Just did a little checking and the outer spring for the DT as supplied by the Club is 1-3/8 inch o.d., 1-1/2 inch free length, and 0.147 inch diameter wire, with just a bit more than three full working coils.
A set in a 1935 ohv engine I have are identical. Though could have been replaced decades ago by a set intended for a DT, other than they are right-hand wind rather than left-hand like the DT.
A set measured from a 1931-32 F/G ohv model are again like the DT, except the wire is 0.126 inch diameter rather than 0.147, so giving a lower rate.
The road models used duplex springs where as the DT used triplex for the exhaust and duplex for the inlet. Presumably to compensate for the tendency of the exhaust valve spring to weaken over time due to heat. I have not come across any info as to what the SW5 used. The innermost spring of the DT triplex set was the same part number as the 350EW valve spring. The Club just sells (or did) the DT sets as the duplex, though they also carry the 350EW springs too.
If what you have are a set for a F/G31, then the rate maybe a bit low if you are intending on racing, but would be fine for road use. And clearly DT sets would also work in a road going machine, albeit a higher loading on the tappets and cam lobes. As you can see above, the lengths are the same and the longer valves used for the enclosed valve models just compensates for the greater distance introduced by the enclosed head and detachable perches. I did not compare the installed height, which might vary between the two designs and change the seat pressure. But if you can get the DT springs from the club, or the springs that you have match the DT dimensions, then as you are installing them on a DT type head the installed height on the enclose valve head is a moot point.
I did a study of the spring rates, and the new club DT valve springs (duplex) are 143.9lbs*inch. A used (and very rusty) set of DT valve springs had deteriorated to 122bs*inch, of which the innermost spring was contributing 16.2bs*inch to that total. A used set of F/G31 springs (duplex) was measured at 88.8bs*inch.
The following image shows the setup with force dynometer. Once resistance was detected the spring was compressed in 0.05 inch increments and a reading taken to plot the spring rate.
-Doug
