R3W7 Assembly
The Series 5 is their high-end pushrod which is heat treated for added stiffness. Around $19 each.
Not to be pedantic, but heat treating can't change the stiffness of steel. It can make it harder (helps with metal to metal wear and will increase the force needed to bend the metal permanently) but not stiffer.
Stiffness is inherent to most materials and is defined by Young's modulus.
Some types of metal treatment can increase the stiffness of the treated layer (like nitriding, case hardening, or other diffusion treatment methods), but the effect is not very large.
That said, a higher grade of heat treat is rarely a bad thing. Some materials can be made harder than woodpecker lips without becoming brittle. Properly treated 8620 is amazing stuff too, I use it in many of the parts I've designed due to the tremendous strength it can give while remaining amazingly tough (bend before cracking). There's also a million ways to treat it, some are very costly but the results are tough to obtain without resorting to other crazy materials like maraging steels, or tooling specific steels and the like.
