YZ139 Big Bore

Motorcycle Kawasaki KX 139

Please call to order 608-842-0680

Special Price: $549


Quick Overview

YZ139 Fits 1997-Current YZ125


Product Description

Actual Displacement = 139.07cc

Engine Modifications Included:

Cylinder: Bore, Port and Plate.

Power Valves: Precision machined to clear piston.

Head: Re-chamber to work with the Big Bore piston and you fuel octane specification.

Crankshaft: No crank mods required.

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

We have built more YZ139’s over the years than any other kit we offer.  In our opinion it is the perfect bore-only displacement available for the YZ125.

So why should you choose a YZ139 over a YZ144?  At risk of getting too technical, here is the main differences we have found.

Piston weight.   The Max Power YZ139 uses a 57 mm 2-ring Custom Wiseco piston that weighs just 4% more than stock.  The YZ144 uses a 58 mm piston that weighs between 12.7% and 22% over stock.  Most 58 mm pistons from other sources are around 16.5% more than stock.     Our 144 piston is made to be as light as possible.  It still measured at 12.7% over stock and we still don’t care for the way they run in the YZ125.  The extra piston weight just makes 144’s rev slower and vibrate a little bit.

As long as we’re talking piston weight, the Wiseco shelf stock 139 piston for the Honda has been used to make 139’s in the YZ125.  That piston also weighs 20% more than stock and only uses one ring.   The moral of the story – not all 139 pistons are the same.

Port Timing.  The bigger you bore a cylinder the more the transfer ports drop.  The difference between a 57mm bore YZ139 and a 58 mm YZ144 is equal to a couple degrees of crankshaft timing.  While that may not seem like much it is the difference in an engine that has good over rev and one that does not.  In all fairness there are ways to correct this with situation porting but we rarely see it done this way.

Case Modification.  The YZ144 requires it the YZ139 does not.  However you need to check it on 2005-Current YZ125’s.  We have seen it touch slightly on a couple of them in the past.

So you may be thinking that we hate 144’s.  No we don’t, we just like the way our YZ139 runs much better.  The 144 works real well in older KX125’s (previous to 2003) and Husqvarna CR and WR125’s.  That is why we advertise those.

If you have had a YZ144 and liked it you should try a YZ139.  I promise you you will not regret it.  Seriously you get the whole package.  Bottom, mid, and top with over-rev.  The latter being something you don’t hear about too often with big bore engines.

There is a reason why this is our all time best seller.

Check out this 2006 YZ139 review we saw on Vitalmx.com


  1. Jay Jackson says:

    After doing some research I decided to have Max Power build me a YZ139 based on their long standing reputation and I couldn’t be happier. This thing is a rocket. It pulls hard all the way through and still revs out like a 125. In all of my years of racing this is by far the best power increase per dollar I have ever seen. They do everything in house and the workmanship is top notch. They actually return calls and emails and will give you all the tech advice you need. Thanks Max Power!

    Jay Jackson,

    Battle Ground, WA.

  2. Rory C. says:

    Have had the 139 in for a few weeks now and it is amazing. My bike now actually wants to wheelie when it hits the powerband. I race harescrambles on the eastcoast and after the fist race on it I can tell it keeps me riding smoother since the low end power actually pulls.

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