Skip to Content

When & Why To Replace Dirt Bike Valves & Timing Chain

Almost any dirt biker you meet today will agree that four-stroke dirt bikes are expensive when it comes to rebuilding them, especially if you have a shop do it. They have a lot more moving parts in the engine than a two-stroke engine does. This means that there are more things that can go wrong, and when something does break, it results in a domino effect that often grenades most, if not the entire engine.

There are ways to prevent these expensive repairs from happening. So instead of having your bike in the shop or sitting in the garage for weeks, or even months because you don’t have the money, time, or parts, you could still be riding because you did some “preventative maintenance”.

Junk YZ250F Cylinder Head with bent and crushed valves
You don’t want to wait until this happens, do you?

If you do a little research, you’ll see that valves can get fairly expensive, considering you usually have to buy four or five of them. What will cost even more is when one of them breaks and puts a hole in your piston, resulting in a destroyed top-end. As far as timing chains, this is probably the most disregarded part in the top-end when it comes to maintenance.

Many people think that timing chains will last hundreds of hours without failure because they’re just spinning on the crank and cam(s). Well with the crank spinning from 1,500 rpm up to 12,000 rpm, and constantly accelerating and decelerating, the cam/timing is going to wear out eventually. When it does have to snap or break, it will seize the engine and more than likely destroy most of the parts.

Now I’m not trying to scare you or to try to convert you to a two-stroke. I just want to inform you so that you know how to prevent something like this from happening. Now that you know why to replace these things, you should know when to replace them, otherwise, it will do you no good.

Are You Racing Your High-Tech Four Stroke?

4 stroke top end maintenance
Putting this 250 4 stroke engine back together in the garage.

If you race and actually ride your 250/450F hard, then it’s going to cause more wear on parts a lot quicker. Some bikes are different, and some valves can be defective, but I’m not going to say any names (Hon…da…..cough…cough).

When racing, I recommend that you check the valves every 15-20 hours. Doing so will give you an idea of how long they may last. If everything seems good and they are still in-spec then you can screw everything back together and ride some more. If they are slightly out-of spec then you know they’re starting to wear out, but should still be good. HotCams makes a Valve Shim Kit (VERY useful if you deal with a lot of 4-strokes) which makes adjusting easy.

When to actually replace valves is often the harder question. Some bikes can go a couple of hundred hours with the valves still in spec, but that is very unlikely if you are racing. I suggest you replace the valves around 40-60 hours.

Hard starting is a common indicator of at least a valve adjustment because they are tight and out-of-spec. If you are racing then it would be safe to just replace them when this happens. It may seem like a lot of work, but trust me when I say it’s cheaper to adjust or replace your valves every so often than letting them go until you drop one and grenade the engine.

What If I Trail/Play Ride?

As I said earlier, I’ve heard of many people going 150+ hours on their four-stroke motocross bikes and still be good, especially on 450F’s because you don’t have to ride them as hard. I recommend, as long as you aren’t riding every single day, checking your valves about once a year. If they are in-spec then you should be go to good for some more riding. Adjust them if they are out of spec.

If you’ve adjusted your valves 2-3 times, then it’s probably time for some new ones. If you wait another time to adjust them it could be catastrophic…. If you have never had to adjust the valves on your four-stroke dirt bike, it’s kinda up to you on how long you want to go. I would replace them every couple of seasons if I’m just trail riding, or after 100-200 hours.

What Is THE MOST Disregarded Part In Four-Stroke Engines?

Timing chains are the most forgotten part when doing maintenance on a four-stroke dirt bike engine. Why is this little chain so important?  The big deal is that it can break and blow up your engine, ’nuff said! Now if you replace it every so often it will prevent that from happening.

I explained earlier how timing chains get worn over time because of the abuse they get from the high-rpm engines. Not only can they break from fatigue, but after so much wear and tear from spinning on the crank and cam(s) it will get loose.

When the cam/timing chain gets loose it can slip and cause the teeth on the cam sprockets to jump. The timing is then changed, resulting in poor performance. If it jumps too much then the valve(s) can eventually hit the piston because the timing is so far off. And when that happens, it’s over for good.

Now do you get the idea about why it is so important to replace the timing chain? I recommend replacing it every 100 hours of ride time. That’s usually every year, unless you ride a lot more.

It’s a cheap part, and fairly easy to replace. It can save you hundreds in the long run if you just replace it after so long. Just call it ‘cheap insurance’.

Now you may disagree with me, but it’s pretty much a proven fact that Yamaha has been making the most reliable 250 and 450cc four-stroke motocross bikes. The other colored bikes would not last quite as long. But, within the past few years they have all stepped it up and got A LOT better.

So keep this in mind when you’re in the market for a used dirt bike. If you are looking to buy a new 250f or 450f, then it’s pretty much pick a color and ride.

Have fun, and don’t forget to do your “Preventative Maintenance“!

jared miller

Tuesday 23rd of February 2021

hello i saw a post on facebook and the guy said the bike had a snapped valve its a 2002 Yamaha yz250f but what is a valve and how mush does it cost to replace\fix it.

Kelley Fager

Wednesday 24th of February 2021

Hey Jared, thanks for the comment! A valve (intake or exhaust valve) is a part in the cylinder head of the engine. Unfortunately, if that snapped then there's usually several other expensive engine parts that need to be replaced. That can cost hundreds or even thousands of dollars, depending on the parts needed and if you need to pay someone to do the work.

Astrid Leicht

Sunday 18th of October 2020

My daughter just got her first dirt bike, a 02 Yamaha ttrL 125, do you think we should get a tune up on it? It looks fairly good maintained, just needs a new back tire, and we noticed the choke doesn't stay up, looks like they tried to glue the bottom part!

Kelley Fager

Tuesday 20th of October 2020

Hey Astrid, that's awesome to be hear about your daughter's first dirt bike! Yeah, unfortunately the choke mechanism on the TTR125 is prone to failure. Does it still start easily and run well? I wouldn't get a tune up unless there's a problem.

With that said, I would make sure that the engine oil is full and clean (not black), as well as the air filter being clean. Checking these two things will help make it last a long time.

Jack Scuderi

Monday 29th of June 2020

Thanks dude! I have a ssr play/trail bike and I don’t abuse it. People were telling me that you have to adjust them like every 10 hours! Lol thanks

Kelley Fager

Tuesday 30th of June 2020

Glad it helped! A simple trail bike like that should be able to go for a lot more than 10 hours before needing a valve adjustment. Just remember to keep oil in it and change it when it gets dirty, as well as keeping the air filter clean. Just doing those things should make it last a long time!


Tuesday 27th of November 2012

My crf230 stopped running. it died while i was out on the trail. I suspect it may be the timing chain. if the timing chain went what is the worst possible damage i'm facing. If the piston still moves and i replace it will it run again?

Kelley Fager

Thursday 29th of November 2012

There's no real way to know until you tear the engine down. Worst possible scenario would be a blown top and bottom-end from a collision of parts. If the bike still kicks over, then you might get lucky. I'd recommend having a friend that knows what they're doing look at it if you don't have the experience. Good luck!


Sunday 9th of September 2012

how many revs does a motocross two stroke do? and how many does a mx four stroke do? minimum to maximum