Help 167 yz

Jan 7, 2021
I've been contemplating doing the max power 167 kit for my 2019 yz 125 The thing that is bothering me is the reliability of it .
A top end every 3 hours what is that? Not a racer just a rider normally i get a year out of a top end it would go way longer but I get bored in the winter and tear it down and replace top end.
But they recommend 3-10 hrs service top end. Why is there plating bad or is it that the cylinder is now to weak and flexes?
3 hrs about what I spend breaking a motor in -why aren't they more reliable
Jul 27, 1999
Geometry plays a big part. It gets increasingly difficult to support the rings as you increase the engine size and the associated port time/area in a cylinder with ports cast to feed a 125. With big bores you also get bigger heavier parts that lead to increases in stress in the crank and associated bearings.

144cc has proven over the last 40+ years to be the best compromise between increased size, power and reliability.

Eric Gorr's 144 engines have longer service intervals than the OEM setup and is likely what you are looking for.

Eric Gorr Racing - Big Bore 125s

From his website :

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 ground 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.
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