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How To Tell If Bottom End Is Bad On Your 2 Stroke Dirt Bike

Wondering if your 2 stroke dirt bike has a bad bottom-end? This article will help you figure that out, how to inspect it, as well as how to help prevent it from happening sooner. 

2 stroke bottom end noise

Do you hear a whining or a grinding noise in your engine while riding? There’s a chance it may be the bottom end. 

Unfortunately, it’s hard to know for sure without taking the engine apart. With that said, if there’s a new noise that’s getting louder, you need to stop riding and figure out what’s causing it. 

Colorado Trip 2016 3 How To Tell If Bottom End Is Bad On Your 2 Stroke Dirt Bike
I would not want to hear a new engine noise out on this type of a ride…

Continuing to ride with a loud noise may end in your engine blowing up and resulting in a major crash or getting stranded. 

When to replace bottom end on a 2 stroke

If the crank has seized, then you absolutely need to replace the bottom end. If the bearings are worn out or the rod is bent, then it will either need to be rebuilt or replaced.

How long does a 2 stroke bottom end last

The bottom-end crankshaft will last longer than the top-end, but the exact amount of hours is not certain. It really depends on how you ride and how well you maintain your 2 stroke dirt bike.

You may have to rebuild the bottom end every other time you rebuild the top-end. On the flip side, you may get 500+ hours on the stock bottom-end.

The harder you ride (high RPM), the shorter it will last. Also, not using enough 2 stroke oil or having a clean and properly oiled air filter will greatly reduce the life of the bottom-end.

Funny story

Bad crank bearing symptoms 2 stroke

Bad crank bearings can make extra engine noise when they’re bad. They size the bottom end sooner than later if not replaced. Worn bearings will also have extra movement when installed in the crankcases.

How to check crank bearings on a 2 stroke

You must have the top-end removed from the engine. This includes the cylinder and head. 

To check the crank bearings, simply wiggle the crankshaft connecting rod. There should be zero movement up and down, but a small amount of side-to-side play in it. 

How to tell if a 2 stroke engine is blown

It can be difficult to tell if your engine is blown up for sure without taking it apart. With that said, here are the most common symptoms of a blown 2 stroke engine:

2005 Honda CR85RB 1 How To Tell If Bottom End Is Bad On Your 2 Stroke Dirt Bike
This CR85 engine is locked up but I’ll have to tear it down to figure out why…

How do you know when your 2 stroke dirt bike needs rebuilding?

If the engine is seized or the compression is low, then it’s time to rebuild your 2 stroke engine. It may only need a new piston ring (or 2), but it could need a new piston kit and/or a crankshaft rebuild. 

The only way to tell for certain is to start tearing down the top-end for inspection. 

2 stroke worn top end symptoms

A worn top-end will generally have low compression, foul spark plugs, be down on power, and is harder to start. 

What causes a 2 stroke to lose compression?

Wear and tear on the engine is what causes any dirt bike engine to start losing compression. However, there’s other factors, and they can be prevented if you pay attention and do some simple maintenance

For one, you need to keep the air filter clean. Running a dirty air filter for too long will cause dirt to get into the engine. Why is this bad? Because dirt eats away at the metal and causes it to wear out much faster than an engine that has clean air running through it. 

How to prevent catastrophic engine failures

Having an engine break down on you sucks, but what’s worse is when it happens while you’re out riding miles away from your truck or camp. That’s why I want to show you 3 simple tips to help prevent this from happening with my free guide. Click here to download it.


Monday 17th of April 2023

My son has a 2016 KTM 100 and has had to have the top end done twice in the last two months. He does not cold seize it and does not race. We just got back the bike and he took it out and again the bike lost compression. Any ideas on what’s going on?

Kelley Fager

Tuesday 18th of April 2023

Hey Kristy, I'm terribly sorry to hear that - it must be very frustrating! Did you bring it to the same shop every time? What gas are you running in it?


Sunday 10th of July 2022

Thanks for the article! I did have a question though. I'm looking for a KTM 2 stroke for trail riding, so a 250-300 xc or xcw. I found a 2018 with 250 hours, 110 on the top end, and I wondered how I could tell if it needed a bottom-end rebuild without pulling the cylinder and feeling for play.

I imagine that with 110 hrs on the top-end it would need a rebuild but if there was a way to see if the bottom end needed to be rebuilt before I purchased the bike, I might be able to negotiate the price.

Thank you!

Kelley Fager

Saturday 23rd of July 2022

Hey Tevah, sorry for the delayed response! I actually just included some info about this in my latest article here on knocking noises. You can pull the pipe off and tap on the piston - if it wiggles then there's probably something loose in the bottom end. Honestly, 110 hours isn't that many on a 250 or 300 if it was just casually trail ridden. If it still starts easy and has good compression, I probably wouldn't do anything to it until at least 200 hours or more. These bikes are reliable if properly maintained.