-JdK- While checking over my 1927 Model 18 last summer I discovered that one of the valve springs was broken. Rather annoying as they come in a different size compared to the later OHV models and are difficult to find. I managed to buy a few sets but as all the original Norton stock is now gone they are all replica and -not surprisingly- all different. I sent them off to Viktor for a closer look.
|Dimensions of the 1920s Model 18 valve springs and the recorded data. In blue the old/used spring; in green the replica spring from Czech Republic. In purple and red the two replica springs from the UK.|
Viktor does everything with great care and took a scientific approach. The tension of the springs was measured at 41 mm; that's how far the springs are compressed when the valve is closed. And at 30.8 mm; that's the compression at full lift when using a standard W7 Model 18 cam.
The outcome is that the strength of the springs differ a lot, see the summary in the table above. Both UK replica springs constantly apply around 10 kg extra pressure on all the delicate parts of your Model 18 valve train and this is bad news; it will result in a lot of extra wear, especially on the cams and the followers.
My 1927 Model 18 has been tuned with a high compression piston and IT cams, it goes like stink and it likes to rev. It does so on the softest springs I could find; the old/used springs in the table above. So I suggest there is no need at all for stronger springs in a flat tank OHV Norton.
If you have ever new springs made you could probably ask the supplier to use the data as presented in the figures above.
Do realize that the 41mm/30.8 mm dimensions are for a c1927 Model 18. For other models/years valve stem length, position of the collets and dimensions of both the top and bottom collars etc. may differ which may affect spring pressure.