Loosening the Bolt at the Top Dead Center of Compression is the First Step to Install the Bore Up kit!

Increasing engine displacement is the simplest way to get more power! Bolt-on bore-up kits for the 4-stroke 50-90cc class were introduced in the 1970s during the popularity of small-displacement motorcycles, and were a popular choice for tinkerers. A number of these are still available, but the first thing to do, regardless of model, is to set the piston to top dead center compression.

The 4-stroke uses a chain to set the intake and exhaust timing. What about 2-strokes?

In contrast to the horizontal type engine of the Honda Monkey and Super Cub, the engine installed in the Ape and XR series is called the vertical type because the angle of the cylinder is upright. The horizontal type, which is often used in export models, has been converted to a fuel injection system and is still in production, but the vertical type has been discontinued as a carbureted version. (*Note: Only the Ape 50 is available with fuel injection)

A four-stroke engine operates in four stages: intake, compression, combustion, and exhaust. The intake and exhaust valves are responsible for these processes. When the intake valve opens, the mixture flows into the combustion chamber, and when the intake valve is closed, the mixture is compressed and ignited by the spark plug, resulting in explosive combustion.

This explanation focuses only on the movement of the intake and exhaust valves, but the actual movement of the valves is linked to the piston. Even though the intake valve opens and the air mixture flows in, the air does not move spontaneously. As the piston moves down the cylinder as the intake valve opens, the pressure inside the cylinder drops and the air is sucked in. From the piston's point of view, it can be said that the intake valve opens when the cylinder is lowered, and the mixture is sucked in.

As the piston rises to compress the mixture, the intake valve closes, sealing the combustion chamber and compressing the mixture. Then, just before the piston reaches the top, the spark plug is sparked, causing explosive combustion that expands the volume of the mixture and pushes the piston down with great force.

Since the piston is pushed down and has momentum, it rises again in the cylinder beyond the bottom. However, what is inside the cylinder at this point is the mixture that has been burned off, so even if the plug sparks at the top, it cannot burn. Therefore, when the piston rises from the bottom, the exhaust valve is opened and exhausted to the muffler side. Then, as the piston descends past the top, the intake valve is opened and a new mixture is drawn in.

If the exhaust valve is opened while the piston is rising in the compression process, the mixture that is trying to burn will escape. Or, if the intake valve does not open while the piston is descending for intake, the engine will not start because there is no mixture inside the cylinder = no material to burn.

In a real engine, the top of the engine is called the top dead center, and the bottom of the engine is called the bottom dead center, with the intake valves opening before the top dead center and the exhaust valves opening before the bottom dead center, but in any case, the position of the piston and the opening and closing of the valves must be perfectly synchronized.

The camshaft chain synchronizes the position of the piston and the movement of the intake and exhaust valves. The camshaft chain is applied to the camshaft sprocket on the camshaft side and the crankshaft on the piston side. Since the piston and crankshaft are united through the connecting rod, it is thought that the movement of the piston and the intake and exhaust valves are interlocked by the camshaft chain.

Some 4-stroke engines do not use a camshaft chain. For example, the Kawasaki W650/800 uses a bevel gear mechanism to link the crankshaft to the intake and exhaust valves. The Harley-Davidson uses pushrods to transfer the crankshaft's rotation to the intake and exhaust valves.

In contrast to these 4-stroke engines, 2-stroke engines without intake and exhaust valves divide the process of intake, compression, combustion, and exhaust according to the height of the piston and the holes in the cylinder wall (ports). I would like to explain this mechanism on another occasion.

If you align it with the top dead center of compression, it's easier to get the position of the piston and intake, and exhaust valves.

Before disassembling the engine, remove the headcover and the engine left side cover in order to check the piston and intake and exhaust valve positions.

The T mark indicates the top dead center of the piston and the F mark indicates the ignition timing. Since the ignition timing comes before the top dead center, the flywheel turns counterclockwise in this image.

The sprocket bolt is off, but when the T-mark on the flywheel is aligned and the circle can be seen on the top of the camshaft sprocket, it is compression top dead center. When the T-mark is aligned and the circle is not visible, it is exhaust top dead center and the valve is open.

Bore-up kits, which increase displacement by enlarging the piston diameter, have been popular since the 1970s as tuning parts for small displacement engines. 50cc to 88cc is more than a 60% increase in displacement ratio, so it's no wonder the motorcycles are faster.

Many parts suppliers offer bore-up kits for both horizontal engines such as the Honda Monkey and Super Cub, and vertical engines such as the CB50 and Ape/XR. In the old days, these were carburetors, so a carburetor setting was used to increase displacement, but now that fuel injection has become the norm, kits are available with a sub-computer to control fuel injection.

When removing the cylinder head from the engine, it is important to align the pistons and intake/exhaust valves as explained earlier, regardless of the type of kit used, or even if it is for overhaul purposes rather than a bore-up kit. If the crankshaft and intake/exhaust valves are connected by a camshaft chain, disassembly in the correct position will result in a loss of timing alignment reference during assembly.

I mentioned in the previous section that it is important that the piston and valve movements are synchronized, but what happens if the valves are open at the top dead center when the intake and exhaust valves should be closed to trap the mixture?

At top dead center, the piston is closest to the combustion chamber, so if the valve is open, it could come in contact with the piston, which could bend the valve or damage the piston. If this happens, it's not just a matter of not sucking in the mixture, it could also lead to tinkering and destruction.

In order to prevent such problems from occurring.Consideration for assembly when disassemblingYou have to do it. How exactly do we do that? Keep the piston at compression top dead center during disassembly. The first thing to do is to close the intake and exhaust valves. That way the intake and exhaust valves are closed and the piston is at the top.

When looking at the relationship between the valves and the camshaft, the intake and exhaust valves are closed at top dead center of compression = the camshaft is not pushing the valves = the valve springs are not contracted, so there is no extra force applied when removing the camshaft.

However, this is a story for single cylinder engines, and the situation is different for engines with two or more cylinders. For example, in a four-cylinder engine with cylinders 1 and 4 on the same surface as cylinders 2 and 3, even if cylinder 1 is at top dead center of compression, the other cylinders will not be at top dead center of compression, and the intake and exhaust valves will be open somewhere.

When the camshaft cap is loosened to remove the cylinder head in this condition, the camshaft is pushed by the tension of the valve spring in the part where the valve is open. It is necessary to loosen the camshaft cap bolts evenly so that the force is not concentrated in one place. The same goes for installing the camshaft. If you tighten only one part of the cap, the camshaft pushed by the spring will be tilted and it may cause trouble.

It can't be helped that there is no timing when all four cylinders reach top dead center of compression at the same time, but it's also good to know that there are differences between single-cylinder and four-cylinder engines.

Align the T mark on the flywheel with the alignment mark on the crankcase.

When the piston is at top dead center, the two camshaft sprocket bolts are horizontal and interfere with the top of the cylinder head, making it impossible to use a socket wrench. Once the two nuts are loosened, the camshaft sprocket bolts can be loosened with a socket wrench.

The two camshaft sprocket bolts are the same size but made of different materials, so remember the position when removing them and put them back in the same position. Be careful not to drop the bolts.

Returning to the single cylinder engine, many engines have a mark on the flywheel to indicate the top dead center and the position of the piston. Piston position on flywheel? As mentioned earlier, the pistons are assembled to the crankshaft via the connecting rods, so the flywheel at the end of the crankshaft gives the piston position.

The image shows a Honda Ape with the letters T and F engraved on the outer edge of the flywheel which appears when the left hand engine cover is removed. The piston is at top dead centre when the T mark is aligned with the matching mark on the crankcase side. However, as explained in a previous article, there are two top dead centre points on a 4-stroke engine, compression top dead centre and exhaust top dead centre, so the cylinder head must be at compression top dead centre to be removed.

The camshaft sprocket that turns the camshaft is the one to check here. There is a round mark stamped on the camshaft sprocket near the outer circumference where the camshaft chain hangs, and if you can see a circle when you align the flywheel with the T mark, it is upper dead center of compression.

With the flywheel and camshaft sprocket aligned this way, the cylinder head can be removed without stressing the valve spring area because the pistons are at top dead center and the intake and exhaust valves are closed. Once the cylinder head is removed, replacing the cylinder and pistons with custom parts is not a difficult task to bore up.

Many motorcycle owners are uncomfortable with disassembling and assembling the engine, but if you can lower the hurdle to engine tuning by knowing the theory of disassembly as preliminary knowledge, you will be able to enjoy your motorcycle more.

Once the camshaft sprocket is removed, remove the four camshaft carrier nuts that hold the camshaft in place. These nuts are important because they hold the ends of the stud bolts that go through the crankcase, cylinder and cylinder head.

Since the intake and exhaust valves are closed at compression top dead center, the rocker arms are not pushing the camshafts. Therefore, the spring tension is not applied to the camshaft carrier and it can be removed smoothly. Up to this point, the engine can be removed with the engine on the frame, but when the cylinder head is removed, the frame interferes and the engine must be lowered from the frame.

If the engine is lowered from the frame, the cylinder head can be removed. It is possible to disassemble the engine by itself from the beginning, but it is easier to loosen it if the engine body is held when removing the four nuts of the camshaft carrier, so the process was carried out in the state of the vehicle.

Pull the cylinder out and the piston will come out from inside. At this point the piston is at top dead center. When you reassemble the pistons for bore up, it is OK for the crankshaft to turn, but when you set the new cylinder, put the cylinder on and restore the sprocket, make sure the T mark on the flywheel is aligned with the alignment mark on the engine side, and the circle mark on the camshaft sprocket is right above Hang the camshaft chain so that the circle mark on the camshaft sprocket is directly above.