Where is the crankcase on a motorcycle

Why does the bottom bit, the crankcase, also pump air? Everything outside of the cylinder is just there to make things work, not pump air surely?. Indeed there's no point in pumping air around the crankcase. The crankcase is a sealed unit to keep the oil in and dirt, water and other nastiness out.

As the piston rises and falls the underside of the piston is also going up and down. As the underside of the piston is within the sealed crankcase the air within is compressed and relieved as the piston whizzes up and down. There is quite a lot of air within the crankcase, certainly compared to the cylinder itself.

As such the pressures achieved are quite minimal and as you can see in the video I can easily seal up the crankcase with almost no effort using my thumb or finger. Logically if the underside of the piston rises and falls the pressure will - slightly - rise and fall within the crankcase.

So why do we need a crankcase breather? The pressures in the top part of the cylinder, the business end, the air pump end, are immense. No matter how well a piston is sealed by it's rings within the cylinder bore some of that pressure escapes into the crankcase. Do not pry and damage where the cases meet or they will not seal properly and you will have an oil leak.

If you are having no luck, walk away and try again later. Sometimes the case will loosen itself after being unbolted for some time. Try to get some penetrating oil near the knock pins of the crankcase if you suspect they are rusted in. Keep everything in order and clean when your engine is apart. When you are ready to put it back together, follow the installation sequence as specified in the shop manual. If you are unsure, it can help to assemble things in a dry run to make sure you can shift through the gears and that everything spins properly.

Place a small amount of liquid gasket along the crankcase surfaces and shut them back together. Make sure all the knock pins are in and the cases seal properly. Do not bolt the cases shut if they are not sealing back together correctly — something is likely wrong. Refer to the shop manual for the proper torque values and tightening sequence for the crankcase bolts.