Our readers keep the lights on and the garage heater humming. As an Amazon Associate, I earn from qualifying purchases.
Specs are compiled from manufacturer listings and verified buyer reviews and can change over time — please confirm the key details on the product page before buying.
Your stock Honda XR50 or CRF50 was fun for about a season. Now you want it to pull a wheelie without a running start, climb hills without bogging down, and keep up with the bigger bikes on the trail. That is exactly what a big bore kit does — it swaps the stock cylinder for a larger piston, bumping displacement from 49cc to as high as 88cc depending on the kit. The result is a surge of low-end torque and usable top-end power that transforms a putt-putt play bike into a legitimate trail machine.
I’m Mo Maruf — the founder and writer behind GoCarLife. This guide is built by comparing the manufacturers’ published specifications and the patterns across verified customer reviews, so you get each pick’s real strengths and trade-offs instead of marketing spin.
Below, we break down three distinct routes to that upgrade, from the proven brand-name kit to the budget rebuild assembly, so you can confidently choose the right xr50 big bore kit for your bike and your wrenching comfort level.
Our Picks at a Glance
How To Choose The Best XR50 Big Bore Kit
The right kit is the one that matches your riding goal, your mechanical skill, and the exact year of your engine case. The most important first step is knowing that the crucial number — the 39mm bore size — is shared across essentially all of them, but the quality of the cylinder wall, piston pin fitment, and included hardware vary enormously between a generic kit and a premium assembly from a brand like BBR.
Bore Size, Displacement, and Real Power
Some kits, notably the BBR 88CC kit, achieve a larger displacement (88cc) through a longer piston stroke or a slightly larger cylinder, giving you a more dramatic power gain. The important spec to verify is the pin bore — most of these kits use a 13mm wrist pin. If your engine has a 10mm pin, the kit will not fit without machining the connecting rod, which almost nobody wants to do in a garage.
Included Components vs. Hidden Costs
Some kits ship as a complete top end — cylinder head, cylinder body, piston and rings, wrist pin, clips, and both a base gasket and a head gasket. Others arrive with zero gaskets, which means you need to source those parts yourself before you start the job. If you are planning a same-weekend install, make sure the kit includes the full gasket set or order one separately ahead of time. The same goes for instructions: many kits expect you to already have a service manual or a YouTube video queued up, so do not rely on the box for guidance.
Brand Reputation and Fitment Quality
Aftermarket parts for these little Honda engines have a famously wide quality spectrum. A well-machined cylinder slides onto the studs with a light hand-press fit. A poorly-machined one fights you the whole way or, worse, has a cam journal that is undersized and ruins the cam bearing within a few heat cycles. Reading the pattern across owner experiences is essential here — a cheap kit that “works perfectly” for one builder may be a “won’t fit” dud for another, and that inconsistency is the risk you pay to save cash.
Quick Comparison
| Model | Best For | Displacement | Kit Weight | Cam Included | Amazon |
|---|---|---|---|---|---|
| BBR Motorsports 88CC Bore Kit★ Best Overall | Guaranteed wheelies & brand confidence | 88cc | 2.65 lb | Yes | Amazon |
| labwork Top End Kit 39mm | Budget rebuild with potential | ~72cc (39mm) | 8.37 lb | No | Amazon |
| FLYPIG 39mm Rebuild Assembly | Complete parts kit for a full top-end job | ~72cc (39mm) | 8.23 lb | No | Amazon |
In‑Depth Reviews
1. BBR Motorsports 88CC Bore Kit W/CAM for Honda CRF50F XR50R
The one that delivers the most satisfying straight-to-wheelie upgrade you can bolt on in an afternoon.
This is the kit that experienced owners reach for when they want the upgrade to just work, with no second-guessing. It bumps displacement all the way to 88cc — a noticeably bigger jump than the common 39mm kits — and the included BBR camshaft is the secret to that extra top-end pull that makes your 50 feel like a completely different machine. At just 2.65 pounds, the cylinder itself is a lightweight aluminum unit (painted exterior), a striking difference from the 8-plus-pound generic castings you find in cheaper kits. Buyers report: “Installed within 4 hours in my garage started first kick great top end my 50 does wheeles everywhere.” That first-kick start is the tell — BBR’s machining tolerances let the piston and rings seat correctly from the moment you put the socket down. The torque-to-yield checklist here is not a bad idea, and owners note you will want a pound-inch torque wrench for the head studs, but the process is straightforward enough for a confident home mechanic.
The trade-off is the price and availability. This is a premium piece, and it often sits at a higher cost than the budget alternatives, plus it has been on the market since 2012, so some sellers may carry older stock. Still, the consistency across the reviews — every single rated owner gave it a full 5 stars — tells you this is the kit you buy when you want “super fast powerful” with no asterisks.
What gives it the edge
- 88cc displacement plus a performance cam means a real power jump, not just a refresh
- Weighs 2.65 lb, compared to 8.23 lb for the FLYPIG kit and 8.37 lb for the labwork kit — less unsprung weight on the top end
- Every single verified buyer gave it a perfect rating, and the install stories all share “first kick” and “wheelies”
The honest fit note
- Premium price tag — this is the most expensive option in the lineup, and the difference is in the cam and the casting quality
- You will need a service manual and a torque wrench; there are no paper instructions in the box
Reach for this if: you want the most proven, most powerful bolt-on that reliably starts first kick and turns your bike into a wheelie machine — the brand confidence is worth the extra spend.
Choose something else if: your budget is tight and you are comfortable with the potential gamble of a generic kit, or if you already have a cam you prefer to run independently.
2. labwork Top End Kit Cylinder Head Piston 39mm Replacement for Honda XR50
The cheap ticket to 72cc that gets you running, but do not expect the fit and finish of a premium brand.
The labwork kit is the entry-level price point for a 39mm piston top-end rebuild. It includes the cylinder head, cylinder body, piston and rings, wrist pins, and clips — and, importantly, the head gaskets pictured. That gasket inclusion is a genuine plus at this price bracket. At 8.37 pounds, the cylinder is a heavy cast-iron piece, compared to the 2.65 lb BBR aluminum cylinder, but it will hold up fine on a play bike that does not see racing duty. Owners who had success reported: “Runs great so far on a 1973 honda z50AE block” and “Works great” — so when the stars align, the kit delivers a running bike for very little money. The “labwork” brand is a house-brand part, not a specialty manufacturer, which explains the inconsistent feedback.
The honest reality is that some units arrive with no gaskets at all, despite the listing photos showing them, as one buyer put it: “Came with zero gaskets rings piston head that’s it.” Other owners describe the quality as low, saying they would buy a reputable brand next time. This kit is best approached with the understanding that you may need to source an aftermarket gasket set separately and that machining quality can vary from one batch to the next. If you are a tinkerer who just wants to get a cheap bike running for a season of backyard riding, this is your play. If you need reliability for a kid’s bike that you do not want to tear down again next month, know the risks.
Where it makes sense
- Lowest-cost route to a 39mm bore on a budget beater bike
- Includes head gaskets in the package (at least per the listing description)
- Several owners mention a straight swap that started right up on first kick
The recurring caveat
- Inconsistent build quality — some arrive with missing gaskets or poor machine work
- No installation instructions included, so you need a manual or a good YouTube video
- Heavier than premium options at 8.37 lb compared to the 2.65 lb BBR unit
Grab it for: a low-investment rebuild on a beater bike or a project where you are willing to chase fitment issues to save cash.
3. FLYPIG New Performance Big Bore 39mm Cylinder Rebuild Assembly Kit
The full-parts-pile option that gives you everything in one box, if you get a good one.
The FLYPIG kit is the most complete package among the budget entries. It ships with the cylinder, cylinder head, piston, wrist pin, bearing, piston rings, base gasket, head gasket, and circlips — every consumable you need for a full top-end job, no trip to the parts store. The 39mm bore and 13mm pin bore match the standard fitment for XR50, CRF50, and Z50R models (though FLYPIG explicitly warns it does not fit 1985 Z50R or 1981 Z50). The kit tips the scales at a substantial 8.23 pounds, so you know you are getting a full cast-metal top end. Owners who got a good unit report: “Absolutely perfect fit on a Honda xr50. Put it together put it in time adjusted the valves and started right up.” That is the experience everyone wants — open the box, bolt it on, go ride.
But the consistency is where this kit reveals its nature. Amazon’s own review summary flags fit and quality as “mixed,” and approximately one in five owners had a genuine lemon. One buyer’s blunt review: “This product does not fit, the machine work is very poor quality and will not work.” Another owner noted it is really just a stock 49cc replacement top end, not an actual “big bore” upgrade at all, because the 39mm size is identical to a standard replacement. That is a fair point — a true big bore kit for these engines typically goes beyond 39mm or adds a stroker cam. The FLYPIG is best understood as an inexpensive complete top-end rebuild that refreshes a tired engine. The fitment gamble is real, so buy from a seller with a solid return policy.
What you get in the box
- Most complete parts inventory — cylinder, head, piston, rings, wrist pin, bearing, base gasket, head gasket, and circlips
- Satisfied customers note a perfect fit that fired up immediately after basic assembly
- The mid-range price sits below the premium BBR kit but above the bare-bones labwork unit
The inconsistency you should know
- Fitment is the number one concerns — some units have poor machining that simply does not work
- One owner considers it a standard 49cc replacement, not a genuine big bore, so manage your power expectations
- Heavier cast-iron construction at 8.23 lb compared to the 2.65 lb aluminum BBR kit
Best suited for: a mechanic who wants every single part in one shipment and is prepared to test-fit and possibly return one unit before getting a good one.
Not the right choice if: you want a guaranteed weekend install with zero hassle, or you are specifically looking for a true displacement increase beyond the stock 49cc bore.
Understanding the Specs
39mm Bore vs. 88cc Displacement
Kit Weight and Cylinder Material
A cast-iron cylinder (8+ pounds) is heavier and cheaper to produce. An aluminum cylinder (around 2.6 pounds) saves unsprung weight on the top of the engine, which helps suspension response and makes the whole bike feel lighter. Aluminum also dissipates heat better, which is important for sustained riding. The BBR aluminum kit weighs 2.65 lb, while the cast-iron options weigh 8.23 lb and 8.37 lb.
FAQ
Will an XR50 big bore kit fit my CRF50 or Z50R?
Do I need to upgrade the carburetor with the big bore kit?
How long does an installation typically take?
What tools do I need for this job?
Should I replace the camshaft too?
What is the difference between “big bore” and “top end rebuild”?
My kit arrived without gaskets — what do I do?
Do these kits work on a 1973 Honda Z50AE?
Why is the BBR kit so much lighter than the others?
Is a big bore kit street legal?
Final Thoughts: The Verdict
The real answer for most people is the BBR Motorsports 88CC Bore Kit W/CAM. It is the crowd favorite for a reason — every single verified owner gave it full marks, and the included cam delivers an easy power gain that cheaper kits cannot match. If your budget just does not stretch that far and you are comfortable with the fitment gamble of a generic 39mm kit, the labwork Top End Kit can get you back on the trail for the least cash. And if you prefer to have every part in one box and are willing to accept the inconsistency of an off-brand casting, the FLYPIG Rebuild Assembly is worth a roll of the dice with a flexible return policy. Whichever route you take, plan for a solid afternoon of wrenching, a re-jet session, and the most fun you have ever had on what used to be a slow little bike.
How We Picked
We do not accept paid placement. Every pick is matched to a real buyer and a real use-case; we do not hands-on test units.
Sources & Methodology
Specifications: manufacturer listings and product documentation. Review insights: verified customer reviews, as of July 2026. Pricing: not shown on this page (it changes often); check the current price via the retailer link.
As an Amazon Associate, GoCarLife earns from qualifying purchases. This does not affect which products we feature.