Practical Engine Building: Step 8: Cylinder Head Assembly
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Step 8: Cylinder Head Assembly
03.54
| 00:00 | - At this point the cylinder head should be stripped and cleaned ready for assembly. |
| 00:04 | The required work is going to depend to a large extent on the particular cylinder head design you're working on, however we'll try to focus here on the fundamental concepts. |
| 00:14 | It's typical to start by fitting a new set of valve guide seals to the cylinder head when it's been stripped and machined. |
| 00:21 | Once the valve guide seals are fitted we can then fit the valves into the guides, followed by the valve springs, retainers, and collets. |
| 00:29 | These processes are thoroughly detailed in the practical engine building skills section of the course, so you can refer to them there for more information. |
| 00:38 | Once the valve train is correctly installed in the cylinder head, we may have the option of assembling the head onto the block bare, and then adding rockers, lifters, and cams, or assembling the head completely prior to fitting the finished head onto the block. |
| 00:53 | The technique you can employ will often be limited by the specific head design, as in some instances with overhead cam engines, you'll need to have the cams, rockers or finger followers removed in order to gain access to the head bolts or studs. |
| 01:09 | For example with a push rod engine, we'd install the head bare and then the push rods and rocker arms can be added once the head's installed. |
| 01:18 | One important consideration with the valve train is the valve lash or valve clearance. |
| 01:24 | This is the distance between the cam lobe and the valve actuation mechanism, such as the rocker or bucket. |
| 01:31 | This clearance is critical to the correct valve actuation and needs to be set or adjusted to suit the entire valve train. |
| 01:38 | There are two fundamental ways that valve lash is dealt with, either with a mechanical shim or adjustment system, or alternatively with a hydraulic lash adjuster or lifter. |
| 01:49 | These different adjustment techniques actually require completely different cam designs in order to operate so it's not a straightforward job to swap between one style and another. |
| 02:00 | In a mechanical system there is always some amount of positive clearance between the cam lobe and the valve actuation system, whereas in a hydraulic system, there will be no clearance, and the hydraulic lash adjuster pumps up with engine oil to take up the lash clearance. |
| 02:16 | The advantage of the mechanical system is that valve clearance is fixed and can't change, whereas in some instances hydraulic valve adjusters are known to pump up at high RPM, holding the valves open. |
| 02:30 | The disadvantage with a mechanical lash adjuster however is that it is labour intensive, requiring periodic adjustment as the engine ages, and components wear. |
| 02:41 | Hydraulic lash adjusters on the other hand, require no maintenance work and are essentially a fit and forget item which is why they're favoured in a lot of production engines. |
| 02:51 | With this is mind, if your engine uses a mechanical valve lash adjustment system, this lash needs to be checked and set. |
| 02:59 | In some instances, such as with overhead cam engines, this clearance can be set during the machining process however with pushrod engines, the lash must be set once the head is installed and torqued down on the engine block. |
| 03:13 | If you're using hydraulic lash adjusters in an overhead cam cylinder head, these will need to be bled down prior to installation as otherwise they may hold the valves open while the head is being installed, and this may cause valve to piston or valve to valve contact. |
| 03:29 | When it comes time to fit the cams in an overhead cam cylinder head, the cam will run directly on the aluminium surface of the cylinder head in the cam journals. |
| 03:39 | What I mean by this is that there is no bearing that the camshaft runs in, and for this reason it's important to ensure that the cam and the journals are both adequately lubricated during assembly. |
