What is the difference between two-stroke and four-stroke?
The names count piston strokes. A four-stroke completes its cycle in four movements of the piston; a two-stroke does the same job in two, so it fires twice as often at the same engine speed.
NASA's Beginner's Guide to Aeronautics puts the four-stroke half plainly: the engine "is called a four-stroke engine because there are four movements (strokes) of the piston during one cycle", and it names them intake, compression, power and exhaust. Four jobs, four strokes, one power stroke out of the four. That is the engine in almost every road bike you will sit on.
A two-stroke folds the same four jobs into two strokes by doing more than one at a time. Intake and exhaust happen around the ends of the strokes rather than getting a stroke each. The result is a power stroke every cycle instead of every other one, which is where the reputation comes from: light for its capacity, eager to rev, and unmistakable to listen to.
What we will not claim
Nearly every article on this subject says emissions law killed the two-stroke road bike. It may well have. We went looking for a regulator or a standards body saying so and didn't find one, so we're not going to write it here as a fact. What can be said without argument is that new two-stroke road bikes are now rare in the UK, while two-strokes remain common in motocross, enduro and other competition classes where weight and power delivery are the whole point.
Either engine draws well. The ink and blueprint styles in the Paint Shop suit a stripped competition two-stroke particularly well, and what cc means covers the other number on the advert. Buying for someone who rides? Start at motorbike gift ideas.
Written by Craig Fearn, Throttlefolio. Last updated .
Workshop notes
Questions, answered
Why do some two-strokes need oil mixed with the fuel?
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Because there is nowhere else for the oil to live. A crankcase-scavenged two-stroke passes the fuel charge straight through the crankcase, so the lubricant has to travel with it, either pre-mixed in the tank or metered in by a pump. It is also why a two-stroke smokes.
Are two-stroke road bikes still made?
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New ones are rare in the UK. They are still very much alive in off-road and competition classes. The reason usually given for the road bikes disappearing is emissions regulation, but we have not found that stated by a regulator, so treat it as the received account rather than a settled fact.