TDC is "Top Dead Center". It generally means that a given piston is at the top of its travel and that both valves for that cylinder are closed.
You find TDC by looking for marks on the fan and/or flywheel. See the "how to set your timing" tech article on Pelican for the fan marks; they are at the rear of the fan and you just about have to stand on your head in the engine bay to see them without a mirror. The mark on the flywheel can be seen through a notch on the bottom of the transmission where it bolts up to the engine.
So, you line up one of those marks by rotating the engine. I usually have one rear wheel up in the air and the other on the ground, put the transmission in 5th gear, and rotate the wheel that is in the air. It's good to have a helper to turn the wheel while you look for marks (or they look while you turn). Either that, or a bunch of mirrors.
Then go around and check to see which cylinder has both of the "rocker arms" (those are the things that rock up and down to push the valves open) loose, with a little slack to them. Whichever one has slack in both is the one that is at TDC.
Now you can remove the two nuts off that hold the shaft that the two rocker arms for that cylinder ride on. They'll take an 11mm wrench. You can remove the rocker arms and the shaft. Keep them together, and keep dirt out of them. There will be a bent piece of wire that is sitting in slots in the parts that the studs go through, just pull it enough to get it un-slotted. Oh, and make sure those parts that the studs went through stay in the right orientation. The spring on the shaft will push everything apart; make sure you can line it back up again the way it originally was.
Anyway, then you put on some heavy gloves and grab the pushrod tube from under the motor. Pull outward on it, and twist. It will come loose, and you will smack your hand into the cylinder head. (That's why you use heavy gloves!) You can also use pliers if you are very careful not to dent the tubes much. You don't have to pull the tube all the way out of the head, just leave it hanging by its middle.
You should be able to pull the old seals off the ends of the tube and put the new ones on at this point. (You'll have to slide the pushrod around a bit for the inboard end of the tube.) Clean up the bores where the tube goes as best you can, oil the new seals (or use "silicone paste lubricant"--not RTV goop!), and slide the tube back into the head. Repeat with the other tube.
Then you get the fun part of putting it all back together again.
On some motors, it can be a bit of a challenge to get and keep the inboard end of the pushrod sitting correctly in the concave part of the lifter. If you get the rockers back on with the stands seated against the head and you don't have any slack in the rocker arms, the pushrods aren't seated correctly. Futz with it until it is. (You can sometimes use assembly lube to hold the inner end of the pushrod in place.) I have used part of an L-shaped tool as the fulcrum of a lever so I could push the inner end of the pushrod upwards.
Getting the rocker arms, shafts, and the "stands" (the parts that the studs go through) all on there at the same time, in the right place, with the pushrods seated correctly, can be a bit of a challenge. I have sometimes wished for three or four hands to do that job.
Anyway, once you get it pushed up against the head, snug up the nuts you took off earlier (make sure to put the washers on before the nuts!). Once they're on finger-tight, use a torque wrench and torque them to
TEN lb-ft.
Then hook the bent-wire thingie into the channels on the bottom of the rocker stands. Make sure that the downward-pointing parts of the wire sit on the shoulders of the tube, and do not go down inside the tube. If they do, they'll wear through your pushrods. Oops!
Once you've gotten through this whole thing, you have three more cylinders to go!
I think it may have taken longer to type this all out than for me to do it any more...
--DD
