once you get the pistion totaly exposed,(out of the cylinder). there will be two C clips on each side of the pistion rod. pry them out, be carefull they will fly all the way accrost your shop if they spring out. next you have to get the pistion rod out, some times they go easy others they dont. there is a tool to get them out, other wise use a big punch. next you will need to hone the cylinder, hone them by using a cross-hatch pattren, use a even motion and you should end up with a even patteren. then assemble your pistons, make sure the rings are in the right spot and facing the right direction,( up or dowm there will be a marker). put the piston on the connecting rod and pust the piston rod through,( dont forget the bering). then install the c clips, be carfull they are very easy to lose. after this compress the rings with your fingers and and slide the piston onto the cylinder. make sure there the rings are seated with the open section on the ring pin. rember to coat the pistons and cylenders with some injection oil before reassembly. torque the head bolts and fill with coolant.
Can get puller from shadtreesnowmobile for $20 + shipping. But it you dont want to spend the money, WD-40 (or liquid-wrench) and a punch or a socket (smaller than OD of wrist pin) and extension will do. Use a rubber mallet and dont tap too hard or youll bend the connecting rod - BAD!!!! You can put a block of wood between the connect rod and the cyl pins to counteract the pounding and hold the piston still.
20 + shipping may be worth it - was for me.
I'm a computer geek and I just re-did my 503 and believe it or not it ran on the initial start. SO if I can do it...
get the tool. keep the c-rings on a magnet until use. Account for each one and where they go.
Heres a thought that went through my mind while changing a piston drunk(many moons ago).
"I swear this kit came with 2 c-clips. hmmmm. Now that I think of it, there might have been only one". I took the one out of the other kit, put the piston on and on my first ride(100 feet or so).
I found the missing c-clip. It was in the crankcase!
Bad noise but at least I knew where it was.
$1000 later, the sled ran fine.
This was 15 years ago. I cant imagine what would happen with todays stuff.
Someday, I will tell the "flying needle" story.
The short "rod" is the wrist pin. It should have needle bearings on it. Some bearings are caged and some are loose. The loose ones will also fall into the crankcase and end up like the C-clip that Viktory2k1 had fall, so be careful!
On some stubborn wrist pins, in the absense of a puller, remove one of the c-clips from the piston. Take a threaded rod and slide it through the wrist pin. Put a small nut no larger than the wrist pin opposite the side that the c-clip came off. Take a large, deep socket, bigger than the wrist pin, and slide it onto the threaded rod. Put a washer and nut on the "socket" end and tighten the nut. As you tighten the nut at the socket end, the nut at the other end will pull the wrist pin into the deep socket.
If the piston has been overheated and is damaged, you may not be able to get it out using this method and a puller will be required. If it's just being a little stubborn, it's a cheap "puller" you make with things you probably already have in the shop.