Hello, after a lengthy break, we are back to teaching fundamental motorbike mechanics. The topic of my presentation today is the issue of white smoke coming from the exhaust on 4-stroke motorcycles. Since I spent a lot of time on this subject in the last posts, now I want to concentrate on one specific issue that is the root of this problem—the loss of the piston ring gap.
I have two videos for you mechanic students; one is accessible on YouTube, and the other is exclusive to visitors of this blog. I discuss one of the reasons why a 4-stroke motorcycle smokes like a 2t (2-stroke) engine in the first video down below. It results from the piston ring’s clearance (gap) being excessively slack.
Suddenly, a white smokes like they’re using 2T.
Usually, this issue arises due to the motorcycle’s engine block being too loose from prolonged use. In this instance, the owner “played” the flood, and the incident happened abruptly. After inspection, it was discovered that one of them, the middle ring, had a piston ring gap that was too loose (wiper ring).
What is the typical piston ring gap for motorcycles?
They said the ring gap end should get around 0.004mm for a street bike, but I got it around 0.25mm. I don’t know where they got this, see the video below.
I demonstrate the cylinder block in the second video after it has been “rebored.” Here, I took a measurement of the piston ring gap and could compare it to before the repair to determine how large it should be. Since it has already been put together, the motorcycle no longer emits the white smoke it formerly did.
piston ring gap tool
The piston ring gap tool that I’m using in this video is called a Feeler Gauge—which is also used for adjusting the Valve.
The motorbike engine will have issues such as white smoke, a loss of power, and engine trouble if the piston ring gap is too large. If it is too small, the engine will not start.