If you've got the head off for whatever reason, you should check the guides and the seats.
Even if the guides and seats look good, they are probably caked in carbon build up. A good "while we're there" is check on your valve to head seal and perform a quick valve grind if it needs it.
How do I know?
Flip the head upside down so the combustion chamber is up. The valves are closed by default (no cam pushing them open). Pour gasoline into the combustion chamber and wait to see how long it takes to leak out. It should take a long time, At least a minute or two. I timed my most recent job at no leaks in 30 minutes. Probably overkill, but why not?
Luckily it is easy to do and does not require expensive tools.
Tools
Valve spring compressor
Valve grinding tool (usually a suction cup thingy of some sort)
Materials
Assembly lube (for when you put it all back together)
Valve grinding compound
The basic deal is you remove the valve retainer and spring(s) so that valve can move up and down freely. You'll put paste on the valve seat and then spin the valve on the seat such that the paste polishes those two surfaces until they meet nice and cleanly.
What you want to avoid is getting the grinding paste into the valve guide; the guide is already polished and is a tight fit, we don't want to add friction or change the size of it etc.
I improvised a bit here by putting the suction cup tool into an electric drill and then using that to spin the valve in place with the grinding paste on it. I am impatient, but I was also very careful.
Pics
Getting ready to start. There is our head; the big valves are intake, the little valves are exhaust. You can always confirm that with the ports in the head.
Here is the head all cleaned up with a brass brush. Just getting any excess carbon off. In a perfect world I would polish the combustion chamber and cc it to make sure they are all the same size.
The valve spring pliers push down on the retainer which is a little cap held in by a clip made of 2 "V" shaped pieces that are held by the spring's pressure in that groove by the top of the valve stem. Like a pair of old time ice tongs. Squeeze and the clips fall out.
Same pic, but locating the other side of the pliers on the same valve for stability
Spring and clips are off. The valve will drop down through the guide now. The guides are brass fittings that are just pressed in place. One thing to avoid is to "re-engineer" here - these cars did not have oil seals on the valves and you do not want to put any here. Why? Because some oil has to get into the valve guide to lubricate the valve as it moves up and down. Oil needs to leak down into it.
There is a valve, the springs (I use a dual spring) and the retainers. You can see all the carbon buildup. We'll clean this up and then polish the seat (the angled part near the bottom)
After some grinding, you can see the seat (in the head) and the seal on the valve are nice and clean - not shiny or polished - but not covered in carbon or pitted. You will be able to feel the difference when you turn it in place. Do both valves, put the valves back in using some assembly lube on the stem (you don't need a ton) and then check your seal. I used a brass brush and gasoline to clean off the valve stem as well. Brass brushes are handy because they are generally too soft to put scratches in iron so you can scrub off the carbon etc pretty safely. Your air+fuel mixture is flowing past the valve head to get into the combustion chamber. Any roughness or blockage is not good.
This is gasoline sitting in the combustion chamber. The spark plug is there to prevent a leak obviously (hopefully obviously!) This sat for 30 min and didn't leak at all. My valves and guides all looked good so I re-assembled it all.
When they are all done, you are ready to re-assemble.
If the guides are worn (do the valves wiggle in there?) or the valves look beat up (pitted? scratched?) or you have any other concerns, you can take your head to a machine shop. It is not expensive to have any of this stuff replaced or repaired. Most of all, you don't want to put it all back together just to have it fail again.
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