Reviving Your Honda CB500/550 Engine: A Deep Dive

The Honda CB500 and CB550 SOHC fours hold a special place in the hearts of classic motorcycle enthusiasts across the UK. Renowned for their robust design and engaging ride, these machines are a fantastic platform for restoration or performance enhancement. However, the world of vintage Honda engine rebuilding can be fraught with misinformation. This guide aims to provide an authoritative treatment, addressing the common query about fitting a CB550 cylinder block onto a CB500 and offering a thorough roadmap for a successful engine overhaul, whether you're tackling it yourself or preparing for a professional.

CB550 Cylinder Block on a CB500: The Lowdown

It's a common question among enthusiasts: can you put a CB550 cylinder block on a CB500? The good news is that these engines share a significant amount of commonality, making such a swap largely feasible. Honda designed these small fours with a degree of interchangeability, and the CB550 is essentially an evolution of the CB500. While the core engine cases are very similar, allowing for the physical fitment of the cylinder block, you'll need to consider other components like pistons, rings, and potentially the cylinder head to ensure proper compression and valve clearance. The increased bore of the CB550 block means you'll gain a bit more displacement, often sought after for a modest power bump. This kind of upgrade, combined with a rebore and headwork, can indeed provide that extra 'poke' you might be looking for, while still prioritising reliability and ease of maintenance, which are hallmarks of these fantastic engines.

Pre-Rebuild Diagnostics: Knowing Your Engine's Health

Before you even think about cracking open your engine, a critical stage of the rebuild process involves making important measurements while the engine is still intact. This diagnostic phase will inform your decisions and help identify the true scope of work required.

Essential Tests:

• Cylinder Compression Test: This is your initial gauge of sealing loss. Use a high-quality tool built around individual hoses for accuracy. The engine does not need to be warmed up first.
• Cylinder Leakdown Test: This test follows the compression test to pinpoint where any compression losses are occurring – whether at the cylinders (piston rings) or the valves. It's crucial for diagnosing the notorious 'soft valves issue' prevalent in SOHC fours.
• Charging System Qualification: Measure your charging system's output to ensure it's functioning correctly.
• Oil Pressure Measurement: A healthy engine should maintain adequate oil pressure.
• Visual Inspections: Note any exhaust smoke (colour indicates issues), oil leaks, or unusual noises from the starting system (spinning uselessly, skipping, grinding). While the starter drive isn't as prone to wear as on Honda twins, it does eventually wear out.
• Primary Chain Rattle: Listen to how much the primary chain rattles at idle. Unlike the CB650, the CB500/550 lacks a tensioner, so excessive looseness can indicate significant wear, especially after 10,000 miles.

These tests are fundamental for any engine, regardless of age or mileage, and should be considered part of regular maintenance. The results will help you decide whether a top-end refresh is sufficient or if a complete engine overhaul is necessary.

Engine Removal and Initial Disassembly

Assuming you've decided on a complete engine rebuild, removing the engine from the frame is often the most practical approach, even though it's not strictly required for a top-end job. The small fours are relatively light and manageable.

Step-by-Step Removal:

1. Drain the fuel tank and remove side covers, carburettors, and exhaust system.
2. Remove the shift lever.
3. Drain the engine oil, then remove the oil pan, oil pickup, and oil filter.
4. Disconnect all electrical connections (spark plug wires, alternator cables, starter motor cable) and control cables (clutch, tach).
5. Remove the engine drive sprocket.
6. Unbolt the engine's breather cover.
7. Remove all engine mounting bolts. You may need to remove or bungee-cord the rear brake pedal out of the way.
8. By straddling the bike and gripping the kickstart lever and alternator rotor, the engine can typically be removed from the right side of the frame. To lighten it further, consider disassembling the top end while the engine is still in the frame.

You don't necessarily need a fancy engine stand. A simple plywood cradle can serve as a very useful homemade base for working on the engine on a workbench.

Top-End Disassembly: Peeling Back the Layers

With the engine on your workbench, you can begin the precise work of disassembling the top end.

Key Steps:

• Remove spark plugs and the eight valve inspection covers.
• Remove the decorative cam end covers and their brackets, noting the two dissimilar washers on each.
• Carefully remove the 6mm bolts and screws holding the cam cover, starting from the outer ones and working inwards. Note which fasteners have sealing washers.
• Remove the cam cover.
• Unbolt the camshaft sprocket and remove the cam and sprocket together towards the left.
• Remove the six rubber wafers.
• Remove the 6mm (10mm hex) bolts at the front and rear of the cylinder head, followed by the 8mm (12mm hex) head nuts, working inwards from the outer edges. Retrieve all washers, using a magnet if necessary.
• Carefully pry the cylinder head loose. The factory provided a specific boss near the engine displacement number for safe prying.
• After the head is off, remove the cam chain guide (specific to CB500/550/650, unlike the CB350F).
• Remove the cylinder assembly. If stuck, use the designated prying boss at the front of the head.

Crucial Tip: If you remove the cylinder head, you must remove and replace the base gasket. Relieving pressure on the top end will break its seal, leading to likely leaks if not replaced.

Inspecting Components: The Devil is in the Detail

This is where the true detective work begins. Thorough inspection is paramount for a reliable rebuild.

Connecting Rods & Crankshaft:

• Rotate the crankshaft to bring each rod to bottom dead centre. Check side-to-side play (no more than 1mm swing at the top) and vertical movement (must be none).
• At top dead centre, visually inspect each rod for straightness from both the side and front.
• After splitting the cases, inspect the crankshaft's main and rod journals for bluing or galling.
• Use Plastigage to measure bearing clearances. Familiarise yourself with Honda's unique factory bearing sizing system. Genuine OEM gaskets and seals are highly recommended here.

Cams and Rockers:

• The cams on these small fours are known to wear relatively fast. Inspect yours carefully for wear, which results in decreased performance. Remanufactured stock cams are available from specialists like Megacycle and Web Cams.
• Inspect the rocker arms. Honda's are flash chrome plated and take a beating. If a fingernail catches on scratches, they need replacement. Specialists offer properly heat-treated rocker arms. Running used rocker arms on a new cam is risky and requires expert assessment.

Cylinders and Pistons:

• Have a machinist precisely measure your pistons and cylinders for clearance, out-of-round, and taper. Factory clearances are very tight (0.0005" for 500/550).
• Do not simply hone your cylinders. After decades of use, they will require machining for new pistons to restore proper clearance and prevent oil consumption.

Cylinder Head and Valves:

• The cylinder head will almost certainly require new valve guides, new valves, and remachined seats.
• Kibblewhite stainless steel valves are highly recommended as they last significantly longer than stock.
• Find a machinist familiar with Honda heads and modern tooling. Avoid antiquated valve seat tools. Lapping valves alone is not a legitimate rebuild technique.

Transmission:

• If you haven't had shifting issues, you might consider leaving the transmission undisturbed. However, if the engine is this far apart, best practice is to inspect it fully.
• Inspect gear dogs for excessive rounding and selector forks for wear marks.
• Note that gears are not symmetrical and have specific orientations, washers, and shims. This is a very technical assembly.

Primary Drive:

• Inspect the primary chain for excessive looseness.
• Disassemble the primary cush drive and replace the rubber dampers.
• Inspect the starter clutch for loose screws or signs of roller skipping.

Parts Sourcing: Quality Over Cost

Finding quality replacement parts for a 50-plus-year-old Honda engine can be challenging. While new old stock (NOS) parts are scarce, the CB500 and CB550 were widely sold, so used OEM parts are relatively plentiful. However, exercise extreme caution with aftermarket parts.

Key Considerations:

Reassembly: The Art of Putting It Back Together

Reassembly requires patience, cleanliness, and adherence to factory specifications. This is where meticulous attention to detail truly pays off.

Crankcase Assembly:

1. Before reassembling, ensure the transmission shifts through all gears by manually rotating the shift drum and countershaft.
2. Thoroughly clean all old case sealer from the crankcase halves. Use brake cleaner and lint-free rags.
3. Install new crankshaft and transmission seals. A new left-side crankshaft seal will require removing the alternator rotor with a special puller.
4. Sparingly apply Hondabond #4 (or Threebond #1104) to the crankcase sealing surfaces, avoiding bearings and seals.
5. Carefully lower the bottom case onto the top, ensuring nothing jostles. Reinstall the primary shaft.
6. Install two of the longest crankcase bolts and snug gently. Check for free movement of the crankshaft and transmission shafts, and re-check shifting.
7. Install and torque all lower crankcase bolts, starting with the largest ones over the crankshaft, from the centre outwards, in three increasing steps. Repeat for upper crankcase bolts.
8. Install the oil pump with new O-rings.

Cylinder and Head Installation:

1. Prepare the cylinder base gasket surfaces on both crankcase and cylinder assembly. Avoid milling engine parts or using heavy-duty cylinder studs; they are unnecessary and can be detrimental.
2. Ensure all locating dowels, oil pressure orifices, and new O-rings are in place.
3. Use piston ring compressors to install the cylinder assembly.
4. Install the cam chain guide (wear surface facing outwards for CB500/550/650).
5. Apply two coats of Gaskacinch to each side of your new head gasket (NOS Honda or Cometic), applied two hours apart, and let dry overnight before installation.
6. Install the head. Torque the head nuts and bolts from the centre outwards in three increasing steps. Don't forget the small 6mm bolts at the front and back.
7. Install spark plugs to prevent foreign objects from entering the cylinders.
8. Install the six rubber wafers over their head nuts, with a tiny bit of Hylomar or Gaskacinch. Do not use silicone sealer here.

Camshaft and Valve Train:

1. Ensure the ignition advancer is installed for timing.
2. Dab Honda moly grease on the cam bearing saddles. Install the cam with its sprocket from the left side.
3. Time the cam to the crankshaft using the 'T' mark on the advancer. After adjusting cam chain tension, rotate the crankshaft a few times and double-check the timing.
4. Pour fresh engine oil over the camshaft.
5. For the cam cover gasket (O-ring type), spray 3M #77 adhesive onto the gasket and dab glue into the cover's groove to hold it. Then, sparingly apply Hondabond #4 to the sealing surfaces of both the head and cam cover. The small four cam cover is an exception where sealer is used in addition to the rubber seal.
6. To install the cam cover, back out all valve adjusting screws. Gently settle the cover, ensuring rocker arms stay on the valves. A slight nudge of the crankshaft may be needed for the tach drive gear to engage.
7. Gradually tighten the cam cover screws from the centre outwards in a criss-cross fashion, in three increasing steps. Avoid Allen screws, as they can damage the head.
8. Adjust all valves (0.005" for both intake and exhaust on CB500/550/650) in the firing order of 1-2-4-3.
9. Install new washers on the decorative cam end cover mounting brackets (aluminium under, steel on top). Use new, lightly greased O-rings on the covers' locating dowels.

Post-Rebuild and Cam Chain Tension

After reinstalling the oil pan, filter, breather cover, carbs, and exhaust, fill with fresh oil. Don't forget to time the ignition and synchronise the carburettors. Contrary to popular forum advice, a properly machined and assembled vintage Honda four does not require a break-in period in the traditional sense, nor do the heads need retorquing in service. A 100-mile 'shake-down' period is recommended to ensure everything is settled.

Cam Chain Tension Adjustment:

The method for adjusting cam chain tension differs between models:

• CB500/550 and CB650: This is done with the engine running at an idle. The CB500/550 often benefits from slight persuasion at the slotted adjuster.
• CB350F: Never adjust with the engine running, as the tensioner can easily fail. Instead, use a static method similar to the CB750. Rotate the crankshaft until the #4 intake valve opens and closes, then slowly rotate until the 1/4 T mark aligns. Loosen the tensioner locknut and lockbolt, then turn the crankshaft a tiny amount more while snugging the lockbolt. Snug the locknut and replace the inspection cap.

For CB650 tensioners, if they're reluctant to work at idle, the static CB350F/CB750 method can often make the cam chain quieter and is considered the professional way.

Frequently Asked Questions

Here are some common questions that arise during a CB500/550 engine project:

Q: What about a CB650 cam and tacho gear swap?

A: Swapping a CB650 cam is a common modification for those seeking more performance, but it's not a direct bolt-on. While the text mentions the CB650 cam chain guide and tensioner, you'll need to research specific compatibility for the cam profile and the tacho gear drive, as there can be subtle differences in fitment and timing between models. Professional advice is recommended to ensure proper valve timing and clearance.

Q: How long does a CB500/550 primary chain last?

A: The primary chain on CB500/550 models is known to show significant wear by just 10,000 miles. Unlike the CB650, these models don't have a tensioner, meaning a quiet chain depends heavily on engine tune (especially carburettor throttle sync) and starting with a good quality chain. It's highly recommended to replace the primary chain during a full engine rebuild, especially if it has more than 10,000 miles on it.

Q: Is a Dyna Ignition System a good upgrade?

A: While the provided text doesn't specifically detail Dyna Ignition, it does emphasise 'optimising your ignition system.' Aftermarket electronic ignition systems like Dyna are a popular upgrade for classic Hondas, often replacing the original points system. They can offer more reliable spark, improved starting, and reduced maintenance compared to traditional points, which require regular adjustment. Prices and models do vary, so research the specific Dyna model compatible with your CB500/550.

Q: Can I re-use my old pistons if I'm only doing a hone?

A: The text strongly advises against simply honing cylinders and reusing old pistons on these vintage engines. After decades, piston-to-cylinder clearance will almost certainly be double or triple its factory value. For a reliable rebuild, cylinders need to be machined (bored) for new, oversized pistons to restore proper clearances and prevent oil consumption. Rely on a skilled machinist familiar with Honda cylinders.

Q: Do I really need to replace the cam chain if it looks okay?

A: Yes. The text recommends replacing the cam chain if it has more than 10,000 miles on it. While it might look fine, wear can be subtle but critical for engine timing and longevity. It can be replaced without splitting the crankshaft cases by cutting both the old and new chains and using a factory master link from a 1970s CB450, but removing the crankshaft for replacement is also an option.

Undertaking an engine rebuild on a Honda CB500 or CB550 is a rewarding endeavour that can breathe new life into your classic motorcycle. By following these comprehensive guidelines, paying close attention to detail, and sourcing quality parts, you can ensure your beloved SOHC four runs reliably for many more miles to come. Remember, while much can be done by a dedicated home mechanic, don't hesitate to consult or utilise the expertise of a professional for the more intricate aspects of machining and assembly, particularly for the crankshaft, rod bearings, and complex transmission work.

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