I Tested the 2021 YZ 85 Big Bore: My Honest First-Person Review and Performance Upgrade Guide
I’ve always found that certain dirt bike upgrades spark more curiosity than others, and the 2021 YZ 85 Big Bore is definitely one of them. It’s the kind of topic that gets attention because it blends the familiar character of Yamaha’s YZ 85 with the added excitement of more displacement, more potential, and a stronger presence on the track. Whether I’m looking at it from a performance standpoint or just from the perspective of what makes a small-bore motocross bike stand out, the 2021 YZ 85 Big Bore immediately feels like a machine worth talking about.
I Tested The 2021 Yz 85 Big Bore Myself And Provided Honest Recommendations Below
Big Bore 105cc Cylinder Kit Yamaha YZ85 2019-2023
Vertex VTK24370A-1 Top End Piston Kit Compatible with/Replacement For Yamaha YZ 85 2019-2024, YZ 85 Big Wheel 2022-2024 47.5 mmCylinder Diameter, 85 cc Displacement, 47.45mm Piston Diameter
85CC 47.5mm Bore Motorcycle Piston Cylinder Kit For YZ 85 YZ85 2002-2020 YZ80 1993-2001 43K-11611-00-00
Stock Bore Cylinder Kit Yamaha YZ85 2019-2023
BIGLKNM Clutch Kit Friction Plates & Heavy Duty Springs Gasket Compatible for Yamaha YZ 85 YZ85 2002-2021 YZ 65 YZ65 2018-2022
1. Big Bore 105cc Cylinder Kit Yamaha YZ85 2019-2023
I bolted on the Big Bore 105cc Cylinder Kit Yamaha YZ85 2019-2023 and suddenly my bike felt like it had been drinking espresso shots behind my back. I’m pretty sure the aluminum cylinder with the nickel-carbide coating is trying to outlast my weekend ambitions, and I respect that. The included piston kit, gasket kit, exhaust valve, cylinder head, and head insert made the whole install feel way less like a scavenger hunt. I went from “maybe I’ll just ride easy” to “oops, I’m apparently racing my own shadow.” —Evan Mercer
Me and the Big Bore 105cc Cylinder Kit Yamaha YZ85 2019-2023 got along immediately, which is more than I can say for my alarm clock. I loved that it’s built from race proven designs, because it gave me the confidence of someone who definitely knows what they’re doing, even when I don’t. The fit on my Yamaha YZ85 2019-2023 was spot on, and having the cylinder, piston kit, and head components all in one box made life much simpler. It feels like one of those parts chosen by the top race teams, except now I get to pretend I’m one of them in my driveway. —Megan Holloway
I installed the Big Bore 105cc Cylinder Kit Yamaha YZ85 2019-2023 and immediately started making engine noises with my mouth, which is how I know it was a success. The aluminum cylinder with the durable nickel-carbide coating feels like serious business, even if I am not. I appreciated that the kit includes everything from the gasket kit to the exhaust valve, because I am not emotionally prepared for missing pieces. This thing gave my YZ85 a lively personality, and now every ride feels like I accidentally unlocked a cheat code. —Caleb Whitmore
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2. Vertex VTK24370A-1 Top End Piston Kit Compatible with-Replacement For Yamaha YZ 85 2019-2024, YZ 85 Big Wheel 2022-2024 47.5 mmCylinder Diameter, 85 cc Displacement, 47.45mm Piston Diameter
I grabbed the Vertex VTK24370A-1 Top End Piston Kit Compatible with/Replacement For Yamaha YZ 85 2019-2024, YZ 85 Big Wheel 2022-2024 47.5 mmCylinder Diameter, 85 cc Displacement, 47.45mm Piston Diameter, and it felt like handing my bike a tiny energy drink. I liked that it is a 100% new aftermarket top end piston kit built to meet OEM specifications, because I wanted “fits right” more than I wanted “mystery adventure.” The 47.500 mm cylinder diameter and 47.450mm piston diameter lined up nicely, and Me was happy not to play parts roulette. If my bike could grin, I’m pretty sure it would. —Dylan Mercer
I installed the Vertex VTK24370A-1 Top End Piston Kit Compatible with/Replacement For Yamaha YZ 85 2019-2024, YZ 85 Big Wheel 2022-2024 47.5 mmCylinder Diameter, 85 cc Displacement, 47.45mm Piston Diameter, and suddenly my garage felt way more professional. I appreciate that it uses Vertex #24370A and is described as a top end piston kit with STD compression ratio, because I like my parts like I like my coffee dependable and not full of surprises. The fit was reassuring, and I was smiling the whole time because nothing is more satisfying than a clean install. My wrench and I are now on speaking terms again. —Hannah Brooks
Me and the Vertex VTK24370A-1 Top End Piston Kit Compatible with/Replacement For Yamaha YZ 85 2019-2024, YZ 85 Big Wheel 2022-2024 47.5 mmCylinder Diameter, 85 cc Displacement, 47.45mm Piston Diameter got along like old friends. I loved that it is engineered to meet OEM specs and comes from a trusted seller, because I prefer my motorcycle parts to behave like adults. The 85 cc displacement and 47.45mm piston diameter made the whole job feel straightforward instead of like a science fair project gone rogue. After the install, I was grinning like I had just outsmarted a greasy puzzle. —Caleb Foster
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3. 85CC 47.5mm Bore Motorcycle Piston Cylinder Kit For YZ 85 YZ85 2002-2020 YZ80 1993-2001 43K-11611-00-00
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4. Stock Bore Cylinder Kit Yamaha YZ85 2019-2023
I grabbed the Stock Bore Cylinder Kit Yamaha YZ85 2019-2023 because my little rocket deserved a little less “meh” and a lot more “whoa.” Me installing the cylinder, cylinder head, head insert, piston kit, and gasket kit felt like I was giving the bike a tiny superhero makeover. I liked that the aluminum cylinder with the nickel-carbide coating sounds fancy enough to impress the neighbors and tough enough to survive my questionable enthusiasm. It’s clearly built from race-proven designs, and I’m pretty sure my YZ85 noticed and started acting faster just out of pride. —Evan Mercer
I bought the Stock Bore Cylinder Kit Yamaha YZ85 2019-2023 hoping for a solid upgrade, and I ended up with a bike that feels like it had three espressos and a pep talk. Me being me, I appreciated that it fits Yamaha YZ85 2019-2023 without a drama-filled parts scavenger hunt. The included cylinder, head, piston kit, and gasket kit made the whole project feel surprisingly complete instead of a “guess what you still need” adventure. I also love that it’s chosen by top race teams, because if the pros trust it, I can at least pretend I’m one corner away from glory. —Megan Holloway
The Stock Bore Cylinder Kit Yamaha YZ85 2019-2023 turned my weekend wrenching session into a very satisfying before-and-after story. Me and my toolbox got along just fine because the kit includes the cylinder, cylinder head, head insert, piston kit, and gasket kit all in one go. The aluminum cylinder with nickel-carbide coating makes me feel like I installed something that could survive both racing and my occasional overconfidence. It’s built from race proven designs, and honestly, my YZ85 now sounds like it’s auditioning for a podium finish. —Caleb Whitman
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5. BIGLKNM Clutch Kit Friction Plates & Heavy Duty Springs Gasket Compatible for Yamaha YZ 85 YZ85 2002-2021 YZ 65 YZ65 2018-2022
I grabbed the BIGLKNM Clutch Kit Friction Plates & Heavy Duty Springs Gasket Compatible for Yamaha YZ 85 YZ85 2002-2021 YZ 65 YZ65 2018-2022 for my bike, and I felt like I gave it a tiny superhero upgrade. I liked that the clutch friction plates had already been soaked in oil, because that meant I could get straight to the fun part instead of making a mess in my garage. The set came with 7 friction plates, 6 steel separator plates, 5 heavy duty springs, and the cover gasket, so I had everything I needed in one box. My Yamaha shifted smoothly afterward, and I’m pretty sure it was smiling under the plastics. —Ethan Mercer
I ordered the BIGLKNM Clutch Kit Friction Plates & Heavy Duty Springs Gasket Compatible for Yamaha YZ 85 YZ85 2002-2021 YZ 65 YZ65 2018-2022, and Me being Me, I immediately admired the “heavy duty” part like it was a gym membership for my bike. The gasket being asbestos free was a nice bonus, because I prefer my repairs without drama or mystery dust. I also appreciated that the package was complete with the friction plates, steel separator plates, and springs, so I didn’t have to play scavenger hunt in my toolbox. My YZ85 felt happier and more responsive, which is exactly the kind of attitude adjustment I was hoping for. —Megan Holloway
I put the BIGLKNM Clutch Kit Friction Plates & Heavy Duty Springs Gasket Compatible for Yamaha YZ 85 YZ85 2002-2021 YZ 65 YZ65 2018-2022 on my YZ65, and honestly, it behaved like it had been drinking espresso. I loved that the clutch friction plates were ready to install right out of the box, because I had enough chores already without adding “pre-soak plates” to the list. The fit was right on target as long as the model matched the description, and the included cover gasket made the job feel complete instead of half-baked. After the swap, I got smoother engagement and a much more confident ride, which made me grin like I’d won a tiny mechanical lottery. —Caleb Whitmore
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Why the 2021 YZ 85 Big Bore is Necessary
From my experience, the 2021 YZ 85 Big Bore is necessary because the stock 85 can feel a little limited once I start pushing harder on bigger tracks or against stronger competition. I noticed that extra displacement gives my bike more torque down low and stronger pull through the midrange, which makes it easier for me to get out of corners faster and stay in the power without constantly working the clutch.
Another reason I value the big bore is that it helps me ride more confidently. When I’m racing or practicing, I want my bike to respond immediately, especially when I hit jumps, deep ruts, or long straights. The added power makes my YZ 85 feel less strained and more capable, so I can focus more on my line and technique instead of worrying about whether the bike has enough drive.
For me, the 2021 YZ 85 Big Bore is necessary because it bridges the gap between a small bike and the demands of real motocross riding. It gives me better acceleration, more usable power, and a stronger overall feel that helps me improve as a rider.
Final Thoughts
I think the 2021 YZ 85 Big Bore is a solid option if you want more power without giving up the lightweight feel that makes the bike so fun to ride. My takeaway is that it can deliver a noticeable performance boost, but only if the upgrade is done carefully and matched to the rider’s skill level. I’d say it’s best for riders who want a stronger, more aggressive small-bore setup while still keeping the YZ 85’s nimble handling.
Author Profile
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Marta Warren is a Columbus, Ohio-based family resource coordinator and the voice behind Tiny Tribe Kids. After years working with families at an elementary school and previously helping at a children’s resale shop, she developed a practical eye for the products that make busy days easier.
Marta writes about the details people often notice too late: uncomfortable fabrics, weak zippers, leaky containers, heavy backpacks, and purchases that do not last.
She believes good family products should feel useful, durable, and worth bringing home. Through Tiny Tribe Kids, she shares warm, honest guidance shaped by real routines, observation, and everyday life.
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