The Honda Grom 300 Swap: BST Carbon Wheels, Öhlins, Brembos, and a CBR300R Built on the Kitchen Table

The Honda Grom 300 swap starts the same way every Steve build starts: with a completely stock bike and a plan that escalates faster than anyone expected. The Grom comes from the factory with a 125cc single-cylinder engine making somewhere around 10 horsepower. That's fine if you're a teenager or you're filtering traffic in Bangkok. Steve had other ideas.

The donor engine was a CBR300R unit — Honda's single-cylinder 286cc liquid-cooled motor from their entry-level sport bike. More than twice the displacement, and a legitimate performance engine rather than a transportation appliance. Getting it into a Grom frame requires fabrication work, custom mounts, and a certain willingness to do precision engine assembly on your kitchen table while your friends text you about it.

The Grom came apart completely. Every panel, the stock forks, the original engine, wiring, all of it. What you're looking at above is the whole bike reduced to a frame, some wiring, and two wheels sitting on the floor next to the CBR. The orange and black CBR in the background isn't a parts bike — it's Steve's full-size motorcycle, in frame for scale. The Grom frame next to it looks like a kid's bicycle that got lost in the garage.

That's the CBR300R engine fresh out of the donor bike. Before it goes anywhere near the Grom frame, it gets rebuilt completely — new piston, fresh rings, valve clearance set on new cams. Steve doesn't bolt in used engines. He rebuilds them first.

While the engine was being rebuilt, the chassis got sorted. Inverted forks went in first — gold Öhlins units replacing the stock right-side-up forks. The rear got an Öhlins shock as well. The bike rolled outside at some point during this stage, no engine, no fairings, no reason to be on the driveway in the rain. Classic build shop logic.

The suspension upgrade isn't just cosmetic. A 300cc engine is heavier than the stock 125cc unit, and the stock Grom suspension is tuned for a 125cc bike. Stiffer fork springs went in alongside the Öhlins setup — Steve documented pulling the old springs out on New Year's Eve.

4.24 lbs. That's a BST (Blackstone Tek) carbon fiber wheel on a kitchen scale. For context, a standard cast aluminum motorcycle wheel typically weighs 8–10 lbs. These are less than half that.

BST carbon fiber wheels are used in MotoGP and World Superbike — they're not a Grom part. Putting them on a pit bike is either insane or the most logical thing in the world depending on how you look at it: the Grom is already a light bike, and reducing unsprung weight has an outsized effect on handling when the whole package is already small and nimble. Steve's reasoning was somewhere in that territory. The price tag was somewhere in a different territory entirely.

Orange Brembo caliper on the BST carbon rear wheel, with a Kepspeed axle block. The brake rotor is floating, which you don't usually see on a pit bike. The orange Brembo against the carbon fiber and black frame is doing a lot of work aesthetically — but these are also functioning race-grade brakes, not cosmetic upgrades.

Durran Ranney: "We're talking about tolerance is down to the thousandth of the inch of course you need the kitchen table"

Steve: "garage is to full lmao"

Durran Ranney: "What's a garage, lol"

Durran Ranney: "File fit rings?"

Steve: "took me forever to get piston to wall clearance lol. I did test fit the rings. I normally always get file fit rings"

Durran Ranney: "don't want to lose power to blow by"

The cylinder jug, piston, and telescopic gauge set are laid out on the kitchen table alongside Honda's spec sheet and Steve's handwritten notes. The digital calipers are off to the right. This isn't shade-tree work — checking piston-to-wall clearance on a freshly bored cylinder requires precision measuring down to a few thousandths of an inch. Doing it on the kitchen table is a choice, but it's a legitimate one. The garage was full. The table was available. The measuring tools are the same either way.

The head went back to the kitchen table in March for the cam job. New camshafts went in — performance cams on top of the freshly rebuilt bottom end. The shim tray in the top left of the photo holds dozens of shims in different thicknesses; you measure each valve clearance, do the math, and swap shims until every valve falls within spec. It's tedious. It's also the difference between an engine that revs cleanly and one that burns valves six months later.

The comments on this post:

Daniel Derby: "And to think I have eaten on that table! What a privilege 😱😁"

Gordon N Shae: "Your knowledge of all is mind blowing"

Steve's kitchen table has seen things.

The CBR300R engine, fully assembled and ready to go in. New piston, fresh rings, performance cams with verified clearances, head torqued down. From here it goes into the Grom frame, gets mated to the custom mounts, wiring sorted, and the whole thing buttoned up with the yellow fairings.

Here's what went into the finished Honda Grom 300 swap:

• Engine: CBR300R 286cc liquid-cooled single — rebuilt with new piston, rings, and performance cams
• Front suspension: Öhlins inverted forks (gold) with stiffer fork springs for the added weight
• Rear suspension: Öhlins rear shock
• Wheels: BST (Blackstone Tek) carbon fiber 3-spoke wheels — 4.24 lbs each
• Brakes: Brembo calipers (orange), floating rotors, Kepspeed axle blocks
• Rearsets: Kemimoto adjustable rearsets
• Exhaust: Hindle
• Handlebar: Metal Mulisha crossbar pad, Rockstar branded forks
• Tires: Wide rear profile for improved traction

The result is a Honda Grom that's basically unrecognizable as a beginner bike. The CBR300R engine more than doubles the stock displacement. The Öhlins suspension, BST carbon wheels, and Brembo brakes are each individually more expensive than most used Groms. Put it all together and you have something that weighs almost nothing, stops immediately, and makes legitimately competent power for its size.

The kitchen table survived. Probably.

The Honda Grom 300 swap is one of the most popular Grom modifications for a reason: the stock 125cc engine is the one thing holding the platform back. The frame, geometry, and size of the Grom are genuinely good for tight, technical riding — it just needs more engine. The CBR300R motor is the go-to swap because it fits with relatively minimal fabrication and the parts ecosystem is well-developed.

What Steve did on top of the engine swap is a different category of build. BST carbon fiber wheels and Öhlins suspension on a Grom frame isn't a practical upgrade — it's a statement about what this bike is supposed to be. Fast, light, and completely unreasonable in the best way.

The whole thing was photographed, from teardown in December 2018 to the assembled engine sitting ready to drop in March 2019. Four months, a CBR300R worth of engine work, and a kitchen table that will never be the same.