Reboring Cylinders: What's New?

The Art of Reboring: Breathing New Life into Your Engine

Many classic motorcycle enthusiasts, myself included, have experienced the bittersweet journey of purchasing a beloved vintage machine, only to find it requires a comprehensive rebuild. This was precisely my situation with a Royal Enfield in the 1970s. What began as an admiration for a classic turned into a hands-on immersion into its mechanical heart. The winter months provided the perfect opportunity to tackle the engine rebuild, a task made more manageable by being able to bring the components into the warmth of the kitchen.

During this process, several key areas of the engine required attention. The crankcases, for instance, were subjected to helicoiling or bushing where necessary to ensure secure threads. The crankshaft itself received a regrind, and new main and big-end bearings were fitted. The connecting rod appeared sound, with one notable exception: a careless gudgeon pin extraction had left a score mark through the unbushed small end eye, a detail that would require further attention.

The Vital Role of the Oil Pump

The oil pump, a seemingly sound but somewhat suspiciously designed component on this particular model, was replaced with a new unit. It’s crucial to note that the pump body should be lapped to its housing using a mild abrasive like ‘T-Cut’, rather than a harsh metal polish. Furthermore, only the merest trace of gasket sealant should be applied between the housing and the crankcase. Over-application can lead to blocked oilways, a common cause of catastrophic crankshaft failure, as many owners unfortunately discover.

Cylinder Reboring and Piston Replacement: The Core of the Matter

This brings us to the heart of the engine's restoration: the cylinder. The cylinder was rebored to a larger diameter, and a new, appropriately sized piston was fitted. This crucial step is undertaken when the original cylinder bore has become worn beyond the acceptable limits of the piston rings. Reboring essentially removes a thin layer of the worn metal, creating a perfectly circular and smooth surface for the new piston and rings to work within.

Alongside the rebored cylinder and new piston, new valves, springs, and guides were also installed. While the aluminium valve collars might appear less robust than their steel counterparts, they have proven to be remarkably reliable in practice. To complete the engine's internal renewal, new primary and timing chains were fitted, along with all necessary oil seals to prevent leaks.

Addressing Gearbox Quirks and Clutch Longevity

The Royal Enfield GT and Super 5 gearboxes are notorious for their propensity to produce false neutrals, with estimates suggesting around ten potential locations for these undesirable interruptions in power delivery. With this in mind, considerable care was taken to minimise any slop in the outer linkage and to precisely adjust the inner ratchet to the recommended clearance. The gear dogs, which engage to select gears, were also recut to ensure a positive engagement.

The inner linkage mechanism is certainly a unique design, and it’s easy to see why it might elicit a chuckle upon first inspection. However, on the positive side, the 250 RE models featured cassette-loading gearboxes, a design that predated similar systems found on Grand Prix racing motorcycles by approximately forty years, showcasing an early innovation.

The clutch, while generally adequate, was treated to new friction plates and springs. While some enthusiasts consider fitting an extra plate for added strength, this is generally deemed unnecessary for typical road use, unless one harbours ambitions of significantly bulking up their left arm. It's important to check for buckled plain plates, keeping in mind that two of these are intentionally designed with a slight dish. The clutch mechanism itself is another characteristic RE feature, with the springs mounted externally, sandwiched between the clutch cover and a circular retaining plate – a design that, while peculiar, is effective.

The Importance of Sealants and Frame Restoration

The final assembly of the engine was carried out with meticulous attention to detail, with the primary goal of achieving an oil-tight unit. The legendary tendency of some older Enfields to leak oil is a well-known anecdote amongst enthusiasts. Fortunately, modern advancements, such as silicone sealant, were available to aid in this crucial task, a luxury not afforded to riders in the 1960s.

The engine and frame layout bears a resemblance to the 250 Ducati, though the exact lineage of this design is debatable. It's also plausible that the single-cylinder Honda 250 RS shares design cues from one or both of these models.

In my case, the frame required a thorough shot-blasting to remove old paint and rust, followed by a fresh stove enamelling for a durable finish. The swinging arm was also straightened and re-stoved. The steering-head bearings were in good enough condition to be reused. The aluminium fork sliders were carefully re-polished after a damaged one was cleaned up, and then reunited with their original stanchions.

Cosmetic Touches and Wheel Choices

The upper alloy fork yoke, which also serves as the clock holder, was polished back to its former glory. A new blue-faced speedometer was fitted alongside the original rev-counter, adding a touch of modern clarity to the vintage instrumentation.

Second-hand mudguards were then fitted. The front mudguard, featuring a deeper valance than the standard GT version, offered superior weather protection and was believed to have originated from a Continental model. The rear wheel was rebuilt with a new rim and spokes. However, the original 18-inch front wheel was replaced with a 17-inch wheel from a Crusader model. This decision was primarily driven by cost, as the Crusader wheel was acquired for a mere eight pounds, complete with a tyre. Unfortunately, this wheel also featured a WM-2 rim, which altered the bike's characteristic rakish GT stance. New brake shoes were fitted to the front brake in the optimistic hope that it might perform effectively – a hope that, as it turned out, was somewhat misplaced.

Sourcing Parts and Dealing with Modern Materials

Jack Gray proved to be an invaluable resource for many of the specific GT parts, including the seat, exhaust system, flyscreen, battery/coil cover, and headlamp bracket. The GT headlamp bracket itself is a rather crude item, significantly less refined than many proprietary chrome alternatives. The gear-change outer linkage also exhibits a similar home-made quality. The flyscreen, a functional yet quirky addition, serves a dual purpose: reflecting engine noise back to the rider and providing a surface for taping navigation routes. A rubber gaiter between the chainguard and crankcase, unavailable in the late 1970s, meant the bike was missing this particular piece of protective kit.

The fibreglass tank, despite being treated with a sealer, succumbed to bubbling paintwork after only a few weeks. Similarly, the grey handlebar grips, chosen for their aesthetic, proved to be a regrettable choice and were quickly discarded.

Electrical Considerations: A 6-Volt Dilemma

On the electrical front, a significant misstep was made by opting to retain the original 6-volt system rather than upgrading to a more robust 12-volt setup. The ageing and brittle wiring loom was replaced, along with a new rectifier, brake switch, battery, and headlamp. However, the upgraded headlamp made little difference. After approximately thirty minutes of night riding, the infamous ‘Prince of Darkness’, Joe Lucas, seemed to assert his influence, causing the bulb to gradually fade and fizzle out like a dying glow-worm. Conversely, the battery also exhibited a tendency to overheat during daylight runs.

Despite these setbacks, the project was progressing, and the prospect of actually riding the machine was becoming a tangible reality. The journey of restoring a classic is often one of problem-solving, learning, and ultimately, immense satisfaction.

Frequently Asked Questions about Reboring Cylinders:

What exactly is cylinder reboring?

Cylinder reboring is a machining process where the worn inner surface of a cylinder is precisely bored out to a larger diameter. This removes imperfections and wear, creating a perfectly round and smooth surface for new piston rings to seal against.

When is cylinder reboring necessary?

Reboring is typically necessary when engine compression is low, oil consumption is high, or there are visible signs of wear, scoring, or ovality in the cylinder bore. It's a common procedure during engine rebuilds for older or high-mileage vehicles.

What are the benefits of reboring?

The primary benefits include restoring lost engine power and performance, reducing oil consumption, improving fuel efficiency, and extending the overall lifespan of the engine. It provides a fresh surface for optimal ring seal.

What happens after the cylinder is rebored?

After reboring, an oversized piston and matching piston rings are fitted. The new components are designed to work with the larger cylinder diameter, ensuring a proper seal and optimal engine operation.

Can any engine cylinder be rebored?

Most cast-iron cylinder liners can be rebored. However, engines with chrome-plated or Nikasil-lined cylinders may require different repair methods, such as replating or sleeving, as reboring can damage these specialized coatings.

What is the difference between reboring and honing?

Honing is a process that refines the cylinder surface after boring or for a minor touch-up on a less worn cylinder. It creates a specific cross-hatch pattern that helps retain oil for lubrication. Reboring removes more material to correct significant wear or damage.

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