Author Topic: Valve spring spec tolerances ?  (Read 846 times)

Offline Josh Cox

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Valve spring spec tolerances ?
« on: 07.07. 2009 12:18 »
What are the exhaust valve spring tolerances for a A10 Cast Iron head ?, my Haynes book, which I am rapidly learning to distrust, states, minimum :
Inner spring 1 17/32" / 38.893 mm
Outer spring  1 7/8' / 47.625 mm
Are these numbers correct, I have two full sets of used springs and only one of the inner springs is within these specs ?, can they really all be that old ?.
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Online RichardL

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Re: Valve spring spec tolerances ?
« Reply #1 on: 07.07. 2009 12:46 »

Just reading from my reference material here, the numbers you've given concur with Bacon's "Twins and Triples," with the change to 2" and 2-1/8" in '58 for the RGS and Super Rocket.

Hope this helps.

Richard L.
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Online BSA_54A10

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Re: Valve spring spec tolerances ?
« Reply #2 on: 07.07. 2009 14:06 »
There is no position on the crank when all 4 valves are shut so at least one valve will be open so that valve's spring will deform which because mechanics don't like long words is usually called a "set".

Valve springs are not all that expensive so replace them.
If you are not going to use a bike for a long time then it is good practice to remove all of the tappet screws to allow all valves to close. BSA recommended this to prevent "spring set during the winter lay over".
It also prevents ( well slows any way ) water entering the cylinder and rusting rings to the walls and stops valves rusting open.
If a valve rusts closed then the action of the rockers is usually sufficient to "unstick" it but if it rusts open then there is a good chance that the first kick of spring will cause the piston to hit the valve with the expected damage.
The rule of thumb for maximum valve spring set is to replace them when the set is 5% to 10% of the original length. Nicholson recommends replacement when set is 1/8" or more.
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