Engine rebuild

I am new to dirt bikes so if this is a stupid question take it easy on me! I am looking to buy a used Yamaha 2019 YZF250. The owner says the bike has 90hrs on the engine and has recently did a top and bottom end rebuild. Is it common for the 4 stroke engines to need a top/bottom rebuild so early? I thought the 4 strokes were supposed to be a lot more reliable and durable. What is typical maintenance on the 4 strokes?


Staff member
Engine wear is 100% based on how it's used.
If the previous owner raced desert or MX for 90 hours, then yes, I would expect a top and bottom to be due or done.
If it was ridden on trails slowly with young kids or group riding, I would expect 100 hours for a top end and 200+ for the bottom.
If you read the manual it probably recommends a top end every 10-15 hours of MX use. I would be rebuilding before it was needed, if 'calculated flight' was my primary use for the bike. (MX)
The valves need to be properly adjusted as well, or they will wear faster than normal.

My answer is that regular valve spec, leakdown, ring end gap, and crank clearance readings through the life of a top/bottom end gives the best indication of life expectancy of any given build. You can develop a curve based on change over time to anticipate end of usable life for each of these components.

-BIG DAN:thumb: