Quick Top End List

One of my cousin's is going to do a top end on a PW50 and asked for any input I would have. I was thinking about it a bit and this is just a quick list of my thoughts and what to watch out for.

• The Honda NC50s, many mopeds and scooters, the PW50, etc, have aftermarket cylinders that are available so cheap, its not even worth thinking about the old cylinder, piston, etc. A whole PW50 top end kit, including piston, rings, cir clips, gaskets, upper rod bearing, piston pin, spark plug, cylinder and cylinder head was $49 on Amazon. And if you are mostly stock, these engines are so slow, so low compression, that can live with some cheaper components.
• One of the keys to engine life is keep dirt out of the motor. Be meticulous about cleaning everything. Flush the bottom end with some fresh Kerosene or Mineral Oil, or Vegetable Oil to try to get any loose dirt out of the bottom end. Spend time making sure the gasket surfaces are clean. In general my simple two stroke philosophy is -- keep dirt out of the motor, use high quality oil, spin it fast.
• Get a good set of appropriately sized needle nose pliers to put the cir clips in. Its worth buying a good set.
• If you mark the piston putting the cir clips on, take a fine metal file and just lightly file any high spots.
• Chamfering the ports on the new cylinder with a small file. Too many of the new cylinders I have seen have ports with sharp edges. I just do enough, so my the skin of my finger doesn't catch on the ports as I wipe past them. There are good YouTube videos on this, such as:

https://www.youtube.com/watch?v=UJbtzxtPpSs

• If its a multi ring piston, make sure you know the order of the rings, try not to mark the piston up when putting on the rings.
• Lettering, or dots on the piston rings should face up toward the top of the piston.
• Usually the piston will indicate which side should be the exhaust port side. There will be an arrow pointing to the exhaust port, or an "E" or the part number will be on the exhaust port side of the piston. If you put the piston in backwards, it will often run, but not well. Some pistons are completely symmetrical and some of those don't have any marks.
• Put the ring in the cylinder and measure the ring end gap with a feeler gauge. If its 10 thousands or less, you will where out the cylinder prematurely. It only takes a couple strokes of a file to set the gap. I have had a couple of cylinder kits for the NC50 come with too small of a ring end gap.
• Put some oil on the upper bearing, piston pin, and rings before assembly.
• Make sure the ring has the ring gap aligned with the alignment pin in the piston. It will destroy the cylinder if the gap and the alignment pin are not matched up.
• New gaskets go on dry. You don't need to do anything to them.
• If you have a torque wrench, you really want to torque the cylinder per the manufactures specifications. Then torque again, after you have heat cycled the engine a couple times.
• I usually break the engines in with three heat cycles of about 10 minutes each. The first time you start an engine with a fresh cylinder, it heats up noticeably quicker than normal. So I try to be gentle the first heat cycle.

Putting a new top end on, is something that almost anyone who has any mechanic interest should be able to do. A lot people can probably do it in under an hour, just a few things to look out for.