:08 If you're dealing with a rebuild on a stock engine, then you shouldn't need to worry about clearance between the valves and pistons, assuming the cam timing is correct.
00:17 However, if you're changing to a different piston crown profile, different cam profile, or even to oversized valves, it's essential to confirm that you have sufficient clearance.
00:30 There are a range of considerations we need to understand when deciding on a minimum valve-to-piston clearance that we're comfortable with.
00:38 Like many aspects of engine building, there is much debate on the subject, so I'll explain how I deal with valve clearances, and why I choose particular clearances.
00:50 In simple terms, it may be reasonable to think that, provided the piston and valves aren't contacting during a dummy assembly of the engine, that we have nothing to worry about.
01:01 The reality, though, is that under high RPM operating conditions, there's a lot more going on.
01:07 Firstly, the conrod may tend to stretch slightly at high RPM, bringing the piston closer to the cylinder head.
01:15 If you're using a steel rod, then the stretch will be minimal.
01:19 However, particularly with an alloy conrod, stretch can be significant, and the rod will also grow as it heats up to normal operating temperature.
01:29 This effect reduces the clearance between the piston and valves under operating conditions.
01:36 Next, we need to consider that the piston will also rock in the bore as it moves up and down.
01:41 This can affect the radial clearance available between the outside of the valve diameter and the diameter of the valve pockets, or cut-outs, in the pistons.
01:52 To ensure that we have sufficient clearance as the piston rocks in the bore, we're going to need to ensure this positive clearance under static conditions.
02:02 Lastly, we need to consider the way the camshaft controls the valves.
02:07 In the ideal world, the valves would move as directed by the camshaft, and hence, their movement would be nice and predictable.
02:15 In this case, assuming we have clearance during a static test, we should also have clearance at high RPM.
02:22 It's not uncommon, however, at high RPM, for the cam to lose control of the valve.
02:28 This in particular can happen with aggressive cam profiles where we don't have sufficient valve spring pressure.
02:36 In this situation, we can get valve float, where the valve floats off the cam, and opens further than the cam profile dictates, or we can also end up with the valve bouncing when the valve closes, and hits bouncing back off its seat, and opening again.
02:53 Neither situation is desirable.
02:55 However, when it comes to valve-to-piston clearance, valve float can really affect the clearance that we have.
03:02 In particular, this can be a problem with the exhaust valve, since the piston will be moving up the bore as the exhaust valve is closing.
03:11 Any valve float here could potentially be disastrous.
03:15 Valve float isn't really an issue on the intake valve, since when the piston is near the intake valve, the valve is being opened by the camshaft, and there's no potential for valve float under these conditions.
03:28 So to summarise what we've talked about, our requirements when checking piston-to-valve clearance is to ensure that we have sufficient clearance to allow for conrod stretch, or growth, at high RPM, as well as sufficient radial clearance to allow for piston rock.
03:46 Lastly, it's advisable to allow a little additional clearance on the exhaust valve to provide a safety margin in case of valve float.
03:54 So what numbers can we apply here? When it comes to the intake valve, we know that we don't need to be too concerned specifically with valve float affecting our final clearance, so it's sufficient to use the piston-to-cylinder head clearance, and then add 20 thou, or half a millimetre to this, to arrive at our minimum valve-to-piston clearance.
04:18 Using the piston-to-head clearance as the basis for our valve clearance means that we are automatically going to increase the static valve clearance as we move between a steel rod, and an alloy rod, too, since we're going to be using a larger piston-to-head clearance with the aluminium conrod.
04:37 As an example, let's say that we've elected to use a piston-to-head clearance of 60 thou, or 1.5 millimetres.
04:44 Using this guide, we'd use 80 thou as a minimum clearance between the intake valve and the piston, which is our piston-to-cylinder head clearance of 60 thou, plus our margin of 20 thou.
04:59 When it comes to the exhaust side, I tend to allow a little additional clearance, in respect of the potential for valve float.
05:07 In this case, I allow an additional clearance of 40 thou, or one millimetre, above the piston-to-head clearance.
05:15 In this case, if the piston-to-head clearance was 60 thou, I'd ensure that the exhaust valve-to-piston clearance was a minimum of 100 thou, or 2.5 millimetres.
05:26 As far as the radial clearance between the outside of the valve and the inside diameter of the valve pocket goes, I like to see a minimum clearance here of 50 thousandths of an inch, or 1.25 millimetres.
05:42 It's worth mentioning that too much clearance is always going to be safer than having too little.
05:48 And unless you're building a maximum-effort competition engine, where you need to maximise the compression ratio, there are usually few downsides to having a little additional clearance.
06:00 Likewise, you'll find that for most moderate combinations using off-the-shelf aftermarket pistons and camshafts, you're likely to find that you'll have more clearance than you need, as the piston manufacturer tends to err on the side of safety.