YZ 285 Big Bore

Motorcycles Kawasaki YZ285Dirt Rider YZ285

Please call to order 608-842-0680

Special Price: $599

Max Power YZ285 Fits 1999-Current Yamaha YZ250


Product Description

Actual Displacement = 285.06cc

Engine Modifications Required:

Cylinder: Bore, Port and Plate.

Power Valves: Precision machined to match big bore cylinder.

Head: Re-chamber to work with the Big Bore piston.  You choose the octane.

Crankshaft: No crank mods required.

Crankcase: No crankcase mods required.

Notes: This kit requires you to send us your cylinder, head, and power valves for modification.

The YZ285 is the most popular big bore kit we sell for any 250 because it works so good.  It provides a smooth predictable power delivery with just the right amount of hit to keep things fun.  You will love this kit.

Check out the reviews in Dirt Rider, Motocross Action, or Dirt Bike Magazine.

Going bigger than a 71 mm bore on the stock YZ250 casting requires welding work to cylinder to keep it from popping o-rings.  The transfer ports also start to drop into that area where it becomes more of a bottom end chugger and not nearly as snappy.

Our Custom 71 mm Wiseco piston is made to be as light as possible without sacrificing durability.  It weighs only 15 grams (5%) heavier than a stock YZ250 piston.

If you already have an Athena YZ295 kit it can be livened up to close to a YZ285 style power with some porting and head adjustments.  ($250)  But I’m afraid we cant fix the vibration for you without splitting the cases and balancing the crank for you. ($625)



  1. DirtSaw says:

    YZ285 Kit is awesome
    This is a must do when your old 250 needs to be replated. Spend a few extra bucks. You won’t be sorry. The speed is deceiving. The bike doesn’t sound like it’s reving out so it hides its power. First race out I hit 84.9 mph and still had a hair more left. Everything was a blur…

  2. Vet Racer says:

    Yamaha really should make this bike. It is the best 2 stroke around. I am on my 3rd year and have had no problems, the cylinder plating still looks great. This kit does add horse power but it is still controllable unlike the old 500’s. The best thing it adds is the torque, when your off the pipe with the 250, you get a little power when you turn the throttle, you turn more but no more power, that is where the 285 shines, kepp turning and power keeps coming, you can grunt it up hills from almost a standstill in 2nd, or do switchbacks in 3rd all day long.

    It will get you lazy and short shift the jumps on the track, but that is ok for us vet guys. Down the stretch I am even with the 450’s, and once you hit 3rd gear you will pull from any 250.

  3. raymo says:

    Your friend will be bummed
    I have been running the 285 kit on my 05 250 for almost two years this last fall I put a wide ratio tranny in my Yamaha. I have a friend that just got a 09 450f. No matter where we race he can only beat me on top end IE over 75 MPH we are running the same gearing IE stock for the kx 13-50 I’m running the same on my 285. He just shakes his head,and says that thing is really fast. So last week he ordered a new bomb header and new computer programmer. He says it is only to make it start better. But i know the truth he can’t stand that my lowly 285 yz is a fast as his brand new fuel injected 450f. I highly recommend the staff at Max RPM they have been awesome to deal with.

  4. scorr racer says:

    yz 285
    I love this kit just got ours re done last week and this thing is so quiet and it just shines anywhere no matter what the conditions. woods or moto. it can kick a 450s but any day. Love it Thanks RPM!

  5. trail rider says:

    love the power that the yz285 puts out compared to my stock 250. it lifts wheels shifting down the road all the way to 3rd gear. looks good on my bike and cant stop saying it but its so powerful. i blew my friends honda trx450 away in a drag race. would highly reccomend this kit without doubt. need to wear goggles cause the wind makes your eyes stay shut its really that fast.

  6. andy powell says:

    andy love this kit. I am a 61 year old vet rider who has not drank the 4stroke Kool-Aid. I have rode all of the 4strokes and the power is slow to come on. I am not a clutch person don’t slip the clutch when coming out of the corners. the 285 kit ads torq to the low to mid power. on top it pulls and revs a lot better.

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