OK, so this is one of those, "Bugger me, I didn't know that...", moments.
When I was rebuilding/resurecting my 650 pantah about 7 years ago, there were a couple of spares in a box.. I didn't pay much attention to them till it came to the point of doing the heads.
One of the rockers already in the head was a bit scuffed, nothing nasty, but I thought I'd like to do better. So I looked in the spares box and found another opener that looked very nice. Then I looked closer.. damned thing was completely different.
I had a chat with my Guru, Gowie, and he taught me the very basics that he thought every young, impressionable, naive, ignorant, clueless, "wouldn't know shit from shinola", recent desmos dromos convert should know. And I was all those things. My
lucky day!
The forward exhaust (real) pantahs came with two (eventualy three) different types of rockers.
The first type are a full width rocker, it fits from one side of the cambox to the other and is shimmed for axial play. The closer is easily identified with it's club foot that has a wide notch machined on the underside for valve seal/guide clearance. I guess that since this is the first type made/used, it would need to be called 'normal'. This set is visible at the top of the image. They are perfectly suitable for road work where maximum revs are not held for extended periods on standard heads.
The secont type is the 'racing' rocker, it also fits one side of the cambox to the other and is shimmed for axial play. It was sold under the NCR banner as a 'racing' replacement part (NCR was a 'front' for the factory race shop of course, but we can discuss that bit later if you like). Below you can see the 'beefier' set in the bottom of the image. The 'spine' of the opening rocker, and of the closing rocker, is much taller. The curve of the arm to the cam lobe pad and the valve tip is also more pronounced. The fork of the closer is abreviated/re-angled and is not machined like the 'normal' unit for guide clearance. Overall these rockers have a taller cross section.
Obviously the engineer(s) designed this shape for a much stronger rocker when heavier valves or higher revs, or both, were employed for 'go-fast' work, i.e. racing. I can't pass comment on the mass differences of the two types, I've not weighed them.
I mentioned that both the above were full width rockers, which brings me to the third type. The clipped opening rocker. In discussing this below I'm only referring to flat out, heavy load racing situations. I have not heard of any rocker or pin failures in normal road use... (anyone?)
This type was introduced in the mid to late 1980's and should also be considered a road use only rocker. As it was explained to me, once the rocker is no longer 'full width', that is, the clip system is used to ease opening shim servicing, weakness is introduced in the rocker/pin assembly.
When a full width rocker is asked to open a valve the force (or strain) generated - that's the resistance of the opening rocker, the valve and the closing rocker, to being accelerated - is carried by the openers' pin. That load manifests as a shear moment, vertically, in the space between the ends of the rocker and the wall of the cambox. The pin is very strong in 'shear'.
Now use a clipped rocker, half of its pin support is a bit of pressed tin. When it is asked by the cam lobe to open the valve, the same force now manifests as a flex moment. The area available to flex is around 15- 20mm (I've never measured it). The pin is not strong in 'flex' and the flex moment can occur 4 or more thousand times a second.
Going racing with high revs and big valves? Which set of rockers do you want in your heads?
As usual, feel completely free to correct me if you think I'm way off the mark.
Garry.