Valvetrain Geometry
Were two different lobes tested in the length test? And what was the lash number or was that at zero lash?
Also, I’ve never seen a Crane rocket that was billet. AFAIK they were all made from extruded bar stock.
In fact I think most aluminum rockers are still made from extruded bar stock. The extruding process compresses the grain structure and makes the rocker stronger.
I can't swear to it, but I'm pretty sure that all round and rectangular aluminum stock, or "billet", are extruded. The only difference between bar stock from SpeedyMetals and the Crane rockers is the shape of the die the aluminum was pushed through, as long as the starting alloy is the same. Extruding to near net shape is a savings of machining time, but the general material properties would be the same unless there was some exotic heat treat cycle required. Billet aluminum is cast into a big block (billet) but then heated and extruded to the round, square, hollow, etc shape that the end user needs. It isn't like forged vs cast steel, where the grains structure and strength of a forging are significantly stronger.
Take a look at the picture below. These are some different rocker shaft hold down caps. The two on the top are steel. The two on the bottom are aluminum. In my opinion the caps need to be steel and they need to be stout. A rigid cap will reduce the effective length of the shaft between centers. I can't see the aluminum helping very much. Even though the rocker stand under the shaft is aluminum, a wide, stout steel cap should add rigidity to the system. It would also change the resonant frequency of the rocker shaft. But just like in the example you posted with the steel vs Ti spring retainers, you never know if the frequency change will help or hurt. Rigidity seems like a good thing to me though.
I don't disagree with you but then I also don't understand how the BB guys are using aluminum main caps as an upgrade in high HP builds.
Rigidity is usually but not always a good thing. I was a reliability engineer for ten years before retirement and did a lot of Long Term Reliability and HALT testing (Highly Accelerated Life Testing) on a variety of products. It was a lot of fun and we beat the **** out of everything and the engineers always complained that nothing could survive, but that was the point. Find the failure modes in a short amount of time. Anyway, you would be surprised what resonant frequency can do to a supposedly rigid design. Sometimes, adding compliance and altering the natural frequency is a better idea. There are always caveats though, and in a mechanical system like the SBM valvetrain, less flex is generally going to be better at the higher stress levels.
I've always wondered how much a thicker rocker shaft of the right heat treated material would increase stiffness of the system. With pushrod oiling, you could run a solid shaft no? Solid should be more resistant to flex between the hold downs with strong springs. On the other hand, I would have thought it's been tried before and if it isn't popular, maybe the difference is insignificant.