Valves

Internal Combustion engines have number of different valves, this article is concerned with the combustion mixture intake and exhaust valves. Further we'll only be concerning ourselves with poppet type valves.

First up, lets define the terms:

Face	This is the skirt around the base of the uppermost part of the valve.
Seat	Is the ground area of the cylinder head which matches the shape of the face. Together the Face and the Seat provide the tight gas tight seal necessary for engine operation.
Stem	Is the metal "stick" which controls the opening of the valve. Usually a cam turns and lifts the stem which in turn lifts the valve.
Guide	Is a tube in which the stem moves. Together the guide and stem are such close tolerance that they provide an oil and air tight seal.
Spring	Holds the valve closed against the vacuum present in the cylinder during various parts of the stroke. For example, you do not want the intake valve to open during the power stroke, nor do you want the exhaust valve to open during the charging stroke.

When an engine is new the fits are carefully machined and in close tolerance to each other. Over time the engine wears and this tolerance is lost. Mechanical wear will gradually defeat the seal between the seat and face and between the stem and guide. In addition the heat of combustion combined with the corrosive compounds produced during combustion will slowly destroy the valves, especially the exhaust valve.

To return the valves to good condition we'll reface or grind them.

Refacing

To do this you will need a hand drill or an electric screw driver. Note: while most valves have a groove suitable for a flat blade screw driver, sometimes they do not. They might have two pin holes (I use a loop of fencing wire to do these ones) or they may have no marking/hole at all. For ones without a marking I use a Dremel rotary tool and a 1mm grinding wheel and make a groove. Carefully smooth and polish the edges of the groove if you do this to minimise subsequent carbon buildup.

Obtain some valve grinding paste from the local auto parts store. This paste will be used to lightly grind the face and seat. Remove the head from the engine, and take it away from the engine to your work bench. As always when working cover the engine with cloth and stuff all ports in the head with cloth to keep it clean.

Removing the valve from the head

The first thing to note is that you should only remove one valve at a time unless you are planning a total overhaul, this is because the wear in each valve face and seat as well as each stem and guide match. To mix them up would be disastrous and would lead to a need to machine the head and put in new valves.

To remove the valve, you will need a spring compressor. Valve springs are typically fairly tough, so this tool while not essential will make your life much easier. Take a look at the end of the valve stem, you will see that it has a small plate or washer and a locking device (which is probably a simple cotter pin). Compress the spring using the spring compressor then remove the locking device and plate. You should now be able to draw the valve out of its guide by pushing up on the stem. Remove the spring compressor still holding onto the spring and put it aside.

Does the valve need refacing?

Normally, you can tell if the valve needs refacing by looking at how it is sitting in its seat and by the condition of the edges of the face and seat. If they are pitted, scorched, burnt or scratched then you need to reface the valve. You will also need to reface the valve if it has got out of round. The valves tend to rotate in their guides during operation so both the face and seat must be round.

If a visual inspection suggests that no work needs doing, check the round of the hole. Use some engineers blue, or if you cannot get that then I use a permanent texta (marker). Cover the face and seat with blue and reseat the valve. Rotate the valve through a couple of revolutions and then pull it out again. The colour should be completely removed all around both the seat and face. If it is not, then the hole is out of round and must be ground. Note that it is ok for some colour to remain on the surface above the seat. Note that you can also use a soft lead pencil and mark it as per the diagram.

Inspect the stem of the valve to ensure it is straight and in good condition. If it is bent or badly worn then it should be discarded. Check with a machinists square to be sure the stem is at a 90 degree angle to the valve.

Refacing the Valve

Place a cloth in through the valve hole to prevent the grind paste from getting where it is not desired. This stuff is designed to grind metal, so imagine what it would do if it got between the moving parts of your engine. Apply the course grade paste to the face in a thin layer using your finger. Place the valve into the seat and rotate it back and forth under a light pressure around half a turn, then back. Do this for five seconds. Lift the valve and turn it a quarter turn then repeat. Do this through all four quarters. Remove the valve from the seat and carefully wipe all the paste from the valve and from the seat.

Next apply the finishing compound and repeat the operation. When done remove the valve and wipe away all the paste. Repeat the refacing check with engineers blue to see if the job has been done well. If not repeat. Another way of testing is to mark the face of the valve with a lead pencil about every 5mm, replace the valve in the set and rotate it through a single turn then remove the valve and check the marks. If they are all removed then the job is done ok.

Reaming the Valve

If the valve or seat are particularly bad you will have to ream the seat and install a new oversize valve. When selecting the new valve pay particular attention to the guide. When selecting the new valve, be sure to get the same size stem. If the guide is in good condition (you can tell by looking for obvious wear on the valve stem) then you can use the same guide, otherwise you will have to press out the guide and install a new one along with the new valve. If the guide is machined into the head, then you will have to drill out a larger hole and press in a new guide.

To ream the seat you use a special valve reaming tool. Take great care that you do not oversize the hole so much that even your new valve will not fit.

Cleaning the stem

The stem may have a build up of burnt oil and carbon, particularly the exhaust valve. Wedge the valve between blocks of wood in a vice and use 600 grit wet and dry sand paper to clean the stem. Take care not to bend or deform the stem. Light pressure only!

Grinding the stem end

The stem end away from the face should be ground true to ensure correct valve operation. Check the grinding with a machinists square to ensure the grinding has been done well. I usually use a bench grinder for this. Check that the tool rest on the bench is at 90 degrees to the wheel face then switch on your grinder (you did put on goggles and gloves now didn't you?) and grind the head of the stem.

Final check before reassembly

Give the valve a final once over to make sure you did not bend the stem in any way. Lightly oil the stem with machine oil and drop it into its guide. If the reface is done well, and the guide and stem are in guide condition the valve will hit the seat and bounce up then back down again. This indicates a near perfect finish. If it falls with a thud or does not fall cleanly then suspect the guide or stem.

Cleaning out

Wash the valve and kerosene or diesel (some people recommend petrol, but I try to avoid it due to flamability concerns). Flush out the valve guide and area around the guide but take extra care to ensure none of the grinding paste gets into any part of the head. You should also clean the spring.

Reassembly

Again lightly oil the valve all over. Put the compressed spring back in place and drop in the valve. Put the locking device in place and put the locking pin back in. Decompress the spring and remove the spring compressor. Push the valve stem with your finger it should not show any side to side movement. Try to action the valve taking note of the spring pressure. All the valves should be the same. Try and rotate the valve through a full turn with the screw driver and make sure it works freely without binding.

Last modified Sunday, 20-Jul-2003 15:28:00 BST

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