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Change of AC cylinder

by Francis Sevigny on March 26, 2022

On a 50cc scooter, the engine is the first part to be replaced due to its price/performance increase ratio.

This tutorial was done on a Bws Sport. However, the procedure remains the same on all models. We have removed the bare minimum of parts. You can remove the rear body and the trunk to see better and create more work space.

*Note: the change of the cylinder must absolutely be accompanied by a readjustment of the carburettor, unless replacing the same cylinder. The carburetion must be adapted to the cylinder, under penalty of tightening/jumping your engine*

Necessary tools :
- Ratchet wrench with at least one 10mm and 12mm socket.
- Spark plug socket (13/16, or other tool to remove a spark plug).
- Phillips head screwdriver (star) number 2 and number 3
- "Long nose" pliers
- Torque wrench (ideal, but not mandatory)

For engines requiring squish adjustment (the space between the piston crown and the engine head (cylinder head) when the piston is at top dead center (TDC)).
- Tin wire about 1mm in diameter
- A caliper/micrometer or any other precision tool capable of measuring to the nearest .01mm.

1. Remove the battery cover. It is retained by 2 Phillips head screws (star), they are very easy to locate.

2. Remove the muffler. It is retained by 2 bolts with 10mm head screwed to the cylinder as well as 2 bolts with 12mm head screwed on the side of the lower engine.



3. Once the muffler is removed, the fan cover and the engine cover must be removed. To do this, you must unscrew the 4 Phillips head screws (star) located in the places circled in red. Once the 4 screws have been removed, all you have to do is pull lightly on the fan cover to dislodge it and remove it.


The engine cover no longer has a fastener, you can now remove it (unscrewing the spark plug first makes the task easier).
These 2 pieces of plastic are important, they direct the air pushed by the fan around the cylinder for good cooling. 

4. You are now ready to remove the original cylinder. To do this, locate the 4 10mm head nuts in the 4 corners of the cylinder head and unscrew them.

5. Once the 4 nuts are unscrewed, you can remove the motor head (it can get stuck over time, with a little effort it comes off easily).

6. You can now slide the cylinder out. As for the head (breech), it sometimes requires a little effort to come off the gasket (gasket).

7. Remove the air box from the left side of the scooter. It is held in place using 2 number 2 Phillips screws (star), as well as a ring at the level of the carburettor. Just unscrew the 2 screws and pinch the ring to remove it.

8. To remove the piston you must first slide the pin in the center of the piston. This pin is held in place by 2 locks (one on each side of the piston) called "circlip". To do this, we must remove a circlip on one side, a pair of ''long nose'' pliers or a small flat screwdriver does the job well. Once one of the two circlips is removed, you can push on the piston pin from the other side using a flat screwdriver or any other tool that rests on the pin. Once the pin is out, nothing holds the piston in place. When removing the piston, do not forget the needle cage which is located inside the hole of the crankshaft connecting rod.

9. Once the piston and the needle cage have been removed, carefully remove the gasket (gasket) from the lower engine. If the gasket tears and sticks, it will need to be scraped off with a blade or other tool (be careful not to damage the gasket surface!).

10. Now is the time to do the reverse operation, i.e. install your new motor! Here we have a DR.Evolution 70cc, an engine known for its good performance and legendary reliability.

11. Install the rings on the piston. There is no secret, you just have to open them and lower them on the piston at the same time (be careful not to break a ring by forcing too much!). Pay particular attention to the direction of the segments (the dark one at the bottom, and the notch in the opening of the segment at the top).

Align the opening of the segments with the notches provided to prevent them from turning, it will make the task easier for later. If the segment is not squashing into the groove as it should, check to see if they are upside down.

12. Fit one of the 2 circlips, preferably the one on the air box side because the one on the exhaust side is easier to fit when the piston is fitted (on the top of the piston, an arrow points towards the exhaust outlet). escapement, this is the way to get your bearings). There are several methods for installing circlips. With those in the shape of G, one seizes the small leg of the G, one places a part of the circlip in its groove (groove) and one turns it while pushing to make it return to its place. (The important thing is not to bend it and to make sure that it is well seated in its groove). To help you, you can temporarily slide the axis in the center of the piston. It will act as a wall and prevent the circlip from going too far inside the piston when you install it.
*This step is important since an incorrectly installed circlip causes serious damage to the engine!*

13. Pour some oil into the cap of your oil bottle and dip your needle bearing and piston pin in it.

14. To reassemble the piston, it is the reverse operation. The trick is to partially retract the piston pin then put the needle cage in the crankshaft connecting rod. Then place the piston so that you can slide the piston pin into the needle cage and push the pin in all the way in until it rests against the first circlip that you posed.

*Make sure that the arrow on the top of the piston points towards the cylinder exhaust outlet (the holes in the piston, or the notch depending on the kit you have, points towards the intake)*

15. Once the shaft is in the bottom, fit the other circlip. Make sure it is firmly seated in its groove. You can push the piston pin from left to right to confirm. Better to check now than to have a nasty surprise when starting up!

16. Clean the gasket surface and then slide the gasket which seals the lower engine and the cylinder. Be careful to put it right side up! In the case of our cylinder, there is a 5th hole in the gasket that lines up with that of the oil pump. It is rather rare on the other cylinders on the other hand.

17. Lubricate the inside walls of the cylinder with 2-stroke oil.

18. You have slipped the cylinder into place! The best method is to slide the cylinder over the top of the piston to the rings, then press the rings against the piston with your fingers and slide the piston into the cylinder. As with the installation of rings on the piston, there is no miracle method. Make sure that the opening of the segments is in line with the notches, as said in step 11.

Finally, push the cylinder all the way down.

19. Install the gasket that seals the cylinder and its head (head). Slide it into place as best you can without distorting it and lay your head on top. The shape of the head should naturally follow that of the cylinder.

20. Tighten the 4 cylinder nuts. Tighten the nuts diagonally, in X. Start with one corner, then make the corner completely opposite, followed by the other 2 corners. Do not tighten them all at once, it is important to tighten gradually. Tighten the 4 nuts to 10lbs/ft. If you do not have a dynamometric wrench (torque wrench), take a small ratchet wrench, butt the nuts and tighten from 1/4 turn to 1/3 turn afterwards. Be careful not to break the studs. If it feels too tight, it probably is!

Our spark plug is already installed, however, it makes it easier to install it at the very end, after installing the engine cover!

21. All you have to do is reverse the operations to refit the engine cover and the fan cover. Change the gasket of your exhaust, you have it in your hands. Everyone will be grateful to you!

22. Screw in the spark plug and reconnect the suppressor (the spark plug cable).
The stock spark plug will do on a small cast iron 70cc with the stock exhaust, but a BR8HS would be best.

You have now completed changing a 2-stroke engine on a scooter! As mentioned at the beginning of this tutorial, it is now necessary to readjust the carburettor according to this new engine.

*We wrote and imaged this tutorial a few years ago. We've decided to re-post it so it's useful to as many people as possible.*


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