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Definitely built for racing. Is that a bronze head? That dual float carb was usually used for alcohol or dope fuel and seen on the old JAP speedway engines. It has a '49 and older timing cover but the mag is mid '50's. The barrel and head do appear to be something special. Maybe someone's one off racer.
Bill B...
PS: Just noticed the drive side and the breather with the model "KM23" stamped. The breather I believe is prewar and a KM23 is a '39 500cc Silver Star.
Last edited by Boomer; 03/12/19 7:07 pm.
Boomer
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A quick online search comes up with many articles of the 1939 BSA KM23 Silver Star.
Gordo
The roadside repairs make for the best post ride stories.
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Given the bronze "beautification" of various tidbits on that engine along with the fact that, to the best of my knowledge, BSA didn't list a bronze head upgrade, my guess is it's the stock cast iron head with a coat of bronze paint. The fact that by then BSA had the technology to cast Al heads (for the Gold Star), and that 1939 was pretty much the end of the line for anyone using bronze for heads anyway, also makes painted cast iron more likely.
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Is there anything inside it?
Were the oil pumps so unreliable that it was important to have a gauge to check that the pressure hadn't dropped from the usual 5psi?
At the very least, it's a lovely historical monument.
The cast in breather? at the top of left case is unusual, is that moulding found on later cases?
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By the XB engines in 46/47 the breather was lower down behind where the crank came out. The pre-war knob was gone but that new knob stayed there for years. Gordo ![[Linked Image]](https://i.postimg.cc/HVf4nwX0/IMG-0518.jpg)
The roadside repairs make for the best post ride stories.
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Gordo,
According to what is stamped on the crankcases it is an M series heavy weight not a B series sports engine.
M's breather remained on the top of the case till it moved down to the timing cover with the upgrade to tele forks.
Nick
Pre WW II M series de-luxe models had a tell tale button where that pressure gauge is.
Actually I think they were a bit higher up.
They remained on the De-luxe models only till WW II when they were dropped, never to appear again.
Originals are only found under rocking horse poo and the repo ones must be made from solid gold.
Bike Beesa
Trevor
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