Clas­sic Cob

From the shed

Old Bike Australasia - - CONTENTS -

When at­tend­ing the re­cent All Bri­tish Rally at New­stead I came across a 350 BSA with an al­loy head which had a dif­fer­ent shaped rocker box. On speak­ing to the owner it tran­spired that this ma­chine was fit­ted with hair­pin valve springs. He be­lieves that this is one of only ten produced by the Small Heath firm. Not be­ing into the BSA mar­que I had no idea that they had ever made one. He ad­vised that he also had an al­loy head and bar­rel which he be­lieves, but is un­able to con­firm, was pos­si­bly made in Queens­land. I was able to pho­to­graph the ma­chine and he later for­warded me pho­tos of the al­loy ver­sion. Want­ing to know more about as to what is the ben­e­fit of hair­pin springs, if any, I con­tacted an old ac­quain­tance to find out more. His ver­sion is that hair­pins came about due to the early coil spring ma­te­ri­als not be­ing able to han­dle the high rpm be­ing achieved by mod­ern en­gines. With high valve lifts be­ing in­tro­duced in the 1920s and 30s there were also high fail­ure rates. An­other ad­van­tage was that these al­lowed bet­ter clear­ance around the top of the valve spring which helped in the cool­ing of that area. By re­mov­ing the coil spring from the top of the valve stem it was pos­si­ble, in some cases, to shorten the valve stem thus re­duc­ing the weight of the valve which is crit­i­cal in high rpm en­gines. All of the Manx Nor­tons from the early 1930s on used hair­pin springs were very suc­cess­fully. These days how­ever coil spring ma­te­ri­als are so good that they can op­er­ate re­li­ably at ex­tremely high rpm even with the high valve lifts. The owner of the ma­chine was able to for­ward to me some lit­er­a­ture on the hair­pin valve BSAs. It tran­spires that at the end of 1939 BSA de­vel­oped a B29 Sil­ver Sports with a cast-iron head and equipped with hair­pin valves. A small few made their way to the civil­ian mar­ket. At the out­break of war there was a mil­i­tary ver­sion – the B30 produced – but af­ter the first batch left Small Heath, the army re­jected this ma­chine and went with the M20 BSA and other makes. Af­ter the war an ex­per­i­men­tal high per­for­mance ver­sion which was ba­si­cally a B29 was pre­pared for the 1947 Ju­nior Manx Grand Prix. BSA still had the rigid frame at this time so for this event the en­gine was in­stalled in a frame with pro­pri­etary Feri­dax McCand­less piv­oted fork rear sus­pen­sion and a large 9 inch front brake. It was rid­den in this event by Ir­ish tri­als and scram­bles rider Bill Ni­chol­son. Although the ma­chine was quick it was not push­ing for the lead and un­for­tu­nately ran out of fuel and did not fin­ish. Ni­chol­son also rode the ma­chine in hill climbs and af­ter sign­ing a con­tract was em­ployed in the com­pe­ti­tion sec­tion at Small Heath. In the 1970s an all al­loy en­gine num­bered EX291 with elec­tron crankcases was lo­cated in the UK. The en­gine num­ber EX291, is in­ter­preted as Ex­per­i­men­tal B29, No.1. It is be­lieved that this is the en­gine used by Ni­chol­son and was built by Jack Em­mott who was in charge of the ex­per­i­men­tal en­gine shop at BSA. Ni­chol­son about that time is quoted in the Mo­tor­cy­cle Sport July, 1983 that he re­called that there were about three or four of those en­gines in the ex­per­i­men­tal shop when he was work­ing there. Ap­par­ently coils later re­placed the hair­pin valve springs in late 1948 when they re­vived a model name that had been dropped in 1940 and this was to be the B32GS 350cc Gold Star, first of the post-war Gold Stars. Views are held that the post-war Gold Stars were de­scended not from the M24 500 Gold Star of 1938-39, but from the 1940 B29 350 Sil­ver Sports. There is some men­tion of this en­gine in the book Goldie which I do not have ac­cess to.

ABOVE The hair­pin head. RIGHT Photo taken at the All Bri­tish Rally of the hair­pin valve spring BSA.

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