Because I'm at a bit of aloose end and Ivan was asking about clearances perhaps people would like a bit of info on valves.
So what is the purpose of a valve?
- To make a gastight seal in the ports when closed.
- To offer as little opposition as possible to the flow of gasses when open
- To offer the minimum of fricton when operating.
For our purposes we'll just look at poppet valves, those mushroom shaped things in the cylinder heads of our old shitboxes. There are several other types, probably the best known ones Sleeve valves as used on certain aero engines but we'll concentrate on poppet valves.
All valves are forged. They have to be, because when they are formed by the stamping process the granular nature of the metal, a special sort of steel with manganese, silicon, nickel and chromium in it, forms a very dense, striated structure which helps prevent it sagging when it gets hot.
Believe me, it gets hot !!!!! Your exhaust valve, when the engine is running hard will glow dull red. Overtax it's ability to cope and the head will fall off, this makes the 'Dogga-Dogga' noise and is generally very expensive.
The seating face of the valve is always cut, in conjunction with the seat, as a flat face, in the case of our Guzzis to an angle of 45 degrees. The sealing face should always be wider than the seat it sits on and the seat should sit in the middle of the sealing face with an area both above and below the sealing surface that doesn't contact seat. This helps ensure a good seal while allowing a bit of area for expansion and contraction of various components. It also reduces the risk of bits of carbon and shite getting trapped betwixt valve and seat and inadvertently holding the valve open.
The valve is closed by a spring, or pair of springs. Pairs are used to combat 'surge', a condition where at a certain speed there may be a natural harmonic in a single spring that will cause the valve to float, that is, not remain closed when it should. By using paired springs with different harmonics this problem can be diminished, but rarely fully eradicated. This is what causes valve float at high RPM when the springs can't overcome the weight of the spring efficiently in the limited time that the valve is supposed to remain shut. The obvious solution is to fit heavier springs but these will put further strain on the neck of the valve and increase the loadings on the valve train components. The springs are retained to the valve by means of a collar and two little devices called split collets. These are like two halves of a hollow cone that fit into a machined groove in the stem of the valve. To install them the spring is pre-loaded with the collar on top until it is pushed down below the groove the collets sit in. The collets can then be installed around the valve, in their groove and the pressure released so the collar presses on the outside of the collets. This can be a tricky operation for the inexperienced and has to be tackled carefully. If the collets don't engage properly they are prone to shooting off across the room when thespring pressure is released to the accompaniment of bulk cursing and/or pain if the spring and collar come off like bullets.
The edge of the valve should not be sharp. There is usually a depth of 1 to 1.5 mm around the head of the valve itself known as the 'Margin' this is there because sharp edges will tend to get hot, sometimes so hot they can promote pre-ignition, the lighting of the charge before it's meant to ignite. This can cause severe damage to pistons and is usually visible in the form of 'nibbling' at the edge of the crown. The valve moves up and down in a 'Guide' these are usually a separate component from the cylinder head casting to allow for their replacement without the need for replacing the whole, more expensive, head. Usually made of cast iron or some form of bronze Guzzis use both, the big blocks opting for bronze the smallblocks cast iron. Both have advantages and disadvantages. Cast iron has excellent wear properties but bronze transmits heat better. The only way a valve can dump heat when running is through the seat, when the valve is closed, or from the stem to the guide. For this reason it is important that stem to guide clearance be kept to a minimum but as the environment where a valve and guide interact is very hostile it is often hard to reach a decent compromise. The other functions of the guide are to keep the valve central to the seat, if the valve guide is allowed to become unduly worn the seating may wear oval preventing good valve sealing.
It is VITAL that when valves are closed there should be a gap between the valve and it's operating mechanism. this is known as the 'Valve Clearance' If there is insufficient clearance the valve will be held slightly off it's seat and the burning gasses the vale is supposed to seal inside the combustion chamber will leakout past it at great pressure. Given that these are nearly at the temperature of an oxy-acetylene flame no valve, no matter what it's quality will last more than a couple of minutes. The other thing that clearance allows is oil to get between the cam and it's follower during the period the valve is closed. This is a very high load area and without sufficient cooling and lubrication these parts will fail.
There y'go, that's all you really need to know about valves for beginners.
65, Osborne St.
Bungendore. NSW 2621