In search of 55hp

I didnt. I just got the top clamp so far.

Does anyone make a 475 kit for the YZ? I know Tom Morgan made one for the RMZ and it added a bunch of HP. Might be a way to go if they do.

Here is a big bore offer that claims serious power. I tried their work with a 315cc ice cube on my yz250f and it was nice. I had no reliability issues. If you go that route, let us know how it works.

478cc big bore: http://www.maxpower-engines.com/big-bore-kits/yz478f.htm

Here is a big bore offer that claims serious power. I tried their work with a 315cc ice cube on my yz250f and it was nice. I had no reliability issues. If you go that route, let us know how it works.

478cc big bore: http://www.maxpower-engines.com/big-bore-kits/yz478f.htm

That looks awesome. Is there a downside to a kit like that. $560.00 isnt that bad for 8-10 hp.

I can't verify the accuracy of their 8 - 10hp claim, but if you e-mail them, they will send you a real dyno. The 8-10hp could be at various rpm levels vs true peak power. Regardless, I am sure that the power increase would be huge. Do not worry about quality or reliability with those guys. If that is what you are looking for, you won't be sorry.

The downsides that I can think of are as follows:

1) The high compression bigger piston will be harder to kick over.

2) The bike will only be allowed legally in the Vet or Sr. class if raced in MX.

3) You may need to run 100+ octane for the best results.

4) You have to either send them your cylinder or pay an extra $300 (worth it) for a already bored out cylinder.

5) You should run an aftermarket exhaust that is less restrictive for best results.

6) My 315 liked to eat clutches, but I'm a two stroke guy (trying again with the '07 yz450f). Hinson's complete kit solved my problem.

None of the above reasons would talk me out of it. However, be sure you know what you want. As bad as I want to do this because I'm a hard head, I also own a Service Honda CR500AF that wears me out within the moto. Too much power without the skill to use it properly is tiring. You may need to buy dress shirts with a longer sleeve.

To anyone that has this kit or if you go forward, please report back. Good luck.

$860 for the kit and a new cylinder seems like a bargin for 8-10hp when you consider most pipes cost that. I remember MXA testing a big bore RMZ and they loved it. It was from Tom Morgan and they said it just added more hp across the board and didnt change the power spread at all.

I would love as well if someone has experienced this, although handling is more important issue for me.

I can't verify the accuracy of their 8 - 10hp claim, but if you e-mail them, they will send you a real dyno. The 8-10hp could be at various rpm levels vs true peak power. Regardless, I am sure that the power increase would be huge.
A favorite trick used by parts people to show off their stuff is to take a run or two with the stone stock, as delivered unit and compare that figure with the completely set up, tuned and jetted version sporting their equipment.

The power gains should follow at the close to the same power/liter ratio as the bike has now.

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