I thought the original owner told me they were SBC rockers and that is what I ordered. When they would not fit over the studs (7/16?) I went down to the engine shop and we talked about it. When he looked at my original rocker arms he said they were 1.7 BBC and that no one made a 1.5 BBC rocker. However, he suggested I enlarge the slot in the SBC lifters which is what I did on my Bridgeport, using the nuts and rocker washers that were on the engine when I got it (they fit fine in the sb rockers). They appear to fit fine. No interference, no rubbing of the pushrods on the holes in the head and the end of the rocker is on the valve.
Did I say I don't know what I'm doing???
You keep referring to your rocker arms as lifters,makes me think you are not too familar with parts of an engine??? Don't know.
Anyways,,, I would think you must have bad rocker arm geometry running SBC 1.5 ratio rocker arms.
The rockers arms must be hitting on the edge of the top of your valve stem???? (Really easy to see w/a roller tip rocker.)
It should be hitting the center of the valve stem,easier to see this when you have roller tip rocker arms,which I would recommend anyways,because the six cylinder has short valve guides & have a dendency to wear more (the guides)+ with better rocker arms you get more even lift, not like the factory stamped units.
They are supposed to be 1.75 ratio, but they are not accurate,some could be 1.6,1.68 ,1.72,,,etc.
Comp cams sells roller tipped ones that are not too much if you are on a budget.
Not sure what type of answer you are looking for,but I would definately not run 1.5 ratio rockers arms,the geometry has to be off. Not contacting the the top of the valve correctly.
MBHD
