Editing BSA A7

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The early engines are most easily recognized by their screw-in rocker box caps. One unusual feature on the prototype engines was the arrangement of the crankshaft. This was built up from three sections and would have allowed the use of one piece rods with lead bronze bushes. Each crankpin was extended and tapered to fit a corresponding hole in the central flywheel. The three parts were drawn together by a very special bolt fitted from the right and machined with a differential threads, that is, two threads of different pitch , the nett effect was to give the bolt a very fine pitch and so high clamping power, but to retain the normal thread section and strength.  
The early engines are most easily recognized by their screw-in rocker box caps. One unusual feature on the prototype engines was the arrangement of the crankshaft. This was built up from three sections and would have allowed the use of one piece rods with lead bronze bushes. Each crankpin was extended and tapered to fit a corresponding hole in the central flywheel. The three parts were drawn together by a very special bolt fitted from the right and machined with a differential threads, that is, two threads of different pitch , the nett effect was to give the bolt a very fine pitch and so high clamping power, but to retain the normal thread section and strength.  


The construction concept can be found on Scott engines using a normal clamp bolt while the differential screw is used in Hirth crankshafts which are built up from individual components locked together with radial teeth. This complex and expensive design was dropped in favor of convention by the time units reached the public. The rocker boxes were separate alloy castings each retained by five fastenings. Each pair of rockers ran directly, without bushes, on a long spindle across the box which was supported by an internal rib in the centre and secured by a dome cap nut on the right-hand side. Early models relied on oil mist lubrication but from early 1948 the shafts were drilled to take a positive oil feed taken off the oil return pipe.  
The construction concept can be found on Scott engines using a normal clamp bolt while the differential screw is used in Hirth crankshafts which are built up from individual components locked together with radial teeth. This complex and expensive design was dropped in favour of convention by the time units reached the public. The rocker boxes were separate alloy castings each retained by five fastenings. Each pair of rockers ran directly, without bushes, on a long spindle across the box which was supported by an internal rib in the centre and secured by a dome cap nut on the right-hand side. Early models relied on oil mist lubrication but from early 1948 the shafts were drilled to take a positive oil feed taken off the oil return pipe.  


This form of rocker cover design was arrived at after many different arrangements had been investigated. One such design was for a one piece cover over the whole of the valve gear. Accessibility would have been great and would have dispensed with the pushrod comb, so beloved amongst A7/A10 owners, however the problem of keeping such a cover oil tight was beyond the 1940's technology. Fifty years later such covers are common with OHC engines which have a much more copious supply of oil. The cylinder head was formed as one casting with a separate manifold bolted to it, to which the carburetor was bolted. Originally there was no drip tray but after several fires this essential piece was designed to take the petrol drips away from the sparking magneto. The cast iron cylinder barrels were deeply recessed into the crankcase mouth with the pushrod tunnel cast in and the head attached to it by nine bolts with a copper and asbestos cylinder head gasket fitted between the two.
This form of rocker cover design was arrived at after many different arrangements had been investigated. One such design was for a one piece cover over the whole of the valve gear. Accessibility would have been great and would have dispensed with the pushrod comb, so beloved amongst A7/A10 owners, however the problem of keeping such a cover oil tight was beyond the 1940's technology. Fifty years later such covers are common with OHC engines which have a much more copious supply of oil. The cylinder head was formed as one casting with a separate manifold bolted to it, to which the carburetor was bolted. Originally there was no drip tray but after several fires this essential piece was designed to take the petrol drips away from the sparking magneto. The cast iron cylinder barrels were deeply recessed into the crankcase mouth with the pushrod tunnel cast in and the head attached to it by nine bolts with a copper and asbestos cylinder head gasket fitted between the two.
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The real oddity was the stand. This device was a combined main and propstand. It was telescoped up the seat tube where it was retained by a tension spring. On paper it must have looked a good and neat design. All the rider had to do was to press the peg on the end of the stand down until its ratchet was released by moving a small lever. However, in practice, it had three major drawbacks, the first was to make the machine stable the machine had to be lifted a little to bring some weight on to the stand, this was very similar to hauling yourself up by your bootstraps. The second was it was susceptible to little fingers, what was an easy exercise when putting it on to the stand would then form an irresistible urge to every small boy for miles around. One touch on the small lever and the stand would retract, leaving the bike to fall over. Thirdly and potentially the most serious for the rider, was if the retaining spring broke then the stand would fall down and because it was angled forward would become a first class sprag.  
The real oddity was the stand. This device was a combined main and propstand. It was telescoped up the seat tube where it was retained by a tension spring. On paper it must have looked a good and neat design. All the rider had to do was to press the peg on the end of the stand down until its ratchet was released by moving a small lever. However, in practice, it had three major drawbacks, the first was to make the machine stable the machine had to be lifted a little to bring some weight on to the stand, this was very similar to hauling yourself up by your bootstraps. The second was it was susceptible to little fingers, what was an easy exercise when putting it on to the stand would then form an irresistible urge to every small boy for miles around. One touch on the small lever and the stand would retract, leaving the bike to fall over. Thirdly and potentially the most serious for the rider, was if the retaining spring broke then the stand would fall down and because it was angled forward would become a first class sprag.  


At least with a conventional stand when the spring breaks, it clatters along the road for a while before throwing the rider down the road at the first raised manhole cover. Therefore the odd centre stand was dropped pretty quickly in favor of the conventional in the first year.
At least with a conventional stand when the spring breaks, it clatters along the road for a while before throwing the rider down the road at the first raised manhole cover. Therefore the odd centre stand was dropped pretty quickly in favour of the conventional in the first year.


When launched the A7 delivered 26BHP at 6000 RPM and the price was listed at £135 plus £36.9s.0d. purchase tax with a further £5.1s.7d. for the speedometer which was listed as an extra but was a legal necessity. All told the new BSA twin looked well and compared favourably with the only other twin then available, the Triumph. It look sufficiently similar to overcome the conservative nature of the motorcyclist but was sufficiently different to distinguish it as a BSA with its distinctive tank and timing cover. These early machines continued on until 1950 when a new larger capacity twin was announced called the [[BSA A10 Golden Flash|A10 Golden Flash]].  
When launched the A7 delivered 26BHP at 6000 RPM and the price was listed at £135 plus £36.9s.0d. purchase tax with a further £5.1s.7d. for the speedometer which was listed as an extra but was a legal necessity. All told the new BSA twin looked well and compared favourably with the only other twin then available, the Triumph. It look sufficiently similar to overcome the conservative nature of the motorcyclist but was sufficiently different to distinguish it as a BSA with its distinctive tank and timing cover. These early machines continued on until 1950 when a new larger capacity twin was announced called the [[BSA A10 Golden Flash|A10 Golden Flash]].  

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