Valvetrain Geometry

gzig5 said

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.


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.

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I thought the compressive forces of drawing (or pushing I forget how it’s done…maybe it’s both ways) the bar stock through successive dies changed the grain structure or probably more correctly lets the grain flow bend and shape around the part, making it stronger than if rocker or whatever part was machined from a round or whatever shape of materiel you start with rather than just machining it.

Do I have that correct?

Also, there are times when you need the part to have some characteristics that absorb some shock loads like a main cap. I’d rather have the cap move and take some shock rather than have a stiffer cap on there and have that shock end up fracturing the block.

If the block was stronger to begin with then a billet steel cap would make the bottom of the block more rigid and stronger without causing the shock loads to fracture the block.

In the valve train, flexing always occurs but it’s generally a bad thing because that flexing can cause a part or multiple parts to excite and then you get the valve train out of control.

I forgot to mention earlier I’ve seen steel rocker stuff go crazy at a very low rpm (it started a 6k) and the engine would not get through it.

That was a 377 inch SBC Chevy that had stud mounted rockers and a stud girdle. So we took the girdle off and it went several hundred rpm higher and ran into the same issue. They dyno operator said I have a set of aluminum rockers. Let’s try those. I said WTF? That makes zero sense. And I’ll never forget what he said and that was then that’s why we should try it.

This guy was and still is a dyno operators dyno operator. At his age (today) I’d bet he has over a million pulls on his dyno. So we (the engine owner and I and ultimately it was his call even though it was my build) said go for it.

It ran well past 8k without an issue and we were stunned. My question was how did that even work? And his answer was that sometimes the resonance frequency of the multiple parts go into chaos at or very near the same rpm and when that happens you need to change something that has enough mass to affect a change.

It could have been a change in pushrod diameter or wall thickness or a spring change but we didn’t have those parts laying around.

Sometimes the behavior of these parts makes no sense. Im sure that that stud mounted rockers was affecting the situation.