Obviously when using a roller cam, you have to keep the roller wheels lined up with the cam lobes. If not BAD things are going to happen.
On factory Ford roller motors, the lifters have flat spots machined into them perpendicular (90*) to the cam lobe and lifter wheel. Then there are little "dog bones" which are basically 2 horseshoes joined at the arc, that hold onto those flat spots on the lifters. Then there is a "spider" which has 8 legs (go figure!) to hold all the dog bones in place, and it's bolted to the lifter valley via 2 bolts above the cam.
NOT a factory Ford roller lifter, but very similar.
FORD roller lifters, without the dog bones.
Ford Cam, dogbones, and spider:
Now, aftermarket lifters have a "link bar" system in lieu of the all of the above. We still have to keep the lifter wheel perpendicular to the cam lobes, but there's a much simpiler way to do it.
Link bar roller lifters:
Aftermarket roller lifters are permanently paired. Yes, they come in "2's". The flat spots are only machined on 1 side of each lifter, and their connected via a flat piece of steel. Lifter #1's connection is simply a pivot point for the bar. Lifter #2 (of the pair) is connected to the bar in similar fashion, but it's connection is on a slotted hole, to allow movement.
They both accomplish the same thing, we keep our lifters correctly oriented on the cam. But the aftermarket lifters allow us to install roller cams into non-roller blocks.
Hope that explains it a bit, but Cam Motion can help you out if you tell them what you're up to bro. I HIGHLY recommend a solid roller if you have the cash laying around.
