I have a 4 bolt mains block that was fitted at the factory with all the boses for the retaining spyder for the roller lifters but the engine was fitted with a flat tapped hyd. cam.I would like to build the engine and use a roller cam,what are the limitations of the factory lifters as far as lift on a performance application?Should I be looking at aftermarket lifters or are the factory lifters good for .525-.575" lift range at the valve?Thanks

To increase the valve lift, they make the cam SMALLER: they can't make it any bigger because then the lobes would be too large to install it, so instead since lift is the DIFFERENCE between the highest point and the lowest point, they grind the back side of the lobes lower. Then once some given lobe lift (smaller cam diameter) is reached, the lifters will fall out of those stupid figure-8 things. It depends on how far the block was spot-faced for them, as to exactly what lobe lift that will be. But somewhere around .375" of lobe lift (about .550" at the valve) is about as far as you can usually go, and still be safe.

You may also find, if the block wasn't fully machined for that goofy roller setup, that the top of the lifter bosses wasn't spot-faced for the figure-8s at all; in which case you can't really use them. They won't sit flat, and the lifters will slide out the bottom of them even sooner. A stock cam might not even work right like that... probably won't in fact.

They make "retrofit" lifters that are taller so that the link bars clear the taller lifter bore boss as cast into those blocks. That's probably what you'll end up needing.