One problem often encountered when rebuilding classic motorcycles is the availability of replacement parts. On the upside, BSA, Triumph and Norton engines are straightforward four-stroke, single or twin cylinder engines with overhead valves. The lower end consists of a typical crankshaft with bearings and piston rods. This combination of basic components equates to an absence of complexity in terms of rebuilding. Obtain the necessary parts and rebuild the BSA motorcycle engine on a workbench using the manufacturer's shop manual and mechanics tools.
Remove the top-end components, including the rocker arm covers, overhead valves and cylinder head. All bolts are removed clockwise using sockets and a ratchet or open-end wrenches.
Position the flat tip of a screwdriver or steel chisel on the seam between the cylinder sleeves and the crankcase. Tap the end of the tool with a hammer to unseat the cylinder gasket. Lift the sleeves off the crankcase.
Insert the end of a flat screwdriver into one end of a piston sleeve and against the ring clip at the opposite end of the sleeve. Tap the end of the screwdriver with a hammer to pop the ring clip out. Tap the end of the piston sleeve with the screwdriver and hammer until the sleeve is halfway out of the piston. Remove the piston from the end of the piston rod.
Refer to the workshop manual and split the crankcases. Remove the main crank and crank bearings. Loosen the connecting rod bolts and remove the piston rod from the crank.
Have the cylinders and cylinder head serviced by a motorcycle shop. Request new valve guides, keepers and valve springs with the cylinder head servicing. Have the cylinders honed or bored to accept the particular size of the new pistons.
Place the rocker arm covers and crankcase casings in a parts-cleaning pan. Clean the parts using engine cleaner and wipe the parts clean with rags. Receive the serviced parts from the motorcycle shop. Obtain the new pistons, piston rings and crank bearings.
Attach the piston rod to the crankshaft and thread the connecting rod bolts clockwise into the base of the rod. Refer to the workshop manual and tighten the connecting rod bolts to the specified torque using a torque wrench.
Install the main crankshaft and bearings in the crankcase. Assemble the cases and tighten the case bolts to the specified torque. Attach the new piston to the piston rod using the new piston pins and clips. Put the new rings on the piston.
Position the new cylinder gasket on the top of the crankcase. Attach a piston ring compression tool to the piston. Position the cylinder sleeve above the piston and slide the piston into the cylinder bore. Remove the compression tool and push the cylinder into place on the crankcase.
Refer to the workshop manual and install the cylinder head and valve assembly. Torque the head bolts to the specified torque. Attach the rocker arm cover and tighten the bolts to the specified torque. Fill the engine with fresh motor oil.
Research the availability of a BSA CD-ROM that includes part numbers and a workshop manual for the year and model motorcycle.