The crankcase, where the crankshaft, transmission, and clutch are attached, is called "below the waist" as opposed to the cylinder or cylinder head, which is commonly referred to as "above the waist".
There are two types of crankcase, which is the foundation of the engine, depending on the composition of the parts: vertical division and horizontal division. There is no way to disassemble even a crankcase! However, just knowing how to assemble your motorcycle's crankcase is a good way to learn.
Cars are a separate entity. Motorcycles are one piece, and some are separate. What is it?
The Honda horizontal engine in the Monkey and Super Cub has a split crankcase on both sides. You can see that the transmission is to the left of the crankshaft.
The crankcases on the four-cylinder Kawasaki Z1 are split into upper and lower sections. The crankshaft center journal is held in place by the bearing caps, while the other journals are held between the upper and lower crankcases. The transmission and kick shaft bearings are also held in place by the crankcase.
Although they are both internal combustion engines, there are many differences between car and motorcycle engines, but in terms of structure, the combination of the engine and transmission is the most important.
In a car, the crankshaft, pistons, and cylinder heads are assembled to the cylinder block, and the transmission case with the clutch and transmission is bolted to the block. For this reason, manual and automatic transmission versions can be manufactured with the same engine body.
In contrast, the majority of motorcycles are equipped with an engine that integrates the power source such as the crankshaft and pistons with the transmission system such as the clutch and transmission. Therefore, it is not possible to choose a different transmission type in one engine, but it is possible to reduce the size of a motorcycle by integrating the two functions.
However, there are engines in some motorcycles that have a separate power source and transmission, just like cars. This is the case with the Triumph and Kawasaki W1 series before unit engines, and even current models such as the Harley-Davidson and flat-twin BMW.
These take the power generated by the engine out of the crankcase and put it into the transmission, which is separate from the crankcase, where it shifts gears and runs through the drive chain to turn the rear wheels. In a flat-twin BMW, the crankshaft is positioned vertically in the same direction as the motorcycle body, so the boundary between engine and transmission is hard to tell at a glance, but in a Triumph, W1, or Harley-Davidson, the crankcase and transmission case are separate, and they're connected by a primary chain case, so you can tell the difference.
The right side of the engine has a primary chain cover, while the left side has a separate transmission-like design, but is one piece. This is not surprising considering the current production facilities, performance, and ease of maintenance required.
POINT
- Point 1 - Many motorcycle engine crankcases have a crankshaft and transmission installed in one piece.
- Point 2 - In some older and current motorcycles, the engine and transmission are separate units.
Single cylinders are left and right, four cylinders are up and down, and two cylinders are both.
The Yamaha RZ250's crankcase is split into an upper and lower case. The crankshaft and transmission are placed on the lower side of the case and the upper case is placed over it. The crank bearings are the key to the engine's rotation, so the stud bolts on the bearing caps are thick and strong.
The Yamaha AX125, introduced in 1971, was a parallel twin-cylinder model with a split crankcase, a feature that had been carried over since Yamaha's first two-cylinder model, the YD-1, launched in 1957.
Some of the differences are related to whether the transmission is integrated or separate, but another difference between car and motorcycle engines is whether or not the crankcase is split.
Once the crankshaft is set in the cylinder block, the crankshaft journal, which is the axis of rotation, is held in place by a part called a bearing cap. Then the oil pan is installed underneath it. The rotation of the crankshaft goes into the transmission case, which is separate from the cylinder block.
Motorcycle engines, on the other hand, are housed in a crankcase with the crankshaft and transmission split left to right or up and down (except for separate transmission engines). The number of cylinders is used to determine whether the crankshaft is split left or right or top and bottom, depending on the number of cylinders.
Single-cylinder engines such as the Honda Monkey and Yamaha SR400 are split into a left-right split, with the crankshaft and transmission set in one of the left or right cases and the case on the opposite side. In contrast, a four-cylinder engine is split into an upper and lower case, and since a four-cylinder engine requires a minimum of five journals to act as bearings and crank webs for rotational balance, it is not possible to combine the cases from either side of the crankshaft.
In contrast to these, the parallel twin-cylinder engine is available in either a top-to-bottom or left-to-right split pattern. Current twin models such as the Yamaha YZF-R25 are split into upper and lower cylinders, but some of the older models from the 1960s, such as the Yamaha AT90 and AX125, have split engines. Also, the Kawasaki W1 and the old Triumph have a left-right split engine.
There are also V-twins and L-twins, depending on cylinder placement, but Ducati and Harley-Davidson have a left-right split and the Honda NSR250R has a vertical split.
POINT
- Point 1・The crankcase can be classified into two types according to the coupling method: the upper and lower division type and the left and right division type.
- Point 2 - Two-cylinder engines have a mixture of the upper and lower split crankcase and left and right split crankcase.
Sometimes you need a crankcase separator to split the left and right sides.
This is an example of a crankcase separator. The crankcase has three legs that can be set at any angle to accommodate a wide range of crankcase female thread positions, depending on the model. Many of the tools made by motorcycle manufacturers are specialized for their models and have two legs, but these tools are also designed to be versatile.
It's certainly more stable with three points of support, but as long as the separator bolts pushing the crankshaft don't tilt, there's no need to use all your legs. As with the wheel bearings, the press-fit bearings have some resistance at first, but once they start moving, they push out smoothly.
In the case of this engine, the bearings remained in the crankcase and the shaft was pushed out. Compared to the single-cylinder crankshaft, the two-cylinder crankshaft is longer than the single-cylinder crankshaft, so blows with a hammer are strictly prohibited. When assembling the engine, the installation tool must be used to pull the bolts into the female threads on the end of the crankshaft.
For engines with a split crankcase, the crankshaft and transmission shaft are held in place using the upper and lower joints (the NSR250R cassette transmission is inserted from the side of the case and held in place at the side). There are two types of crank journal bearings: needle roller bearings and plain metal bearings.
In terms of out-of-print motorcycles, the Honda CB750 Four is a plain metal bearing, and the Kawasaki Z1/The Z2 is a needle roller bearing. In both cases, you can remove the crankshaft and transmission from the case by removing the bolts and nuts that connect the upper and lower crankcase.
In contrast to this, the left and the right split engine is a different story. The crankshaft of a horizontal engine for a Honda Super Cub or Monkey can be pulled out by hand, but some models, whether single or twin, have ball bearings pressed into the crankcase for the bearings.
Striking the end of the crankshaft with a hammer will pull it out, but the crankshaft, which is assembled with exacting precision, must not be shocked unnecessarily.
This is where the crankcase separator, a special tool, comes into play. This tool is a combination of two or three legs and a bolt that can be angled at any angle, and it pushes the end of the crankshaft out of the crankcase by placing the legs on a female screw in the crankcase and pushing the end of the crankshaft with the bolt. The mechanism itself is simple, but it is very effective because you don't have to bang on the crankshaft.
The press-fit type crankcase is loose enough to be pulled out by hand on either side, and only one side of the crankcase is usually press-fit assembled, but a separator can be used even if both sides are press-fit. In this case, the crankshaft is not pushed out when you push on the crank end, but the crankcase itself is lifted up.
However, when one side of the crankcase lifts up, the transmission shaft and shift drum that goes through the crankcase also come out, so if you have to remove the circlip or other locking device from outside of the case for those shafts, you have to remove it beforehand. Be aware that the transmission section will not open when you try to lift it up and you risk damaging the crankcase.
The crankcase separator can be used not only to push the crankshaft, but also to pull the case cover out. When you stand the bolts on the cover and tighten the center bolt on the separator, the cover will lift up due to the reaction force pushing the crankshaft end. If the bolts holding the case cover to the crankcase are left in place, the cover is in danger of cracking, so be careful.
As a side note, it used to be common knowledge that the crankshaft bearings of a single-engine with a right and left split crankshaft were ball bearings. However, the Honda CB250R and CRF250 single-cylinder engines use special plain metal bearings, even though they are split into the left and right sections. The bearings are pressed into the crankcase and the bearing and crank journal are in a floating position, so the crankshaft can be removed without the need for a crankcase separator.
When installing the crankshaft that was removed using the crankcase separator, the bearing must be pressed into the crankcase. Since it is not possible to hit the end of the crankshaft with a hammer, you must use a special tool called the Crankshaft Installation Tool to do this job as well. This tool will be introduced at another time.
POINT
- Point 1 - There is a press-fit bearing type crankshaft in the left and right split crankcase.
- Point 2 - Use the crankcase separator to remove the press-fitted crankshaft.
