Hi guys,
Sorry to chime in so late. Part of this is due to my health, which is not at all good, as for the past 4 years I have had to put up with an incurable blood cancer and the amount I can get done in a day is quite little.
The OP's photos in his first post shows that the spring on the left where the collar broke has about 6.5 coils, whereas the other one has about 5.5 coils. I think it's quite possible that the offending spring was coil-bound when the valve was at full lift. I think that's the most likely cause of his problem. A loose assembly of the motor to check this, should show that. To me, by eye, I think even the other spring seems not to have much space between the coils.
Re the valve collars themselves, I've never seen damage like that before, and another poster's view that the fracture was induced by a stress-raiser, namely the inside edge of the seat for the outer spring looks
Re alloy collars, there is no problem at all about alloy collars, providing they are made of a suitable alloy. The ones sold by Ebor Bikes (which I own) are to the design used by Eddie Dow, using Aerospace certified L168 bar stock, and he told me that they were used on racing Cosworth engines of the day, revving to over 11,000 rpm.
Back in the '60s I rode a RGS look-alike with his spring conversion, using DBD34 valve springs. The bike was an absolute pleaser to ride.
The DBD34 springs were designed so that the coils were wound in opposite directions and the inner and outer coils touched lightly. This introduced hysteresis into the spring system, to eliminate the parasitic oscillations which would otherwise occur at particular rpms. Interestingly the WW2 German machine gun, the MG42, which had a long spring behind the breech block, controlled longitudinal oscillations by winding the spring out of 3 strands of spring wire as a means of introducing friction and thus hysteresis.
Also, the DBD outer springs also have more closely wound coils at one end, to decrease the force between the valve and the seat when the valve is closed. This means that these springs should be fitted with their closed coils to the cylinder head, so that in use, as the valve opens, the closed coils don't move as that would just increase the moving mass in the valve-train. The lower seat force when the valve is closed given by these springs differentiates them from many alternatives in the marketplace. The DBD spring used in the Ebor Bikes conversion uses superior wire made from an alloy wire specially made and heat treated for valve spring use.
If high rpm levels are avoided, valve springs don’t have to put huge amounts of force on the valve seat. I remember noting back in the ‘60s, that an RGS motor (with the Eddie Dow conversion) with the outer coil on a spring broken, would still rev to 6,000 rpm before the valve floated!
Anyway, I hope the OP gets his problem sorted out. Whatever spring are used, at full valve lift, a minimum of 0.060 (1.5mm) should be present before the spring becomes coil-bound.
Cheers,
Jon
