A Fresh Start for Brad’s Honda CB900
Honda’s 4 cylinders are among the most legendary engines in motorcycling history, and although we like to get creative when rebuilding these motors, especially whentheyre going into one of our custom bikes. Sometimes the brief is a little simpler, and we’re simply rebuilding the motor back to OEM spec.
That was the case with Brad’s Honda CB900 four.
It was stored for a number of years and was no longer running. He was pulling the bike down to customise the bike and finish it properly but needed us to step in and build his engine. Nothing crazy, just good clean work to restore this once mighty four cylinder.
Tear Down and Inspection
Before pulling the engine down, we cycled it by hand, dropped the oil and replaced it with some frech, cycled the starter for a compression and leak down test. We checked the spark and put some fresh fuel in before attempting to fire it up. The bike fired up without much hassle and ran pretty well. 2 cylinders were making good compression, 2 others were a little low due to some leaky valves.
The request was to rebuild the bike no matter what but its nice to know what we’re up against and these quick test can paint a much clearer picture before the motor is stripped.
As we worked on pulling the motor down a few not so nice conditions were shown, the motor had been stored for some time empty of oil, this was made evident by the parts above a certain level having corrosion on them especially on the gearbox.
Pistons were original, intake valves looked good but the exhaust valve stems were showing some wear. Cams and followers also looked great considering the age of the bike so most of that was retained.
All components on the bottom end looked good and measured up in spec. The only issue down there was the corrosion on the components and the presence of a fair bit of sludge in the corners of the cases. Most of this could be saved with some good cleaning and a blast.
Cleaning
Parts were stripped and catalogued before being blasted, during the process we noticed a poorly repaired stator cover that had been bogged over and painted. That was thrown out and a replacement found.
As we went through the cleaning process a few small parts like cam chain guides and adjusters, along with a number of internal rubber components were identified as needing replacement. External cases were cleaned and prepped for painting once assembled.
During this process the standard carb bank was given an ultrasonic clean and rebuilt with new gaskets and o-rings with the addition of some new fasteners and jetting gear.
Parts Sourcing
This motor was a lot easier than most of our others. With parts pretty readily available. First oversize pistons and rings, Gaskets, seals a full bearing set and the rubber components and chain guides mentioned above, cam chains, new fibre clutch plates and springs, valve springs, crank and conrod bearings, along with some other small parts all arriving pretty quickly. The only part non oem that we sourced was a set of oversized Exhaust valves. The head was getting a full machining service anyway, and the replacement cost was the same so we chose the slight performance upgrade for Brad.
Machining
The honda 900 cylinder gasket surfaces were given a lick on both sides to remove the evidence of time, given a hone to the first oversize and set aside while the head service machine shop did its thing.
The head was given more of a polish than a port job just improving the finish on the casting and smoothing out the gas flow in and out of the engine.
Honing the guides, machining the valve seats and fitting the new set of oversize exhaust valves.
Re-building the engine
Assembly moved smoothly due to the tedious prep work that goes into preparation for the engine build. And with reasonable condition of the motor, the hardest part (the cleaning) was behind us.
Starting with the crank and gearbox, the Honda CB900 was methodically worked through piece by piece coming together, checking operation along the way as parts were added and the cases closed up so the work could continue.
The only notable stop we hit was a supplier sending us the wrong cam chains, which we sourced locally from another supplier and kept it moving. Clutch, shift mechanism, stator, ignition, starter, cylinder, head. Step by step through the process making sure each pice is cleaned, lubed and in the right place.
Engine Cycle, Timing and Valve Clearances
At this point we turn the engine over by hand, set the valve timing and get the cam chain, tensioner and guides in place. Valve clearance was checked and a shim kit ordered in to fill a few gaps in our current kit.
If someone could travel back in time and tell manufacturers to standardise their bloody shims that would be great… I’ve got thousands of dollars worth of shim kits and I still don’t have em all!
With that all set and within spec, the timing on the motor was set and ignition static timed in prep for start up.
Painting
This motor was sealed up and prepared for paint once rebuilt, I like doing it this was as we aren’t working on components that are freshly painted with the risk of damaging them.
There are a few downsides here that we work around including having the fasteners painted over, which can be sorted by removing 70% of them and leaving in place only a few so that once paint is complete the fasteners can be swapped out for new one on the covers and case bolts.
That’s a trick I got from my mate Justin at Popbang classics years ago as we were getting started. You also need to take care with getting the cases off (if you need to) just running a blade across the joins helps, but its rarely an issue anyway.
First start and run in
This bike will come back to us eventually down the line but for now the engine is sealed up, painted and installed in the frame. Now its up to the owner to finish up his build around the fresh power plant. When it’s ready to run, we will run him through the start up procedure and help get him on the road.
Another one saved from being a boat anchor, and we’re onto the next.
WE BUILD BIKES WITH PURPOSE.
Purpose Built Moto is home for unique custom motorcycle builds. We offer a unique motorcycle customizing experience in the heart of the Gold Coast.
