This can often be a quick and cheap project. Below is an example of one that won't be, though. Often, you'll go into an old engine and find things aren't worn much and the valves will just need grinding. Other times, you'll need to make everything like new. Your dealer may surprise you and have kits in stock for a 50 year old tractor. Otherwise, Valu-Bilt and many other dealers on the internet can get parts kits for you.
First pull the valve cover (overhead models), head and side covers (flathead only). Life will be easier if you also pull the manifold, unless you break some studs. Go easy...maybe start a week early with the penetrating oil. Manifold nuts are supposed to be brass, but not everybody knows that and you can end up with a rusted mess.
Flatheads: Fords can be especially
easy, or just like the rest, depending on carbon accumulation. To
the valve assemblies. At the top of the spring, as seen
through the side cover, grab the tab with a pair of pliars. Pull
down and out. You should be able to give it a tap and the
valve, spring and guide should slip out the top. If carbon
buildup prevents this, disassemble as follows. (This is how
most non-Fords disassemble, too) Compress the
spring and pull the keepers (You don't absolutely need a special
tool for this, but get one anyway). You might want to stuff
paper towels in the oil drain holes in case you drop something.
Pull the valve up. Pry the spring out. For Fords,
wooden dowel into the valve hole and rap as needed to drive the
guide out. If the keepers don't want to come out, support
the cup washer so that the valve sticks open. The whack the
valve head with a chunk of wood or a brass mallet.
Overhead: Loosening a few nuts or bolts
will usually allow you to remove the rocker arms from the head.
Mind now, that the shaft may be in several pieces and a wrong
move will dump springs, shafts, rockers and towers into the far
corners of the shop.
This is how we face the valves.
Lap the valves. Drop in a guide (Ford), put a dab of valve grinding compound on the seat, drop in the valve and spin it. Sears and the flea market junk shops offer a valve grinding tool (only the price is different) that you crank like an eggbeater. The back and forth rotation these make is a good idea, but ours fell apart after just a few valves. Use the kind that looks like a stick with suction cups on the ends. In reality, they are sticks with suction cups on the ends. Work it back and forth in your hands until the seat is smooth. Don't get so much compound on that it drips into places you don't want to wear out. Some compounds come in coarse and fine. Start with coarse. Don't get the valves mixed up after grinding. Do a good job of cleaning up the gritty compound when done.
For Fords, put new seals on the guides. If this operation seems difficult, it is because it is! It can be done, though. We think a minister or polititian (with strong hands from lots of handshaking) would be of great help with this. Drop the valve guide into the pocket. The skinny end goes in first. With a hammer and length of broom handle or brass drift, tap the guide into place. Go just far enough to put the retaining clip on and pry it back up. (Some people, including shop manuals, will recommend preassembling the valve, guide, spring, and keepers and installing as a unit. Yes, it can be a little easier, but it requires a special tool.)
All flatheads, make sure the tappets (lifters) are in place. Slip the spring over the guide and put the cup washer under it. Drop in the valve. Now, use a spring compressor (you can do this with two prybars, but it isn't fun) to squeeze the spring enough to slip in the keepers. Slowly release the spring to lock the keepers without removing any fingers.
Adjust the valves.
Flatheads. You will need a
thin openend wrench to turn the adjusting screws on the tappets
and a special tool to keep the tappet from spinning while you adjust.
Adjust the gap between the valve stem and the lifter, measuring
with a feeler gauge. A rule of thumb is that too much gap
is better than not enough.
Dad once said it's better to hear them a little, than not
at all. Unless your manual says otherwise,
go for about .012" on the intakes and .016" on the exhausts.
Re-install the side covers with new gaskets. Replace
the head when ready.