MX LITES 125, 134, 139, 144, 150, 155cc 2-STROKES

MX Lites 125, 144, 153

The FIM and AMA have strict limits for motorcycle racing and we can help you get the most for your racing class. We have experience in mx, off-road, dtx, hill-climb, trials, road-racing, and atv, karts. Some classes like School-Boy 2 have a limit of 125cc and we can build a high compression mid-range top end screamer with porting, plating, and Wiseco GP piston kits. Some classes allow 2mm/80-over maximum displacement limits that include our 134cc kits. Generally speaking the 134 is the best revver of our kits. They’re popular with racers moving up from superminis. The kits of 139, 144, and 155 give more torque to be competitive with 250Fs and are better suited for Vet mx and GNCC racers.


The best value for your money is a top end big bore kit. We can make the biggest difference in performance with the top end because we include porting with our kits with mods to the cylinder head and powervalves to suit your choice of fuel. The package deal starts at $500 and include the top end parts like the piston kit, bearing, and gasket kit. The displacement choices are based on model availability and the choices are 134cc +2mm, 139cc +3mm, and 144cc +4mm. Details of prices and model kits is listed on the Big Bore Masterlist page of our site.

required case mods for YZ and RM 85 and 125 big bore kits.

The YZ and RM 85 and 125 big bore kits require case mods. We provide that for FREE when you send your engine for a big bore kit.

We offer a free service of fitting the big bore cylinder to the crankcases that includes R&R of the top end, case and head adjustments, and evaluation of the engines’ crankshaft. For brands like Suzuki, Yamaha, and TM this is necessary and you can always do it yourself using our photo-instructions on the page titled Super144 Casemods.

All of our kits feature replacement parts that are easily available through multiple sources and 24/7 on-line ordering from our online shopping page.

Check out our videos on Performing case mods to big bore cylinders and Shipping your Engine to Forward Motion here.

What’s Included?

Before and After image of head mods

All of our kits include cylinder porting, head mods, and powervalve service. You get you choice of 3 powerbands and 2 fuel types. All the top end parts are included too like a Wiseco piston kit, top end bearing, and Cometic gaskets.

Powerbands and Fuel Types

The 3 powerband choices

  1. More low to mid-range with a flatter top end. By concentrating the power down in the lower revs it makes a powerband that lugs without chugging yet is responsive enough to steer with the rear wheel. Perfect for technical mountain climbing, single-track, Arenacross, ice-supermoto, beginner mx, short-track and TT.
  2. More mid-range to top end and a bit softer off the bottom end. This choice is best for expert mx, desert, dunes, dirt drags, beach racing, and dtx.
  3. Wider smoother powerband with less mid hit in the mid-range. This choice makes the power wider like a 4-stroke but without the harsh deceleration. It’s great for intermediate mx, off-road, and hill-climb.

The 2 fuel types

We machine your head flat, then machine the squish-band and bowl to match your powerband and fuel choices. The fuel types are unleaded pump 91 and race gas of 98-110. Pump gas is the most popular choice because you can also run race gas. We offer free updates for 1 year on porting changes and head mods.


For details on the exact sizes and prices, see our Big Bore Master Guide

CR125 1989-2007
KX125 1994-2014
125SX 2001+
144/150SX 2007+
RM125 1989-2014
(Casemods required, send complete engine or bare crankcases)

YZ125 1994-2014
(Casemods required, send complete engine or bare crankcases)
CR/WR125 1998-2009
125 MX&XC 2001+
(Casemods required, send complete engine or bare crankcases)
TM125 2000+
(Casemods required, send complete engine or bare crankcases)


Just CLICK & PRINT a copy of the Service Order Form and follow the directions on packaging your parts for shipping. All the options are listed for services, powerband choices, fuel types, spare parts, shipping methods, and payment options. Click here for the service order form.

FAQs on Big Bore and Stroker Kits

Q. Does the power hit really hard in the mid-range and flatten-out quick?

A. Click to reveal:

Typically big bores hit hard and flatten-out and that’s what magazine testers have complained about. The reason is that tuners cheap out and skip a step, cylinder porting. We include porting with all of our big bore kits and YOU CHOOSE the powerband type.

Q. Will I need to buy expensive aftermarket parts to make the kit work?

A. Click to reveal:

Our kits work with all stock and aftermarket parts except FMF SST pipes which make big bores run hot, but FMF’s Fatty pipes work great. European manufacturers like HGS (Nederland), DOMA (Belgium), and DEP SPORT (England) make pipes for 144 big bores. There’s no noticeable increase from a reed valve. Silencers make a big difference, long for top end, short for low end. Flywheel weights are only good for single-track off-road. A 1-tooth larger countershaft sprocket makes a big difference and compliments the greater torque of a big bore.

Q. Will a 144 pull a 250F?

A. Click to reveal:

Our 144 ported for mid-range to top end has more peak power than a stock 250F but the four-strokes still have a bit of an acceleration advantage in hard dry conditions and through rhythm sections. The main advantage of a big bore 125 is the bike is much lighter, there’s no brutal deceleration, and its easier to ride over rough terrain and holds a tight line through turns. Big bore 125s are far cheaper to operate than 250Fs.

Q. Does the crankcase need to be grinded to accept the bigger piston?

A. Click to reveal:

Most models don’t need case mods with the exception of Yamaha YZ, Suzuki RM, and TM125s. We offer free R&R of the top end with crank inspection and case mods on applicable models.

Q. What about the carb jetting and pre-mix oil and ratio?

A. Click to reveal:

Technically you need to adjust the carb jetting when the air density changes but most 144s can use standard 125 jetting. Normally you have to jet a 144 leaner but some models like the 2006 and newer YZ125s need a richer needle and main which we have available. On the flipside, Suzuki RMs need to be jetted much leaner. Jetting has most to do with where you ride and the average air temperature. On certain models, we supply specific jet needles and main jets with our big bore kits. Regarding pre-mix oil and ratios, you don’t need to alter that, but we recommend Maxima Castrol 927 at 32:1 because it mixes with pump gas.

Q. What about the gearing?

A. Click to reveal:

Always gear for more speed because of the increased torque. Typically you should increase the countershaft sprocket by 1 tooth.

Q. Will the 144cc piston make my bike vibrate? Does the crank need to be re-balanced?

A. Click to reveal:

Our custom Wiseco Pro-Lite forged piston kit is the same weight as a stock cast piston, so there are no vibration problems caused by the 144 piston. However if you’re building a big bore, long rod stroker for top end, then you may want to have Crank Works in Tempe AZ change the balance factor for best results.

Q. Will the engine wear out faster or be less reliable?

A. Click to reveal:

We build-in reliability with thicker cylinder plating, a coated forged piston, and polished port edges. Our 144cc kits are more reliable than a stock 125.

Q. Is the price of the 144 piston really expensive like RPM’s pistons?

A. Click to reveal:

The price of the 144 piston kit is priced lower than a 125 piston, plus our custom Wiseco has anti-wear coatings, more surface area to transfer the heat, lightening slots in the inside of the piston to balance the crankcase compression ratio, longer in length for better stability in the bore, and a smooth intake to boost the engine’s over-rev. Replacement cost is $99 or $150 for the piston kit, gasket kit, bearing, and 3-day Priority Mail shipping! The average service life is 10-30hrs for ring and 20-60hrs for piston changes.

Q. Are their any alternative pistons for the 144?

A. Click to reveal:

Eric Gorr of Forward Motion invented the 144 and because of the explosive growth in 144 conversions, many other companies have copied his custom Wiseco design with some minor changes in material, shape, and weight. However the copycat pistons are more expensive.

There are two types of pistons; forged from billet and cast from aluminum alloy. Athena makes the only cast 144 piston, whereas the forged piston choices include our custom Wiseco, Wiseco GP, Vertex, and Wossner.

Our piston is a custom Wiseco and they make their own GP model 846MO5800 which is a bit lighter but 1.5 times the price. The Vertex 23335A uses the same forge tooling as my piston with a different crown shape that is more flat like the Wiseco. The Vertex is the heaviest forged 144 piston. Wossner’s forged 144 piston was chosen by Yamaha for their GYTR kits. We sell our custom Wiseco, their 846MO5800 Wiseco, Vertex, and the cast Athena pistons (for the YZ and KX Athena kits). The Wiseco, Athena, and Vertex are more expensive than our piston at $155.

Q. How fast can I get my kit and do you have core exchanges?

A. Click to reveal:

Turnaround time is fast because we have many core cylinders in stock for immediate trade-up. And if you have a new cylinder and want to modify it, we’ll stamp your name on it and the time varies by season, longer in the wintertime because that’s our busy season.

Q. What if I live outside the USA, in Canada, Europe, Australasia, or South America? Can I get a big bore kit and buy parts from Forward Motion?

A. Click to reveal:

Yes, we specialize in international business. Just CLICK & PRINT a copy of the service form and send it with your parts. Send your parts by airmail (Par Avion) or a popular shipping company like UPS, FEDEX, DHL, or Purolator. State on the customs documents that you are sending your engine block to America for repairs. We’ll resend your customs docs back with your order and declare the value under $200 USD so you don’t pay taxes. We’ll send your proper invoice by airmail, not email because the EU monitors private email. We’ll email the airbill tracking number so you’ll know when to expect the package. We are experts at international shipping and documentation.


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