Unlike the crankcase of a 4-cylinder engine, which is divided into upper and lower parts, the crankcase of a single-cylinder engine is divided into left and right parts (some models had a single-cylinder engine with a crankcase divided into upper and lower parts). The crankcase consists of a crankshaft, two transmission shafts, a shift drum, and a kick shaft, and some engines incorporated a balancer shaft. Among these single-cylinder layouts, the 2-stroke engine has a simpler structure than the 4-stroke engine. In this article, I will focus on the upper part assembly of an old 2-stroke engine and show you how it is done.
Replace bearings for overhaul
When press-fitting the bearings into the crankcase, instead of installing them in the cold, warm the crankcase after degreasing and cleaning (using a high-temperature dryer is convenient) so that the bearings can be assembled as if they are being sucked into the bearing holder rather than being press-fitted. Once the case temperature is down, use a bearing driver to tap or push the outer ring to make sure the bearing is firmly seated in the bottom of the holder. If the bearing is not available as OEM parts, you can buy it by referring to the standard number on the side of the bearing, but you should be aware that some models have special bearings that have been modified from the STD bearings.
Bearing cap gasket is handmade
After the left and right crank cases were tightened and integrated, case boring was done and the crankshaft and transmission shaft bearing holders were machined. This engine has a structure where the cover is tightened and fixed to the crankshaft holder, but there were no special gaskets on hand, so I decided to make my own by making a stone rubbing with copy paper and a pencil and cutting it out of gasket paper.
Kawasaki's unique special tools
When installing the crankshaft, it must be in a low pressure fit situation against the crank bearing inner ring. Kawasaki of this era (from the early 60s onward) had special tools for pushing a wedge-like spacer into the crankshaft while straddling the connecting rods to avoid damaging the highly accurate crankshaft, and for hammering the crankshaft under conditions where distortion was less likely to occur. After straddling the connecting rod and setting the special tool in the gap between the crank webs, insert the crankshaft into the bearing and set the pipe-shaped special tool from the other side to low-press-fit the crank. In this case, the crankshaft was incorporated by lightly tapping it with only the weight of the hammer, rather than gouging it.
After installing the transmission unit, check the operation.
This engine has the crankshaft, transmission unit, and kick shaft installed in the right side crankcase. After setting the crankshaft, set the transmission unit with shift drum and shift fork at the same time. Once the bearings and drum holder are settled, set the kick shaft and try to turn the shift drum with your fingertips. Make sure that each gear can be shifted smoothly at this stage as well. Also, make sure the kick gear engages properly at this stage.
Apply oil spray and check crank center
After checking the transmission entry and kick gear engagement, thoroughly apply oil spray to the crank bearings, big end of the connecting rod, and all parts and bearings of the transmission. In the past, I would have used an oiler to pour the oil, but nowadays there are high-performance oil sprayers that are much easier to use. After oiling, degrease the crankcase joints, apply a solvent-based liquid gasket (fuel-resistant liquid gasket), and join the left and right crankcase together. At this point, tighten the crankcase, making sure that the crankcase sender alignment and the center of the connecting rod are almost exactly the same.
Tighten the bearing caps diagonally and averagely.
After tightening the left and right crankcase to the specified torque, check the crankshaft and transmission for smooth operation. If the crankshaft rotation feels sluggish, tap the end of the left and right crankshafts with a plastic hammer to recheck the bearing seating.. This work often results in smoother crankshaft rotation. If the rotational resistance remains the same, the accuracy of the single crankshaft must be rechecked. The same goes for the transmission shaft, check for smooth shifting with the case tightened. If it is reluctant, try lightly tapping the transmission shaft end with a flat punch to settle the bearings and shift fork. After confirming that each shaft moves smoothly, tighten the crank end cap diagonally across the gasket (control the torque while tightening it averagely), and check the crankshaft rotation again before moving on to the next work.
- Point 1: Be careful not to deviate from the assembly accuracy of the crank unit when installing the crankshaft.
- Point 2: If you don't have a gasket, you can make your own DIY gasket.
- Point 3: When assembling, do not rush ahead, but check the operation of each item as you go.
There are many engines, not only single-cylinder engines of old 2-stroke motorcycles, that have a "2-split" crankcase configuration. There are many examples of the current newest models that have almost the same parts configuration. In the case of multi-cylinder engines with 2 or more cylinders, the crankshaft is often assembled between the upper and lower crankcase for ease of parts production and assembly, but single-cylinder engines still use an assembled crankshaft. In many cases, the crankshaft and transmission parts are built in such a way that they are sandwiched between the left and right crankcase.
When overhauling such engines, the bearings that have been hammered into the left and right crankcase must be removed and replaced with new bearings. Heat is a great thing to use in such cases. In the past (and it still works in the present), the cleaned crankcase would be placed on the stove, and when the crankcase was warmed up, a new bearing with a thin coat of oil would be placed in the bearing holder. When the crankcase is cold, the bearings will not fit unless they are press-fitted, but when the crankcase warms up to a certain temperature, the crankcase expands and the bearings "pop in" very smoothly. When you experience and witness this "crankcase warming strategy", you will once again be reminded of the amazing power of thermal expansion.
After inserting the bearings into the crankcase using thermal expansion, wait until the crankcase cools down to a certain degree, i.e. until the bearings cannot be pulled out with just your fingertips. Once the bearing is stable, place the crankcase on a stand and use a bearing driver to tap the bearing outer ring on average. Even if you think you have inserted the bearing into the bearing holder, the heat shrinkage (during the cooling process) will actually tighten the bearing and often cause it to lift slightly out of the holder.
When setting the crankshaft in the bearing, do not tap the crankshaft to insert it into the bearing inner ring. This may distort the centering and runout of the crankshaft. Nowadays, there is a special tool for pulling in the crankshaft end, but in this era, Kawasaki had a special tool for setting the crankshaft into the bearings while maintaining the crank width (by lightly tapping the crankshaft). Currently, it is possible to assemble the crankshaft into the crankcase while maintaining the accuracy of the crank itself by using the threaded end of the right side crankshaft and using an assembly tool that pulls in from the outside of the crankshaft crankcase.
When combining the left and right crankcases, is it possible for the crankshaft and transmission shaft to rotate smoothly? In particular, can the transmission be shifted smoothly? Check the crankshaft and transmission shaft by turning it with your fingertips. Don't rush the assembly process, but be sure to check the shifting condition when the crankcase is assembled. I'm sure there are many Sunday mechanics who have forgotten to do this, and then proceeded with the assembly, only to have the whole thing come apart again later on. After assembling the left and right crankcase according to the tightening torque, turn the crank end with your fingertips to see if it turns smoothly. Can the transmission shift from neutral to each gear with a click? While turning the transmission shaft, try turning the shift drum to see if the transmission can shift from neutral to each gear. If the bearings are not seated properly, the movement will be sluggish, and you can often solve the sluggish movement by tapping the end of each shaft with a plastic hammer.
In the case of 2-stroke engines, as a general rule, "fuel-resistant liquid gasket" should be utilized for the mating seating surface of the crankcase, as the mixture (fuel, engine oil and air) is suctioned into the primary compression chamber. When applying a liquid gasket to the mating surface of the crankcase, use "fuel resistant" for 2-stroke engines, or the usual "silicone-based liquid gasket" for 4-stroke engines.
