The Importance Of Top End Engine Rebuilds

How-To Guides & Tutorials  |  13 August 2020

The Importance Of Top End Engine Rebuilds main image

A top-end engine rebuild is the first major piece of maintenance you'll need to undertake on your motocross or enduro bike, which is why it's incredibly important to ensure you do it at the right time to prevent further and costly damage. 

In this guide, we cover the importance of top-end engine rebuilds for both two-stroke and four-stroke dirt bikes.

When should I rebuild a four-stroke top-end? 
Rebuilding a four-stroke engine really comes down to how you ride your dirt bike, and the type of riding you're doing. 
If you're a regular racer, and especially if you're riding a 250 four-stroke, then you'll want to rebuild your new top-end between the 25-30 hour mark. 

If you're riding a 450, or riding the bike under an easy load majority of the time, then you'll want to rebuild the top-end between 30-40 hours. You should also rebuild the top-end when you do the bottom end rebuild.

When should I rebuild a two-stroke top-end?
​​​​When it comes to two-stroke top-end rebuilds, a general rule is that the piston and rings should be replaced every 25 hours, which will keep you in the safe range. 

Again, this comes down to how the bike is being ridden and what type of riding you're doing, but with two-stroke rebuilds being relatively affordable, it's always best to play it on the safe side with regular rebuild intervals.

If the bike is having a relatively easy life, you can replace the rings at 15 hours, and the piston between 30 and 50 hours.
You should also rebuild the top-end when you rebuild the bottom end.

Why do I need to rebuild the top-end?
It's incredibly important to rebuild you're the top-end of your dirt bike's engine within the correct intervals, as failure to do so could result in it seizing up. 

An engine seizure can be catastrophic to many parts throughout the engine, which will come with a significant price tag to repair - and in some cases, scrapping the entire engine - in comparison to the cost of maintaining rebuild schedules.

Another impact of not rebuilding your engine's top-end is that your bike will experience a decrease in power, which is usually a good sign that it's due for a freshen up.

What parts do I need for a top-end rebuild?
A top-end rebuild requires a new piston and piston rings, and depending on whether it's the first rebuild or one or two down the track, you might also be required to replace the valves and cams upon inspection on a four-stroke. You'll also need a new head gasket, which is generally included in top-end rebuild kits.

Do I need to run the bike in after rebuilding the top-end?
Yes, absolutely! There's a number of new parts, especially the piston, that need to break in and get up to temperature before you start ripping full intensity motos. You can read our full guide on running in your engine here: How to Run in a New Dirt Bike Engine

Can I rebuild the top-end myself? 
The short answer is yes, however this really comes down to your mechanical knowledge and confidence. Two-strokes are easier to rebuild than four-strokes, but if you're not confident in your ability, we advise taking your engine to a professional technician to complete the job and prevent the possibility of causing further damage.

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