Rebuilding Classics: Restoring Honda CB Engines

Honda CB motors: A favorite at Purpose Built Moto

Honda produced a range of bikes in the 1970s and 80s that would eventually become icons in the bike world. At Purpose Built Moto, the Honda CB motors are among our favorites, particularly the mid size models being the CB400 and CB550 four.

Our engine rebuild shop has had a steady flow of Honda four motors arriving in various states of repair and leaving as gleaming, rebuilt motors ready for another 50 years of bitumen service.

In this blog, we will take you through what goes into rebuilding a Honda CB750 four, CB550 four and CB400 four. Sharing a few common upgrades and tricks we’ve found to build a vintage honda engine that will be reliable, and perform better than the day it left the factory.

1978 Honda CB750 Four

This classic bike smells of nostalgia. All the original internal components, such as pistons, rings, valves, camshafts, and bearings, have been rebuilt or replaced to ensure optimal performance and reliability. A mild port and polish has been done on the cylinder head, which improves the airflow and increases the power output. The engine’s capacity has been further boosted by installing an 836cc big bore kit, which enlarges the cylinders and raises the compression ratio. The ignition system has been upgraded with a Dynatek electronic ignition, which provides more accurate timing and eliminates the need for points and condensers. The clutch has also been strengthened with upgraded springs and plates, which can handle the increased torque and prevent slippage. The engine’s appearance has been brought back better than new by vapour blasting the cases in house, which removes dirt and corrosion and restores the original shine. The external covers have been polished to a mirror finish, and any minor case repairs to broken fastener points have been done with care. On the electrical side, we make sure to test the charging system during the rebuild, and replace any faulty parts. On completion and wiring we also recommend a reg/rec upgrade on our builds to make sure your engine is delivering a steady DC charge when running, this alone can avoid reliability issues with your ignition and lighting.

The CB750 four is prolific with restorers and vintage honda fans, so often our rebuilds are back to original spec, or at least it looks that way. If thats not of too much concern to you we recommend a carburettor and ignition upgrade as standard practice on all of our custom builds. These 2 modifications can greatly increase the performance and when it comes to bang for your buck, this is our pick.

1975 Honda CB400F

This bike is the prime example of what pocket rockets were back in the day. All the internal parts have been replaced with new ones, such as pistons, rings, valves, camshafts, and bearings. When rebuilding vintage motorcycles, if you have the bike torn down to the cases, most of your expense is in machining and labour. We always advise our clients that replacement of all bearings, bushing and seals is essential for us to make the bike as reliable as possible. For the cost of the bearings, its not worth running the risk of a failure on a seemingly good, but 50 year old component.

The cylinder head has also been ported and polished, followed by a 466cc big bore kit, which increases the cylinder size and raises the compression ratio. The valve springs have been upgraded with Kibblewhite springs, which are stiffer and can handle higher revs. The ignition system has been improved with a Dynatek electronic ignition and coils, which provide a more dependable spark with less maintenance. The clutch has also been reinforced with new plates and heavy duty springs, too keep up with the extra juice we’ve put into the might 400 powerplant. The carburettors have been refurbished to their original condition, and tuned for optimal performance. The engine’s look has been completely restored by vapor blasting the cases, preparing them for painting, and applying a high temperature gloss black paint.

A rebuilt and refreshed CB400 four is a joy to ride, with a good amount of power for the Sunday fun run up the mountain. Again when it comes to bang for your buck performance wise, we recommend a carb upgrade, performance ignition, and a big bore kit, which is often less expensive than genuine oversize pistons and rings.

1972 CB550F build

This engine was part of our Shannons Dream Bike Build in 2022, the heart of the 550 honda has been torn down, inspected for any faulty components, and replaced with new or better parts. No extreme modifications have been done, but rather the potential of the CB550 engine has been unleashed. Again we’ve used a Dynatek ignition system and coils to make sure when our rider has reliability from his bike for years to come, and with a little power boost too. The stock carburettors have been replaced with performance Mikuni’s, paired with some significant port and polish work to the head and a high flow exhaust system that adds the finishing touches to the CB550’s unique note. This Honda four has got to be my pick of the family. With a great amount of power once the performance mods are done, in a small and lightweight frame the combination just works for a rider like me. I do understand the appeal of the bigger CB750’s but my personal favorite will always be the 550.

This particular engine has been finished to perfection with a few aftermarket add ons like finned tappet covers, and re-chromed covers topping off the vapor blasted crank cases and barrels. Everything else on the outside of the engine has had the battle scars of 50 years repaired and refinished to a fine polish.

From the origin of the CB line beginning with the CB77 to the CB1100 and everything between, these motors were signature of an era that keeps going. Nothing gets us going like a bike that all age ranges have and will continue to love. Purpose Built Moto is the place to bring your timeless Honda four for a well deserved refresh. We build engines the same way we build our custom motorcycles, with no corners cut and to ensure our riders can enjoy these vintage motorcycles with confidence until all the oil runs out.