Why Your Motorbike Emits White Smoke: the Piston Ring Gap Explained

Greetings everyone, we’re excited to be back teaching fundamental motorbike mechanics after a lengthy break. In this post, we’ll be discussing the white smoke coming from the exhaust of 4-stroke motorcycles, specifically, the root cause of the problem: the loss of the piston ring gap.

We’ve covered this topic in previous posts, but today we’ll focus on one specific issue. We have two videos for you, one accessible on YouTube, and the other exclusive to our blog. The first video explains why a 4-stroke motorcycle may smoke like a 2-stroke engine. This occurs when the piston ring’s clearance or gap is excessively slack, resulting in white smoke

White smoke coming out after losing ring gap
piston ring gap

The Surprising Reason Your 4-Stroke Motorcycle Emits White Smoke Like a 2T Engine

Usually, this issue occurs due to the motorcycle’s engine block being too loose from prolonged use. However, in some cases, the issue arises suddenly, as in the case of one of our clients whose motorcycle emitted white smoke after he “played” the flood. Upon inspection, we discovered that the middle piston ring had a piston ring gap that was too loose (wiper ring).

So, what is the typical piston ring gap for motorcycles? They say the ring gap end should be around 0.004mm for a street bike, but we measured it at 0.25mm. We don’t know where they got this information from, but we discuss this further in the video.

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Typical Motorcycle Piston Ring Gap?

In the second video, we demonstrate the cylinder block after it has been “rebored.” Here, we took a measurement of the piston ring gap and compared it to before the repair to determine how large it should be. After the repair, the motorcycle no longer emitted the white smoke it had previously.

piston ring gap tool

In the video, (for those who don’t know) we used a piston ring gap tool called a Feeler Gauge, which is also used for adjusting the valve. It’s important to note that if the piston ring gap is too large, the motorbike engine may have issues such as white smoke, a loss of power, and engine trouble. On the other hand, if it is too small, the engine may not start.

In conclusion, the piston ring gap is a critical aspect of motorbike mechanics that requires attention. We hope this post has been helpful in shedding some light on this issue.

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