Needle bearings are fine when used to support a rotating shaft, so long as the rotational load isn’t being hammered from a direction perpendicular to its tortional direction.

Lets exaggerate the case in point by replacing the babit shell bearings in the connecting rod with a full complement of needle bearings.   The needles are nestled between one another with just enough clearance, between the rod journal and the connecting rod, to maintain a free turning assembly.

When the engine starts and the crank turns, the needles start to roll between the two bearing races, which are the crank journal and the connecting rod.   When the piston reached top and combustion occurs, all of the force of the piston is driven through the connecting rod, putting an enormous pressure on the upper 10% of the needle bearings lying flat between the races.  

Unlike the shell bearing, having a non compressible liquid (oil) to prevent the two surfaces from making contact, the needles get no cushion effect from oil because the ‘knife-edge’ contact between the needle and the two races.   There is no chance for those needles on top to get a cushioning effect from oil, not even if they were completely submerged in oil.

It’s metal on metal until the load is lifted and they are repositioned into the ‘no load’ zone, replaced with an equally small portion from the other 90% of the needles which are merely filling a gap, while waiting for their turn to be on top for another blow.  

Now, lets introduce the big sledge hammer know as ‘DETONATION’.   This destructive force that can destroy blocks and snap connecting rods, is now flattening the unprotected needles into the race.   After a few episodes of this kind of treatment, the precision fit is gone and the assembly is now rolling on needles with flat spots, creating considerably more clearance than when first assembled.

Transplant this hypothetical situation to another area of the engine where the needle bearing roller application has been more commonplace in hi-performance engines.   The application of coarse is the roller lifter.   “Make no mistake about it, “VALVE FLOAT” or even some bad harmonics in the valve train can be almost as destructive as detonation in the combustion chamber. The roller needles in the lifters are flattened, creating excessive clearance that will eventually leads to fracturing the wheel, allowing the bearing fragments and all the needles to mix in with other rotating parts.

Winston cup engine builders may have troubles with flat lifters now, but at least they are able to finish the race.   Making the needle bearing roller lifter large enough to survive 500 miles of pounding may require the lifter bores to be larger in diameter than space would allow in current engine designs.