CB250 Scrambler

+61 416 984 667
naruncheng@gmail.com
Melbourne, Australia

Industrial Design and Mechanical Engineering student driven by a passion for designing products that combine thoughtful aesthetics with practical function. Drawing on mechanical intuition, manufacturing knowledge, and user-centred design principles, he seeks to create products that work as well as they look.

Year

Project

2025

Snoozy Alarm Clock

2025

Podiatry EduKit

2024

Monash Automation

2024

2023

MiLabs Sensors

2023

XERRA Appliances

2023

Kläsk Drinks

2023

Sugar On Tap

2022

Airport Taxi

Personal Project

CB250 Scrambler

2024

 Engineering
 Personal Project
 Completed

This personal project began with a simple but ambitious goal: to build a custom retro scrambler motorcycle, bypassing standard P-plate legal bikes for something with a little more character. The foundation was a 1997 Honda CB250, purchased as an incomplete project. It had great potential with its new tank, seat, and tires, but was non-running and arrived as a collection of parts.

The challenge was to transform this disassembled heap of bits and bobs into a reliable, road-worthy machine. This required a full mechanical rebuild, diagnosing electrical faults, and fabricating solutions to fit the new custom components.

The condition the motorcycle was purchased in.

Luckily, shop manuals were easy to find online. Mechanical assembly began quickly.

Rebuilding the clogged carburetor. Larger jets were swapped in to accomodate better airflow from the pod intake.

Swapping out worn clutch plates and old springs.

Swapping on the new tank. Bike starting to take shape.

Unfortunately, at this point I realised that since the previous owner had cut off a section of the frame of the bike at the rear, the motorcycle will not pass a roadworthy check to be eligible for registration. This meant that, even if I manage to meet all the safety requirements laid out by VicRoads, I would not be able to ride it on the road.

Now a victim of the sunk cost fallacy, the only way I would be able to progress this project is to purchase another identical “donor” motorcycle without the frame damage.

The “donor” bike I purchased off facebook marketplace - an identical 1993 Honda CB250 nighthawk, in fully stock form.

While completely unmodified, this stock CB250 was also not in roadworthy shape. Among the issues were a slipping clutch, a faulty blinker relay, a broken ignition coil and a jammed up rear brake. These were relatively simple fixes, and having addressed these, the bike was sent to a mechanic for a roadworthy certification.

Finally roadworthied & registered.

Having finally obtained a registration for the “donor” bike, the process of modifying the ordinary commuter bike into a scrambler begun.

Removing original bodywork & tank.

Removing stock electricals, and clearing up the area.

Frames stripped down with all components removed.

New tank, bars & knobby scrambler wheels installed.

Given the age of the motorcycle, many componets were showing significant corrision. These componets were restored and reinstalled where they could be saved, and replaced with new parts if not.

Where parts were unavailable or there just wasn’t one I liked, they were custom designed and fabricated.

In particular, I wanted the middle “triangle” of the frame to be clear of the original electrical componentry, a look often found on vintage café racers. After experimenting with a few different options, I decided to have the battery and electricals to be mounted on the underside of the seat. This meant that a whole new housing would have to be designed and made as well as switching to a lithium battery.

3D scan data of a section of the motorcycle frame.

CAD modelling of the new component guided by 3D scan data.

Fitment testing of a 3D printed prototype. Many iterations had to be made to get the sizing just right.

Final part 3D printed with a weather resistant carbon fiber nylon composite material.

Similarly, existing dash options on the market were either too minimal or cheap and plasticky. Instead I made a custom two piece design with a single premium aftermarket speedometer. The top plate was sent to a Chinese manufacturer to be laser cut from stainless steel, while the bottom housing could be 3D printed with the same weather resistant PA-CF composite used for the electrical housing. Waterproof LED lights and switches are then installed for provide indicators for neutral, turn signals, sidestand, highbeams etc.

CAD model of the dash module.

Initial test fit with speedometer.

Assembly with laser cut stainless steel piece.

Final dash assembly installed.

ChatGPT should not be used to find torque specs. I knew this, but did it anyway. While servicing the clutch, I overtightened a bolt to an AI hallucinated torque spec and snapped the OEM cast aluminium clutch lifter plate. Unable to find any reasonably priced replacements online, I took the opportunity to recreate it in CAD and machine a better, stronger billet part.

While in theory the added weight would trade a bit of engine responsiveness for smoother torque delivery, the difference is so small any effects should be pretty much negligible.

Broken OEM clutch lifter plate. Don’t trust ChatGPT.

Machining the part on a CNC mill.

Final billet part frontside.

Final billet part backside.

Final part installed in the bike.

New mufflers and polished exhaust pipes.

A new set of exhaust mufflers and a few other finishing touches later, the scrambler was finally completed.

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