Tuning and Modifications: Installing A Malossi 70cc Big Bore Kit onto an Aprilia SR50 DiTech

1.  Let me first start with the fact that I am NOT a mechanic.  I have been working on motorcycles and scooters for a long time and know a little about what I am doing.  I simply take pictures for other people to use as a guide line to help them modify their own motors.  Any modifications you perform are done completely at your own risk and liability.

2.  Tampering with your motor in any way may void your warranty.  Make sure this is a sacrifice you are willing to make before you begin working on your own vehicle.

3.  Before you begin, make sure you have a nice clean area to work in.  A small amount of dirt can be a big problem when working on the innards of your motor.  Keep your hands, motor, and any other parts you are handling very clean.

4.  This example was performed on a 2002 Aprilia SR50 DiTech.  If you own any water cooled - fuel injected scooter, the same steps will apply.  I chose to remove the entire engine/drive train to do this job.  Many people say that they do it in-frame, but I find it is easier to work on when there is a lot of room to work.  I hate working in tight spaces.  The choice is yours, but my suggestion is remove everything before you begin.

5.  ALL bolts and socket head cap screws are in metric! Don't try to work on your vehicle if you don't have metric tools, you will just end up messing up the heads.

This is the Malossi 70cc Big Bore Kit
You will need to start by removing the farings. You may not want to remove all the farings I did, but I hate clutter, and I want everything off to make it easier. Start by removing the side covers from each side (1). Then pull off the rubber mats that you put your feet on from both sides (2). Then remove the front kick panel (3). Then remove the bottom faring (4). And Finally remove the top faring where your feet rest (5) - you will need to remove the battery to do this. Check the next picture for that step.
Disconnect the battery and remove it. There is a connector that attaches to the fuses underneath the faring. Disconnect it to remove the fuse block.
This is what the fuse block looks like after it has been disconnected.
OK, after you have pulled the wires through and removed your last faring, it is now time to start disconnecting ALL WIRE CONNECTORS, HOSES, AND CABLES from the engine. This is what you should have now.
Start by draining your coolant. There is a small plug just above your intake hose. Simply remove it and the coolant will flow out. Be careful, it really shoots out of there. It won't just drip into a bucket at first, you have to hold the bucket in front of the hole to catch it!
Now disconnect your oil line for your 2-cycle oil. Also there is a breather to help your coolant drain. Remove this screw to help your coolant drain out of the cylinder.
Ok, you will also need to remove your air box to get to the bolt holding your rear shock on. Also remove the connector from your throttle body at this time. (That is the connector that is located on the intake right behind the air box.)
Now remove the connectors from your fuel injector and your temperature sensor. The two connectors are exactly the same, so mark them as to which goes to what. You have to remove the small spring clip to get these off, just like the connector on your throttle body. After you have them off, replace the spring clips so you don't loose them. You can snap the plugs on with the clips attached.

Also you are going to need to remove the torx head screw that holds your fuel lines in.
After the torx head screw has been removed, remove the little steel clip under it. Now you have to unplug your fuel lines. One is the intake from your fuel pump, and one is the return line. No gas will come out of your intake line. But gas will POUR out of the return line. I used a small piece of rubber hose, and plugged one end with a bolt. As soon as I pulled the return line off, I pushed the hose onto the connector. You may also just want to drain the gas into a gas can, the choice is yours.
Now disconnect your coolant lines. There is a hose clamp on the hose attached to the cylinder, but you have to break the clamps off the other hoses. You are going to need some new hose clamps to replace these.
Disconnect your air injector connector. It is hidden behind the coolant line on your cylinder.
Unbolt the rear shock from the transmission housing, and unbolt your rear brake caliper and remove it from the rear wheel.
Remove the spark plug wire. And if you are ABSOLUTELY SURE there is nothing else connected to your engine, then remove the main chassis bolt from the frame. This may require two people to lift the frame off the engine. I was able to do it by myself, but it wasn't easy.

After the main bolt has been removed, simply lift the entire top frame off the engine and set it aside.

Remove the torx head bolt that holds your air injection hose on the cylinder. Then pull the air hose out.
Remove the water line going into the cylinder, this will require breaking the clamp off again. Also disconnect your exhaust from the cylinder.
Now everything should be completely disconnected from the cylinder and head. Remove the four nuts that hold the head and cylinder on.
Slide the head and the cylinder off the four bolt posts exposing the piston. Remove any gasket that may be stuck to the motor.
Now, use a small screwdriver or a pair of needle nose pliers and remove the ring clip from one side of the piston. Push the connecting pin through from the other side to remove the piston.
This is all the stock stuff you should now have removed and ready to replace it with your new big bore kit.
Here is a comparison of the two cylinders. YES SIZE DOES MATTER!!! And as I always say, "There is no replacement for displacement!" YEAH BABY!!!
OK, now we need to remove everything that is attached to your old cylinder head and attach it to the new. There is an assortment of o-rings and gaskets. Replace all that apply, put some 2-cycle oil on the o-rings before assembling, it will help them go together easier. If you see an o-ring, find one the same size and replace it.

Start by removing the two bolts that attach your fuel injector to the head and remove the fuel injector.
Next you need to pound your air injector out of the head... Yes I said pound it out. It is pressed in the head. Flip the head over and use a small piece of plastic to pound the injector out from the combustion side.
Now remove your coolant intake manifold (with thermostat), and remove the temperature sensor.
Cut off the aluminum washer on the temperature sensor and replace it with the new one provided in the kit. Attach everything to the new head. DO NOT TIGHTEN THE TEMPERATURE SENSOR YET. There is not a lot of room with the new cylinder. You have to wait until everything is back together and tighten it so you can plug your connector on. If you tighten it now, you may not be able to get your connector back on, and you can't loosen it after the washer is squished or it will leak!

Don't forget to lubricate the o-rings with some 2-cycle oil before assembling. Push the air injector in first. Seat it as best as you can. Then put the fuel injector on top of it and tighten the two bolts down. This will seat the air injector the rest of the way.
Now you have the head ready, you have to get the piston ready to go on. Insert one of the ring clips into one side of the piston. Then put the rings on the piston. Start by sliding one ring over the top and into the first grove on the piston. Then work the ring down to the second groove. Now put the second piston over the top and into the first groove. Piston rings are hard and fragile. DO NOT stretch them too much or they will snap in half!!! You will notice little pins sticking up inside the grooves on the piston. These pins are to be located where the rings are split after you put them on.
Now the piston is ready to install. Put the piston onto the connecting rod with the arrow pointing to the exhaust port. Push the connecting pin through the bearing cage and up against the ring clip you installed on the other side earlier. Install the other ring clip on the side you just pushed the pin into.
Now you need to put the cylinder on. Start by using a little 2-cycle oil and lubricate the walls of your cylinder. Also, there are bolt studs that came with your kit, these are for your exhaust. Use your old cylinder to determine which length you need, and screw the short end of the post into the cylinder by your exhaust port. I only require one in mine, but there were two in the kit. Put the gasket provided with your kit onto the four bolt posts and slide it down to the motor. Push the cylinder onto the four bolt posts and push it down just until the top of the piston starts to enter the cylinder. Line your ring up with the little pin in the groove on the piston and squeeze the first ring together and push the head down to the second ring. Then do the same for the second ring and push the cylinder all the way down to the motor.
Now install the o-rings provided with the kit into the head.
Put the head on and tighten the four nuts up pretty snug... I don't know what the torque setting is for them, but I just crank down on them pretty good.

Re-attach your exhaust, air injection hose and your water line (This will require a new hose clamp).

After that, you are ready to re-assemble your scooter. Start by putting the main chassis bolt through and re-attaching the frame to the motor. Then hook up your brake line and other hoses, cables and connectors. Remember to check your temperature sensor to see if you can snap your connector on... Tighten the temperature sensor down until it is tight and you are able to clip on your connector.

You will also have to bleed your rear brake line. Take the top faring off around your handle bars and fill the brake fluid reservoir up and bleed the lines.

Now that the cylinder is installed, you will now need to install the computer that came with your kit. Start by disconnecting the connector in the bottom of the computer. Then Remove the three bolts that hold the computer to the frame.

Now push out the metal centers from the rubber grommets and remove them from your old computer and install them on the new one.
Bolt the new computer to the frame and plug it in.
Now you just need to put everything back together. Start by putting the faring that holds the battery in first. Re-install your battery and give the motor a start. No sense in putting everything back together only to find out something is wrong!

I have been told that sometimes you need to have a dealer clear a fault for your scooter to run properly. If your scooter is running at high RPM's and it is running with a real bad cough, this will have to be done. I did not have this problem. My scooter has run fine since it was installed. I do notice a miss here and there when I am running at low speeds, but once I get up around 25MPH, that goes away. I have heard that this is normal. Also the scooter tends to idol a little higher than before. I have heard that this too is also normal.

If you do take your scooter to a dealer to have faults cleared, DO NOT LET THEM RE-MAP YOUR COMPUTER!!!!!! Part of the reason that the big bore kit is so expensive is because of that computer. It is programmed to run the injectors harder for a 70cc kit. The gameboys are not capable of doing this. If they re-map your computer, you will loose the 70cc programming and you will be SCREWED!!!

After you change your bore to 70cc's, chances are you are going to have to change your roller weights and/or contra spring. If you are unclear on how the roller weights and drive train work, click here to understand how it all works.
Also, I HIGHLY recommend getting the final gear drive kit with the 70cc kit. I installed the 70cc kit on my Mojito, and the scooter had great acceleration, but little increase in top end. Also the scooter revved up to its highest rpm's very quickly. After the gear installation I had better top end, and the motor isn't spinning up nearly as fast. So in my opinion, the gear kit is a MUST HAVE with a 70cc kit.

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