Stockers have great geometry nearly all the way thru the valve travel,
well past the stock lift point and to probably over .600" lift. The stock
non-roller piece is well designed, but probably not that great for flex
and of course, the sled fulcrum is a high friction part that is not that
great for anything.
The geometry of the Stock rocker contact point is like this:
RR Comparison in bold (I checked a popular adjustable 1.73 RR)
1. At .000" lift (seat) the rocker pad is inboard side of the valve tip,
(towards the intake manifold) and it is approximately 1-1.5mm or so
from the edge of the tip.
The RR starts at nearly the center, much further from the edge
This is a plus, IMO
2. At mid-lift ranges (.200-.300") the rocker pad is at the center of
the valve tip, exactly where it should be according to my reading.
RR tracks outboard, which is OK (but reversed from stock) and
this could on a high mileage engine cause a new wear pattern, which
would possible be a positive trend, but if the valve guides and stems
are excessively worm, then they would begin to wear in a new region
causing more clearance. But even high mileage (~100,000) Fords
I have seen experience little guide and stem wear. Use your better
judgement here.
3. At high lift (.450-.650") the rocker pad is approximately 2/3 of the
way across the the valve tip, or pretty close to where it should be, almost
the same amount to the outboard side of where it strted on the inboard.
RR never does reverse the tracking pattern on a stock length valve,
the way most valvetrain experts suggest it should. The roller tip relieves
the side loading as much as mechanically possible, but the forces exerted
on the guides are constantly wanting to tip the valve inboard at the
bottom, as opposed to outboard at the bottom like stock rockers do.
No problem IMO, on a low mileage engine.
4. Beyond .650" or so, the rocker pad does not travel as far but does
still track closer to the outboard side, as expected. I would never
recommend exceeding ~.650" with stock rockers, the mechanical
limit being just over .700" before the slot hits the pedestal. (.650"
gives at least .050" clearance)
RR finally reverse the tracking (on the tip) as expected, but well
over usable lift ranges with a stock valve length. At the max approach to
the valve tip edge, it is still better than a stock rocker ever is, the pattern
is just not what is considered "optimum" for roller rocker contact pattern.
5. The stock rocker has almost perfect geometry, (for a fixed, non-roller
rocker pad) the pad never "scrubs" or wipes across the valve tip but it
does "roll" the contact point as well as possible for a non-roller tipped
rocker throughout the lift range as far as I can tell.
RR geometry with the rocker tested calls for a shorter rocker body
or a longer valvestem. An alternative would be to machine the pedestals
and use shorter pushrods, or stock length PR's and adjust the seat cup
to fit. I strongly recommend hardened pushrods for any V6'er running
my Level 2 springs or higher, or anyone's springs that exceed ~225#
open pressure @ max lift. It is just good cheap insurance against bent
pushrods when you have stiffer springs.
I DO NOT SELL RR's
at all, so this has no hidden agenda... I also
never intend to sell them unless I find a "perfect"
rocker that fills the needs better than what is available,
and for most setups this is quite adequate IMO. SSM
and RPM both carry these (same brand I believe) so
contact them for roller rockers!
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CONCLUSION:
I recommend roller rockers, but use caution on higher
mileage engines (IMO)
If the heads are rebuilt then it's a safe bet the guides
and valvestems are up to the task, as this is usually
checked on a freshened up cylinder head.
The RR geometry is not "perfect" but simply
using a roller tip is a slight advantage and
the roller fulcrum (pedestals) reduce friciton
too, and that is where most of the gains come from.
1.8's add usable lift, which is somewhat of a gain
from an airflow standpoint. RR's also supposedly
"flex" less than a stocker but most of those comparisons
are made to the ultra-cheap ch*vy stock junk parts,
most Ford factory stock parts are much stouter than
their ch*vy counterparts...
Rocker Geometry Tech
It is a more traditional 
