Fragile engine

Once revised for installation in the swinging arm frame, BSA's sturdy engine changed very little. That's because its design was good enough to survive the change to unit construction and to have all manner of high compression pistons and big bore carburettors bolted on to it.

But it is the plain main bearing bush supporting the crankshaft's timing side which some believe is the BSA twin's Achilles heel. The copper/ lead lined bush also transmits oil to the big ends. Any wear between it and the crank's journal will allow the precious lubricant to spurt out, resulting in disastrously reduced flow.

Aberystwyth, Dyfed, firm SRM Engineering replace the plain bush with a roller bearing, while drilling the crank's mainshaft to create an end fed oilway. A quill bears on the crank, transferring oil without reliance on the critical fit of a bearing. The conversion costs £287.88 with VAT. This modification was developed in the Seventies by ex-BSA man Les Mason. His Tamworth dealership Devimead offered the service until taken on by SRM.

But Mick Page of Burton upon Trent BSA specialist Burton Bike Bits, believes that there is nothing inherently wrong with the plain bush. He explains that the practice of transferring oil through a plain main bearing has been used in car engines for years. He concedes it must be made of the correct material - originally a Vandervell VP23 product - and that failures of pattern bushes has exacerbated the problem. Regular oil changes should keep wear to a minimum. Although a significant expense, SRM's conversion does give peace of mind.

Memorable symmetry of pre-unit engine - just the right amount of finning.

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