Rail rockers are rocker arms that has like a lip at the end of the rocker that fits around the valve stem on two sides. This keeps the rocker from coming off the valve. On earler style 289 heads rail rockers are not used because they have a small slotted pushrod hole in the head that keeps the rocker arm in alignment with the valve stem. On the early style heads these rocker arms don't have the lip that goes around the valve stem. Rail rockers are used on later model 289 and 302 heads that have the round pushrod hole in the heads. On 289 heads both types of rocker arms are stud mounted.
So,both are stud mounted. I understand what you are saying....but according to the parts house I still need to know which I have because the rail rockers apparently can only handle up to a .480 cam while the other can handle up to .500. I know coil bind will start coming into play here also. So I need to take off the valve cover to check the shape of the push rod hole and the sides of the rockers. Maybe I will just pull this engine and put in the 351 I have on the engine stand. Thank you.
if you're talking about a '65 289 as indicated in your signature line then the
original heads are pushrod guided. rail rockers were used '67 through '78
(maybe some late '66 too?).
I'm no expert but I wonder why there's a difference in max lift for stock style
pushrod guided rocker vs. rail guided rocker. I think the slot for the stud
and fulcrum is the same and I would think the valve tip contact is similar too.
the rail rocker just has rails along the sides of the valve tip, but maybe that
interferes with the retainer at high lifts?