08 YZ125 Bolt on mods

I know from reading the odd post that the stock YZ125 is pretty potent, and pretty hard to improve on, and after spending a few weeks on it, I am very impressed by it in stock conditon.


With this in mind, is it worth throwing on the usual bolt on mods such as pipe, silencer, and reeds? 

I've got the suspension sorted and my favourite tires on it, just wondering if I should look to chase power, or simply ride it as it is.


Am I going to notice the difference if i drop 500 notes on the simple bolt on power mods as outlined above?


Would the guide for the YZ250 pipe test be a realistic idea of what each pipe does?


Could someone with a bit of experience outline briefly what each pipe does, ie PC top end, FMF bottom end etc?

Only thing bolt ons will do is shift the power of the bike where you want it, you may gain 1-2hp or so, if your looking for more power I would invest that 500$ into a 144 kit and jet it properly

Edited by 1000k6

We've recieved some great credible dyno data that shows some decent gains with the FMF Fatty pipe. I can tell you from my seat of the pants experience the gains are not substantial. I also experimented with vforce reeds and notice no difference from stock. Expert tuners can reap more gains with bolt ons than most of us "novice" tuners. So if you want my honest advice keep your yz stock and maintained and thats the cheapest path to consistant reliable power. If you are compelled to spend money , get your suspension reworked. That is truly good money invested that you will feel immediately.

I have a set of V-Force reeds in my 08 YZ125.  I also have a couple PC Works pipes and the FMF Fatty that is on there now.

The stock pipe is fine, even for a faster rider.  The stock silencer is fine too.  There is a very slight gain from both of those pipes, but not enough to make it your first mod.
The V-Force 3 makes more difference than the pipes.  On a tighter track it helps keep you in the meat of the power.  A friend has the Boyesen Rad-Valve on his 2006...  It's a better quality aluminum part, but the performance gain is similar to the VF3.

The biggest difference you'll notice is going to be gearing on a 125.  Buy a range of sprockets and change them per the track and conditions.
I feel like 49, 50 & 51 tooth rear sprockets will have you covered for anything from National tracks to Arenacross.

I'm a B / Intermediate rider and I run a 50 tooth most of the time at the smaller local tracks.  At Washougal and other fast national style tracks, I think the 49 is a better choice.
If you're a C or Novice rider, consider a 51 tooth sprocket for tight tracks and a 50 for national tracks.
I have no idea what a Pro would use, but it's probably close to what a Intermediate would use.

Woods gearing would probably be about the same unless it's really tight single track which might put you into a 52 depending on how much clutch you're willing to use.

Depending where you ride the bike rpm wise will determine just how effective a pipe works.

Most people over rev the 125 thinking that's how it should be ridden since it's a 125....if that's the case no pipe really helps much.

Just ride the thing and keep fresh top ends and clutches and tires on it.  If you really want some gains - a well built 144 is worth every penny.

Edited by harrperf

  If you really want some gains - a well built 144 is worth every penny.

AMEN !  My low/mid ported YZ144 with a 7oz FWW and the FMF Fatty pipe is buy far the most versatile, and forgiving  1/8th liter  engine I have ever ridden ! Took me some component experimentation to get there, but I have what I feel to be the best compramise of top end MX overrev (if needed) or all the way down to casual trail riding with my beginner 15 y/o daughter.

I had an 08 YZ 125 - stock. Now I have a Husky CR150. The power difference is very obvious, and it's well worth the money - it's no contest - and I'm sure if you converted your YZ to a 144, you'd say the same. If you don't want more power, I'd put money into a suspension revalve for your weight/riding style.


If you want more power, you won't find any satisfying bolt on power mods. If you decide to go with the big bore kit. Get a 144 kit from Eric Gorr (and tell him you're from New Zealand - as I think you may have to have special head-milling for the gas you guys use). He ports/polishes/bores your head/cylinder/power valves for a reasonable price. I'd stay away from Athena kits, but they work good too - IF you get the ones with a 1 piece head. The old 2 piece heads cause warping and other issues. They're about the same price as the EG kit, but it isn't ported/polished for a 144 - It's simply a 144 bore with 125 porting.

Edited by JRod4928

144 is the only bolt on that is really worth it.

144 is the only bolt on that is really worth it.


I agree. The 06+ yz125 is perfect all stock.  If you want to spend 500 bucks get the 144, otherwise save your money.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now