I have edited the original post after improving my measurement process by measuring the entire length of the spring and not just the body.
For reference, here is the measurement of the spring constant of the governor springs. I purchased two springs, separately, on eBay. I am assuming these are OEM, as they were advertised as such and one was pictured next to OEM packaging. I also compared them to the springs I had in the bike (“old springs”) from a kit installed 15 years ago.
In the photo, the new (“OEM”) springs are black, and the old ones are silver in appearance. The dimensions are very similar
| “OEM” new spring | Old spring |
| Outside diameter (mm) | 5.8 | 5.8 |
| Free length (inside the hooks) (mm) | 20.7 | 19.9 |
| Wire diameter (mm) | 0.8 | 0.78 |
| Body length (mm) | 10 | 9.7 |
I suspended each spring by one end and hung different weights from the other end, measuring the length of the spring, taking care that the measurement is taken without compressing the spring itself.
I also bent the wire supporting the weight in a wide loop such that the caliper fits under the spring without interference with the wire.
The measurement shows that the old springs are 10-20% weaker than the freshly purchased ones. There are also differences between the two freshly purchased springs, which lead me to think that at least one of them is not true OEM as the difference in the spring constant is too great for it to be a variation in manufacturing.
One observation for anyone who is looking for non-OEM replacement is that the springs have zero initial tension. The zero tension is evidenced in the photo of the springs showing that the coils are separated and is supported by the measurements where the load lines go through zero. In my initial search for stiffer springs, most of the options have some initial tension and that needs to be factored into the desired timing curve.
