LIFTER BORE PROBLEMS
For production engines, the lifter-bore centerline tolerances are not extremely critical. They certainly should be within the factory specifications; but for racing, these tolerances must be much closer. We have seen some blocks come from the factory with lifter-bore "lean" problems so acute that flat tappets may rotate too fast- or rotate in the wrong direction- or they won't rotate at all. In some cases, they ride up on the taper edge of the cam lobe or even ride on the wrong edge of the taper (this taper is built into the lobe to make the lifters rotate)...
There are four definable problem areas caused by inaccurate gang-drilling at the factory. Factory prints call for 1) the lifter-bore axes to intersect with the camshaft axis; 2) all of the lfter bore axes to lie in the same plane and 3) be perpendicular to the cam axis; 4) the bores to be correctly spaced, fore and aft as required, over the cam lobes. Factory tolerance allows for 0.0006-inch out of perpendicular and 0.005-inch out of plane "lean".; But if any of these four specifications are severely violated, some sort of lifter or cam problems may occur. Thus, frequent problems in one or all of these areas is common, and more than likely, they will exist to some degree in every production block...
We have noted that that it is common to find the bore axes off-the-perpendicular. If a problem does occur, it will not effect the entire bank of (lifter) bores. More than likely, one of the gang drills hit a particularly hard spot in the cast iron, walk sideways, and drill a single bore off the perpendicular.
For production engines, the lifter-bore centerline tolerances are not extremely critical. They certainly should be within the factory specifications; but for racing, these tolerances must be much closer. We have seen some blocks come from the factory with lifter-bore "lean" problems so acute that flat tappets may rotate too fast- or rotate in the wrong direction- or they won't rotate at all. In some cases, they ride up on the taper edge of the cam lobe or even ride on the wrong edge of the taper (this taper is built into the lobe to make the lifters rotate)...
There are four definable problem areas caused by inaccurate gang-drilling at the factory. Factory prints call for 1) the lifter-bore axes to intersect with the camshaft axis; 2) all of the lfter bore axes to lie in the same plane and 3) be perpendicular to the cam axis; 4) the bores to be correctly spaced, fore and aft as required, over the cam lobes. Factory tolerance allows for 0.0006-inch out of perpendicular and 0.005-inch out of plane "lean".; But if any of these four specifications are severely violated, some sort of lifter or cam problems may occur. Thus, frequent problems in one or all of these areas is common, and more than likely, they will exist to some degree in every production block...
We have noted that that it is common to find the bore axes off-the-perpendicular. If a problem does occur, it will not effect the entire bank of (lifter) bores. More than likely, one of the gang drills hit a particularly hard spot in the cast iron, walk sideways, and drill a single bore off the perpendicular.