2023 is the last year I will produce PORTED big bore kits with my own two hands.
I will continue to sell and market my custom pistons and gaskets and manufactured cylinders.
Starting January 1, 2023 you need to call or text for an appointment because I have limited space
in my workshop. No more sending engines without paperwork or a clear ability to pay for the work.
Text or call Eric Gorr for a time estimate and appointment (630) 825-5645

Big Bore 125s

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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 $450 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 Master List of our site.

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.


What’s Included?


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 List

CR125 1989-2007

KX125 1994-2016

125SX 2001+
144/150SX 2007+

RM125 1989-2016
(Casemods required, send complete engine or bare crankcases)


YZ125 1994-2016
(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)

FAQs on Big Bore and Stroker Kits

Q. Will a 134 or 144 out-pull a 250F?
A. Our 144 ported for mid-range to top end has more peak power than the a stock 250F but the four-strokes still have a bit of an acceleration advantage in hard dry conditions and through rhythm sections.

The 134 kit is more free-revving than a 144, so expert kids coming off of super minis prefer a 134 to a 144 whereas novice and vet riders like the torque of a 144.

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. 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 fuel? If I get a 144 will I be stuck having to run expensive race gas?
A. You tell us what type of fuel that you want to run and we’ll set the compression ratio and squish to suit. All stock 125s are designed to run 98+ race gas. If we set the compression low for pump gas the engine won’t make as much torque. Raising the compression and reducing the squish clearance for race gas helps throttle response and makes the engine run cooler at peak RPM and that makes more power. We recommend VP brand fuel and their C-12, MSO2, and U4.4 fuels. Oxygenated fuels like MSO2 and U4.4 require richer jetting similar to pump gas with ethanol. Generally speaking, its best to avoid ethanol or alcohol blended fuels because they tend to separate the pre-mix oil from the fuel, which can cause piston seizures.

Q. What about the carb jetting and pre-mix oil and ratio?
A. 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 for hotter weather. Stock jetting is usually about perfect for 60-70 degrees Fahrenheit. If the riding conditions are cold and muddy or you’re riding in sand, you always have to jet richer because the terrain load is greater and the air density is greater because of the cooler temps.

Jetting has most to do with where you ride and the average air temperature. We recommend JD Jetting kits for the excellent instructions and variety of parts in the kits. Contact them direct at

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. Always gear for more speed because of the increased torque. Typically you should increase the countershaft sprocket by 1 tooth to make each gear rev out longer.

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

Q. Will the engine wear out faster or be less reliable?
A. 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. When you choose the powerband for more low to mid range, we’ll TIG weld the exhaust bridge wider for longer piston wear

Q. Are their any alternative pistons for the 144?
A. Eric Gorr invented the 144 in 1976 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. We also stock the catalog pistons from Wiseco #846MO5800 , Vertex #2223B, and Wossner #8160D400.

Q. Does any company make a 2-ring 144cc piston kit?
A. Currently no piston manufacturer makes a 2-ring piston for a 144. A 2-ring piston would have more friction and make less power.

Q. What about aftermarket parts with a 134 or 144 kit, what works best?
A. Heres the scoop on accessories.

Exhausts - Short silencers help low to mid but make the top end fall flat sooner. Long silencers have the opposite effect. FMF SST pipes make the engine run hot at high rpm and FMF’s Fatty pipe with a long silencer is best for the wider powerband choice. DEP and HGS make the best high revving pipes. Pro-Circuit pipes are great for stadium racing where throttle response is important.

Carbs - 38mm is the optimum size for a wider powerband or more mid to top end. Most 125s made after 2004 have 38mm carbs standard. If you want more low to mid range for single track riding then a 35.5mm Keihin Air-Striker carb is the best set-up. Contact for that type of carb.

Reed Valves - All 125s have adequate sized reed valves. Aftermarket reed valves tend to improve just the low to mid range and that’s more important for single track and off-road riding.

Flywheel Weights - These parts help preserve engine inertia when riding in slippery conditions like hard-packed clay and muddy conditions. Off-road riders prefer the Steahly 6 ounce weights.

Q. Can I use a stroker crankshaft with a big bore kit? How does it work?
A. You can always add a stroker crankshaft to a big bore kit just by modifying the head and the crankcases for extra clearance for the piston and connecting rod. No special porting changes are needed, just use 2 cylinder base gaskets. The best crank stroke is 58mm and with a 144 kit that makes a 155, on a 134 kit that makes a 144. Generally speaking a stroker crank is going to improve the low end power and the acceleration. Regarding the use of longer connecting rods, yes that helps top end but requires the use of spacer plates and that can be problematic for sealing the cylinder base from air leaks.

FAQs on 2-Stroke Cylinder Repairs

Q. If the piston breaks can I just powerwash the crankcase and change the oil to get out the broken pieces?
A. The crankcase on 2-stroke engines is sealed so draining the transmission oil won’t help. Most likely the metal particles will contaminate the bottom end to the extreme that you’ll need to rebuild the bottom end with a new crankshaft, bearings, and seals. We offer that service and it averages $550 parts and labor. Additionally you’ll need to powerwash the exhaust pipe (not silencer) because metal may be trapped in there and re-enter the cylinder once you restart the freshly rebuilt engine.

Q. If I send my old piston with the cylinder can you tell me what the original failure was caused from and recommend repairs to keep it from happening again?
A. Yes, we recommend that you send any and all damaged parts, especially if you’ve had repeated failures. Also you should check out the tech article (PDF format) from Eric’s book titled Piston Diagnostic Guide. We’ll diagnose your mechanical problem and recommend action solutions.

Q. My bike has an aftermarket sleeve and now the coolant blows out the vent hose from the radiator. Why does this happen and what can be done to fix it?
A. Cast-iron sleeves are installed with an interference thermal fitting method. The sleeve is chilled in a freezer and the cylinder is heated in an oven and the sleeve in quickly installed in the cylinder where it expands and locks into place. Good sleeving companies install the cylinder in a press and hold the sleeve down until it cools to room temperature and the machine the top of the cylinder flat. On your cylinder the sleeve is squeezing upwards out of the bore, which allows combustion gas pressure to escape into the cylinder’s water-jackets. The excess pressure blows-out past the radiator cap. The only was to fix it is to machine the top of the cylinder and that machine-op is included in the $50 package for cylinder boring/honing.

Q. I just had my cylinder plated for a 2mm oversize and now my bike is slower than before and pings like crazy in the upper mid range. It makes a strange screeching noise at idle too. What’s wrong with my bike and how can I fix it?
A. Plating companies are not engine builders! They don’t have people with the knowledge to do the job right. When a cylinder is over-bored there are many other factors to consider like head and crankcase clearance to the piston, compression ratio, and port heights. Just boring the cylinder 2mm oversize and installing the stock head and powervalves effectively raises the compression ratio to dangerous levels which limits the top end power and makes the powerband flatten-out lower in the rpm range and causes detonation. Additionally the powervalves may contact the piston and that’s probably the screeching noise that you hear when the powervalve is closed because that’s when its closest to the piston face. We can usually fix a top end like that for about $125 plus parts.