Witness the incredible transformation from a seized and sludgy relic, burdened by decades of neglect and corrosion, to a beautifully rebuilt and upgraded engine, now gleaming with ceramic coatings and precision-machined internals, ready to roar to life and dominate the open road!
When the CB400 engine arrived at our Gold Coast workshop for a performance focussed restoration and rebuild, it was less an engine and more a barnacle-covered relic from the deep, looking like it had spent decades submerged.
The sludge wasn't just on the outside, once we dared to crack open the cases, it quickly became clear this project was going to demand far more than we'd anticipated. A lot of the fasteners were so severely corroded they were practically welded together. Our mission: somehow bring the trashed heart of this vintage CB400 four back to life through a complete engine upgrade and rebuild.
Initial Assessment: Unveiling the Challenges
Bit by bit it slowly came apart. The head and cylinders were gummed up, the pistons were cooked, and the bottom end was full of unidentified sludge. All of these issues presented a challenge, but with the list of upgrades and fresh parts going into the motor it wasn’t going to matter in the end.
The Performance Upgrade Hit List
The hit list for the custom CB400 engine read as follows:
- 440cc big bore kit
- Carillo Rods
- Yoshimura cam
- Oversize Kibblewhite valves
- High lift valve springs
- Ported + Polished head
- Re-balanced crank
- Dynatek ignition and coils
- Mikuni RS Smoothbore Carbs
All this plus replacing, repairing or re-making any damaged or destroyed parts we found along the way to ensure a complete CB400 performance upgrade.
Restoring the Foundation: Cleaning and Coating
Once we had the whole thing in bits we could begin the core engine restoration process. We fired up the vapour blaster and hit the engine casings inside and out. The cases were then cleaned, inspected and any damage logged so we could weld up any case repairs needed. This included broken cover mounts, stripped threads, and a few broken fins on the cylinder block. All in a days work when it comes to reviving old motorcycles. All parts were then off to ceramic coat who blessed them with a brand new heat wicking coating to aid with keeping out CB400 cool under pressure. They came up perfect, an absolute treat for the eyes. We’ve gone with a predominantly black colour scheme, with accents of burnt bronze and polished stainless hardware.
Precision Machining: Boosting CB400 Performance
With that out of the way and the aftermarket parts starting to roll in, I went up to spend the day with an engine balancer to have the crankshaft, conrods and big bore pistons re-balanced. The cylinders were re-bored and the head ported, and fitted with new guides and oversize valves at MKR Machining - another excellent local business here in Southern Queensland, crucial for our Honda CB400 engine tuning efforts.
Finally we were starting to make some solid progress. We weren’t just going to rebuild it to factory specs - why go to all this effort without improving 40 year old technology?
Engine Assembly
Motorcycle engine building is a relatively new game for us, and when you're perfecting a new skill everything needs to be by the book. It was imperative each piece was inspected, cleaned, lubricated and installed correctly before we closed up the cases for the last time on this CB400 project.
Once the crank and gearbox had been assembled and installed, the brand new clutch plates had a bath in some fresh Motul engine oil. While that soaked, the piston rings were measured and gapped, among tidying up a few other odds and ends in the meantime.
For the first time in many years, it was starting to resemble a performance engine more than a boat anchor. The new pistons were fitted, and the barrel was delicately slid on. It was time to start assembling the head.
With a brand new Yoshimura cam for our CB400 performance build, we installed new valves, valve springs, re-furbished rockers and new tappet adjusters. Once complete, the last major piece of the puzzle was ready to be put on and torqued to spec.
The Rebirth of a Honda CB400 Engine
It wasn’t without its challenges, but the CB400F engine was ready to return to its original skeleton. The rest of the project is still going through the restoration process, so keep an eye on our social media for the fabrication works as it progresses.
Thanks for taking the time to follow our journey. If you're inspired to transform your own motorcycle, or interested in having a performance engine built by our team here at Purpose Built Moto on the Gold Coast, please get in touch. We specialize in complete custom builds, vintage motorcycle restoration and performance upgrades for riders across the globe. Drop us a line at admin@purposebuiltmoto.com or fill out our contact form and we’ll get back to you.
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