BSA A50/A65: A Comprehensive Overhaul Guide

The BSA A50 and A65 unit twins, introduced for the 1962 season, were a significant departure for BSA, featuring their distinctive egg-shaped engines. While some detractors unkindly labelled them 'Unit Destruction' due to their novel design, these machines, especially the 500cc A50 and 650cc A65 variants, offered a more accessible and cleaner working environment compared to their predecessors, notably the beloved A10. Early models, with a lower compression ratio of 7.5 to 1, proved remarkably reliable; one owner famously clocked up 65,000 miles without an engine strip. This guide will walk you through the essential steps and considerations for a thorough overhaul of these classic British motorcycles.

Essential Tools and Parts for Your Overhaul

Before embarking on an overhaul, it's crucial to assemble the right toolkit. For earlier models, you'll need a good selection of BSF/WHIT spanners and sockets. Later models often incorporated a mix of both BSF/WHIT and AF/UNF sizes. Essential items include:

• A comprehensive set of spanners and socket set (BSF/WHIT and AF/UNF).
• An overhaul gasket set.
• Feeler gauges.
• Grinding paste.
• Scraper tool.
• Valve spring compressor.
• Clutch puller (access to one).
• Valve grinding tool.
• A two-leg puller may also be necessary.

When undertaking a major overhaul, certain components are highly recommended for replacement as a matter of course:

• Piston ring circlips
• Valve springs
• Big end nuts and bolts
• Gaskets
• Piston rings
• Engine oil

The necessity of replacing other parts like pistons, big end shells, bearings, and bushes will depend entirely on the extent of wear discovered during your inspection.

Engine Removal from the Frame

The first major step in any significant engine work is to remove the engine unit from the motorcycle frame. This process involves several key disconnections:

• Remove the petrol tank.
• Disconnect and remove the exhaust system.
• Remove the carburettors.
• Carefully disconnect all electrical wiring.
• Drain the oil from both the sump and the oil tank.
• Remove the head steady and oil pipes.
• The rear chain and chainguard must be removed.
• Disconnect the clutch cable and the rev counter cable (if fitted).
• Remove the spark plugs and HT leads.

With all ancillary components disconnected, the engine unit is held in place solely by its mounting bolts. Removing the engine unit from the frame has a specific knack. A common technique is to lift the rear of the engine slightly, twist, and then lift it out from the left-hand side of the frame, ensuring the rear engine plates are removed with it. Many find it easier to manoeuvre the engine out of the frame once the cylinder head has been removed.

Cylinder Head Overhaul

The cylinder head is a critical area for performance and reliability. While there are minor variations between early (circa 1962) and later (up to 1972) models, the fundamental overhaul procedure remains similar.

Accessing the Cylinder Head Bolts

Begin by removing the rocker box cover. Early models typically use six nuts, while later ones have four bolts and two nuts. To access all the cylinder head bolts, the exhaust rocker assembly must be removed. This provides access to the two front cylinder head bolts. The inlet rocker assembly can generally be left in place. It might be necessary to remove all pushrods to gain access to the centre cylinder head bolt; this is where a socket or box spanner is invaluable.

Valve and Spring Replacement

If the engine has covered a significant mileage, replacing the valve springs is a wise precaution. If you are unsure, the free lengths for the valve springs are:

• Inner spring: 1 7/16 inches
• Outer spring: 1 3/4 inches

Using a specialised valve spring compressor is essential to prevent losing the collets from the tapered spring caps. Once the springs are removed, you can inspect the valves and their guides for wear. Excessive side-play in the valve guides indicates they will likely need replacing, and possibly the valves themselves.

Valve Guide Replacement and Valve Seat Recutting

Valve guides can be carefully driven out using a specialised punch-like tool that fits snugly into the guide. It is recommended to pre-heat the cylinder head before attempting this and to replace the guides while the head is still warm. If new valve guides or new valves are fitted, the valve seats in the cylinder head will require recutting to ensure proper sealing. Valves need replacement if there are signs of significant scoring on their stems or deep pitting or burning on the valve face. Minor pitting can often be rectified by refacing the valves on a specialist machine. All valves, whether new or old, should be meticulously 'ground in' before reassembly.

The Valve Grinding Process

Thoroughly clean all carbon deposits from the cylinder head and ports. For a final clean, use wet and dry paper, and for a pristine finish, a metal polish can be employed. Smear a small amount of oil onto the valve stem and apply valve grinding paste to the valve seat. Replace the valve into its correct seat. Using a valve grinding tool, rotate the valve back and forth in its seat. Periodically lift the valve and reposition it to a new area of the seat. Continue this process until a smooth, matt finish is achieved all around the seat, free from any dark or ungrounded marks. Before reassembly, ensure all traces of grinding paste are completely removed. Oil the valve stems, then replace the springs and refit the collets using the compressor tool.

Pistons, Barrels, and Tappets

With the cylinder head removed, attention can turn to the pistons and barrels.

Piston and Bore Inspection

With the pistons still within the bores, check for any excessive side movement. If significant play is detected, carefully withdraw the pistons slightly down the bore and inspect for a ridge at the top of the cylinder. The presence of a ridge strongly suggests that a rebore is necessary, along with new, oversize pistons. If you are in any doubt, have the bores precisely measured using a dial gauge. Also, inspect the standard (STD) bore pushrods for any damage, such as chipped or loose cup ends, and ensure they are not bent. Replace any pushrods that show signs of wear or damage.

Barrel and Tappet Removal

To remove the barrels, ensure the pistons are positioned at the bottom of their stroke. Carefully lift the barrels off the pistons after removing the holding-down nuts. The tappets within the barrels are typically retained by small circlips; this is an opportune moment to examine the tappets for wear. Also, check the camshaft, located at the rear of the crankcase mouth, for any signs of damage or wear.

Piston Removal from Conrods

Before separating the pistons from the connecting rods (conrods), carefully note their orientation. Mark them if necessary to ensure they are refitted correctly and in their original bores (assuming you are not replacing them). Use a sharp-ended tool, such as a screwdriver, to carefully prise the circlip out of the piston groove. If the gudgeon pin is tight, gently warming the piston, perhaps by wrapping it with a hot, wrung-out cloth, can aid removal.

Piston Ring Replacement

If you are not replacing the pistons, it is highly advisable to fit new piston rings. The correct fitted gap for the new rings should be between .008 inches and .013 inches. Test this by inserting a ring into its bore, off the piston, to measure the gap. Use an old ring to thoroughly clean the grooves in the piston and to 'deglaze' (remove the shine from) the cylinder bore before fitting the new rings.

Clutch and Gearbox Considerations

The clutch and gearbox components require careful attention during an overhaul.

Clutch Assembly

Removing the clutch cover is generally straightforward, as is the removal of the clutch plates themselves. However, you will likely require a specialised clutch hub extractor tool to remove the clutch hub. To remove the mainshaft nut, you might need a special tool (like BSA tool no. 61-3768, which is essentially two clutch plates bolted together) or a method involving fixing a chain around the gearbox sprocket and bolting it in place. A bar inserted through the conrod eyes can assist in removing the crankshaft rotor nut. Remove the stator, rotor, and clutch and engine sprocket assembly together and inspect them carefully for wear. The clutch assembly itself warrants a detailed separate guide.

Timing Side Components

Moving to the timing or gearside of the engine, first remove the kickstart and gear lever. Release the cover, which is usually held by three screws. Remove the points plate and the advance and retard unit. The latter can often be removed using a special tool (like no. 61-3005) or by screwing a bolt into the nut and striking it sharply with a mallet to loosen it from its taper. Next, remove the oil pump and the crankshaft nut, followed by the timing gears. It is generally not necessary to remove the camshaft pinion unless the camshaft itself is being replaced. A two-legged puller may be required to remove the crankshaft pinion. The gearbox itself need not be disturbed unless a full gearbox strip is planned; the unit can remain in place.

Crankshaft and Crankcase Overhaul

The heart of the engine, the crankshaft and crankcases, requires meticulous inspection and potential refurbishment.

Preliminary Checks

Before dismantling the crankcase halves completely by removing all holding bolts and nuts, perform a preliminary check on the condition of the timing-side bush. Grasp the end of the crankshaft and test for any up-and-down movement. If excessive play is detected, it's a strong indicator that the bush will need replacement, even before further precise measurements are taken. A preliminary check on the big ends can also be performed by again checking for up-and-down movement, which should ideally be zero. The maximum permissible wear on all crankshaft journals is typically .002 inches.

Crankshaft and Crankcase Inspection

Once the crankcases are separated and the crankshaft removed, a thorough inspection can be carried out. Regardless of whether the crankshaft is to be reground, the sludge trap plug must be removed, and the oilways cleaned meticulously. The conrods are easily removed from the crankshaft. If there is any doubt about wear on the journals, it is essential to have them professionally measured.

Timing-Side Bush Upgrades and Crank Balancing

Specialist workshops, such as Bri-Tie, often replace the standard timing-side bush with one made from phosphor bronze. This upgrade is designed to suit a crankshaft machined to minimum tolerances, allowing for multiple regrinds rather than jumping between oversize dimensions (.010in, .020in, .030in). While 'stock' bushes are available, a well-made bronze bush is considered a superior solution. Even better are the roller bearing conversions offered by specialists like Devimead and SRM of Cardiff, which are significantly superior, especially for riders covering high mileages. Crank balancing is another enhancement that can be undertaken to mitigate some of the inherent vibrations found in these twin-cylinder engines.

Reassembly and Final Adjustments

The reassembly process demands extreme cleanliness, particularly when working with the big end conrod assemblies.

Torque Settings and Bushing

Torque the conrod nuts to 22 lb/ft, using new bolts and nuts for this critical connection. If any other bushes within the crankcases require replacement, remember that they will likely need reaming to size. Similarly, camshaft bushes should be reamed in line with the crankcases bolted together.

Crankshaft Installation and End Float

Carefully replace the crankshaft into the timing-side casing, taking care to avoid damaging the bush. Bolt up the other crankcase half and check for crankshaft end float. This should be adjusted to between .002 inches and .003 inches using shims. Once the correct end float is achieved, the crankcases can be fully bolted together, using an appropriate jointing compound between the mating surfaces. Crucially, ensure that both the crankshaft and camshaft rotate freely before proceeding with the refitting of other components.

Barrel and Cylinder Head Refitting

When refitting the barrels onto the pistons, the use of piston ring clamps greatly assists the process. Steady blocks placed across the crankcase mouth can also be beneficial. When refitting the cylinder head, ensure all bolts are torqued to 28-30 lb/ft and nuts to 31-32 lb/ft. Always start with the centre bolt and work outwards in a sequential pattern. Refit the two shorter pushrods to the outer tappets and under the inlet rocker arms. The longer pushrods should be fitted to the exhaust rocker arms. Adjust the tappet gaps to .008 inches for the inlet and .010 inches for the exhaust. Replace the rocker box cover, using a new gasket. Refit all other components in the reverse order of their dismantling.

Final Checks and Running Recommendations

As you complete the reassembly, keep these key points in mind:

• When refitting the engine into the frame, ensure it is guided in from the left-hand side.
• Be aware that the oil pipes are crossed over during installation.
• For the engine, a straight SAE 40 or 50 oil is recommended.
• Regular oil changes are advised every 2,000 to 2,500 miles.
• Use EP 90 oil in the gearbox.

By following these detailed steps, you can undertake a comprehensive overhaul of your BSA A50 or A65 unit twin, ensuring its reliability and performance for many more miles to come.

Frequently Asked Questions (FAQs)

Q1: When were the BSA A50 and A65 models first introduced?
The BSA A50 and A65 unit twins were introduced in 1961 for the 1962 model year.

Q2: What were the common criticisms of the early BSA unit twins?
Some critics unkindly referred to them as 'Unit Destruction', likely due to their novel design and the inherent challenges of any new motorcycle technology.

Q3: What type of tools are essential for a BSA A50/A65 overhaul?
A good set of BSF/WHIT and AF/UNF spanners and sockets, feeler gauges, a valve spring compressor, and a clutch puller are among the essential tools.

Q4: What are the recommended valve spring free lengths?
The inner valve spring free length is 1 7/16 inches, and the outer is 1 3/4 inches.

Q5: What is the recommended torque setting for the conrod nuts?
The conrod nuts should be torqued to 22 lb/ft, using new nuts and bolts.

Q6: What oil should be used in the gearbox?
EP 90 oil is recommended for the gearbox.

Q7: What is the recommended oil change interval for the engine?
Regular oil changes are recommended every 2,000 to 2,500 miles.

Q8: Are there any upgrades available for the timing-side bush?
Yes, upgrades to phosphor bronze bushes or even roller bearing conversions are available from specialist suppliers.

Q9: What is the correct tappet gap adjustment?
The recommended tappet gaps are .008 inches for the inlet and .010 inches for the exhaust.

Q10: How should the cylinder head bolts be torqued?
Torque cylinder head bolts to 28-30 lb/ft and nuts to 31-32 lb/ft, starting from the centre and working outwards.