This article is a wikified version of this thread on the forums.
The following instructions explain how to install a cylinder kit on a moped.
Here’s a brief overview of the process for a Puch (and other mopeds). These instructions may vary for Motobecane or other brands. If you plan to case match, it's a good idea to drop the engine and remove the internal parts (or at the very least, block them off so the bottom end is not exposed to metal shavings) before proceeding. There are case matching instructions in sections 3 and 8 of this article. These instructions are for a simple kit throw without engine or cylinder modifications. It’s also recommended that you clean up the new kit (chamfer port windows and check for casting defects) before use.
- Remove exhaust by unscrewing the nuts where it mounts to the cylinder, and the rear bracket. Place aside, keep track of nuts and washers.
- Remove any fuel lines and throttle/choke cables from the carburetor. Remove the carburetor from the intake. If you have an airbox, disconnect that too. Make sure the fuel valve is off.
- Remove the spark plug boot, and remove the four nuts and washers that hold the head and cylinder on. Keep track of them.
- The head and cylinder should slide off, with the intake still attached to the cylinder.
- If you are planning to re-use the same intake and carb, swap the intake onto the new cylinder. Otherwise, place new intake on the new kit.
- Look at the piston that is still hanging from the crank. On either side there are holes, with a clip inside of it. This holds the wrist pin inside. Remove these clips with needle nose pliers. Also remove the wrist pin; it may need some massaging to get it to come out. Remember to support the piston to use the least amount of lateral force on the crankshaft when removing the pin.
- Once the pin is removed, the piston should fall off. Replace the pin inside the piston, as well as the clips as to not lose anything.
- Fit the new ring/rings onto the new piston. Be careful not to break the rings. Look for the pins in the ring groove, this should match up with the ring gaps.
- Fit new piston onto crank arm as the last one came off. Make sure it is right side up. Many pistons have an arrow toward the exhaust port (down) to help make sure you get it on right.
- Put on the new base gasket that came with the kit. (If it didn't come with a new base gasket, make one).
- Cover the new piston with 2-stroke oil, as well as the walls of the kit. Slide the kit over the piston. You will need nimble fingers to compress the rings on the piston while sliding it into the skirt, Make sure everything is straight. Once you get the rings in, it should slide on easily.
- Add head gasket if you are planning to run one.
- Place head back on and re-fit the head screws. VERY carefully tighten to 9 ft/lbs -- it’s less torque than you think.
- You should probably use a new exhaust gasket; re-fit exhaust the way it came off.
- Re-fit carburetor with cables, airbox, gas line. I usually put in a larger jet here, because you know you will need to. Might as well upjet once before the carb goes back on.
- Replace spark plug boot over spark plug. (It may be a good idea to run a cooler plug here) remember to gap it correctly.
- Start bike, and spray carb cleaner at every joint. If there is an air leak anywhere the RPMs will change on the bike when that section is sprayed. FIX ANY AIRLEAKS, ALWAYS. REALLY. NO AIRLEAKS WHATSOEVER.
- Proceed with break in process of choice, as well as fine tune the carburetor. Good knowledge of carb tuning and ignition timing will help you get the most out of this kit/not blow it up.
- Rebuild a Puch E50 Engine -- not only does this article give you a photo-documented guide to case matching, it also shows you how to remove the cylinder, piston, and wrist pin. Jackpot!
- Installing Piston and Rings Correctly|Installing Piston and Rings Correctly -- yes, the classic guide by Fred has detailed instructions and a photo to help you install your piston rings correctly. Read this, be gentle, and stop breaking your rings!
- Performance FAQ -- what size jet do I need? why do I upjet? how fast am I going to go? where can I get all this stuff? Answers to all your (VERY FREQUENTLY) asked questions can be found here.
- Spreadsheet -- further help for your re-jetting needs.
- Performance Tuning -- explanations, options, and a list of articles to answer just about anything you really want to know about tuning your moped for optimum performance.