How to port a scooter cylinder

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How to port a scooter cylinder

Post by noiseguy »

Good article on cylinder porting from Paul Dice, a contributer at MopedArmy. Numbers listed for Express moped are very similar to Spree porting.

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If you are reading this, chances are you have already done the basic performances mods to help speed up your over-the-counter gas saver and are ready for more MPH. Porting out a cylinder, when done properly, will give you more power on less gas than the stock motor ever could. But before you break out your grinding tool, do your homework. The first and most important step of porting is to get a rough idea of what you want your engine to do. Do you need more low-end grunt? Or do you need the extra ponies to finally break the 35mph barrier and head into illegally fast speeds? Most porters will choose the latter, which is what I will focus on. The best way to extract full power from your little motor is to tailor your porting times to your specific driving needs using software programs, but I will give a good, all around setup to get you going faster, quicker. Remember, this will give improvements over stock with minimal calculating or measuring. If you plan on getting the absolute last hp out of your engine, read around, there are plenty of lengthly articles discussing the finer points of porting,

First, remove the cylinder. This is pretty straightforward but there are a few points to pay attention to. DIRT HURTS STUFF!!! Clean the moped well before even getting a wrench. Also, don't take the motor apart while it is still in the bike if your bike is a step through. It will be a lot faster and easier in the long run to go ahead and get the power unit out and on your workbench, BEFORE disassembly.

Next, remove any shrouding, exhaust headers, carburetor manifolds, etc. that are attached to the cylinder. After cleaning any grime from the cylinder base with a wire brush, unbolt the cylinder. The cylinder may require a little coaxing to come off. Tap upward with a small hammer and lightly pry on the cooling fins with a screwdriver. Don't worry about saving the cylinder base gasket, as we will replace it with rtv to increase compression later on. Put every thing but the cylinder in a box and move it far away from your workbench and where you will be grinding. It is a good idea to cover the box up as well. Grinding dust can really eat away at moving parts in an engine.

Now you can get down to business. Get out your tools. You will need a Dremel tool with a good set of grinding bits. Minimally you will want a chainsaw sharpening big and a round nose-grinding bit. You will also need ether a pair of calipers or a really good metal ruler to get accurate measurements inside the tight fitting cylinder.

Cover your work area with newspaper, this can get messy. Get a lot of over head light so you can see what you are doing, it is impossible to do a quality job if you can't see the cylinder. On a 50cc standard size moped engine you will need to measure down from the top of the piston's travel, (.95 inches for honda express and batavus engines, 1 inch for puch engines) down into the bore. The top of the pistons travel is usually flush with the top of the head, but check before you make your mark. This is a very conservative timing point that will work well for all applications. It won't give the best power, but you won't go to far and remove material you cannot put back. You should make the mark with a small scratch from a nail or pick, directly above the exhaust port. The mark will be about a quarter inch or so above the top of the original port. This will be the point that you will grind to from the exhaust port. The transfer ports will be the other ports in the engine that are smaller and don't go through to the outside of the cylinder. Performance gain will not be as noticeable with alterations made to the transfers so if you doubt you skills, leave them alone. If you feel confident and you have a steady hand, measure down from the top of the piston's travel (1.2 inches for bavavus/express, 1.47 inches for Puch) , and make a mark right above each transfer port.

Now, with the marking done, you can bring out your Dremel. Chuck up the chainsaw chain sharpening bit and begin opening up the exhaust port. Enter the tool in from the outside of the cylinder and begin grinding the top of the port up to your mark. It is ok to stop a little short of the mark. Be certain that the top of the port is level just like the original port was. If you get it level before you reach the mark leave it. The most important part is that the area between the actual port and where your exhaust system bolts up, be as large bore and contoured as possible. On my Honda express engine I was actually able to slowly drill out the port with a 1 inch drill bit, I just made sure to stop 1 inch short of the cylinder bore and finish the small areas with the Dremel round tip bit. If you decide to do the transfers, use the end of a chainsaw bit to grind away up to your marks.

Now would be a good time to knock the shavings and dust out of the cylinder and look at your work. The hardest part is over. All that is left is to smooth everything out. With the round point bit, go into the exhaust port and smooth up any irregularities. Use ether the grinding bit or the sharpening bit to go up through the transfers from the cylinder base and open up the transfer port's passages. It is very important to performance that the transfer passages be opened up to allow more air and fuel through. This area varies widely from engine to engine so use your best judgment. As a rule of thumb, don't remove more than an 1/8 th inch of material from the passage walls farthest from the bore, and don't remove more than 1/16 th inch from the bore side as this could cause heat spots in the cylinder.

The final step involving grinding is to chamfer the ports. Using the round nose bit and with the tool at its lowest possible speed setting, very carefully bevel the port edges in the cylinder bore at about 45 degrees to the cylinder bore. The purpose of chamfering is to help the rings make the transition from running in the port to running back in the bore. Think of it as making a ramp for the rings to get out of the port. You should just barley be able to see that you removed material from the bore. Only a slight amount of material needs to be removed to do the job. This is a very important step, do not omit it!

Now, wash the cylinder down in a sink or washtub filled with warm soapy water. Use a plastic brush to clean all of the surfaces of the cylinder. It must be impeccably clean or the grit will quickly ruin the bore. If you can run a clean white cloth along every surface inside the cylinder (don't forget the transfer passages!!!), and have it come out clean, wash it again and get ready to reassemble the motor.

Clean off your workbench, clean it again. You do not want grit in the engine! Clean any remaining gasket from the mating surfaces of the cylinder and base and rub a thin coat of oil around in the bore of the cylinder. Run a small bead of Orange Permatex sensor-safe RTV gasket maker around the cylinder where the gasket used to be. It is important to put it on the cylinder and not the base so that the RTV will not be forced down into the crankcase. Put the cylinder back on the engine. You will need to compress the rings so that they can go back into the cylinder bore. Be careful not to force them, as they are brittle. Torque the head bolts down to 9 foot pounds in a criss-cross pattern if you don't have the original specs.

Finally, simply reverse the process of disassembling the moped and put it back together. For your first tank of fuel through the newly ported cylinder, it is a good idea to mix up a gallon of 25:1 gas to help break in. Ride safe! Pic update. These are before and after shots of the batavus cylinder. It looks like I did alot of cutting on the bottom of the port but that was just to clean it up. Remember, you should not remove any more material from the bottom than is nessesary to smooth it out.
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