Having described my invention, what I claim as new and desire to secure by Letters Patent is

1. A portable line boring bar assembly, for in-line boring the main bearing bores of engine blocks for reestablishing their precise cylindricality and axial alignment longitudinally of such a block, comprising an elongated boring bar for extension longitudinally through the bores of said block; centering means slidably mounted on said boring bar for temporary fitted engagement in selected bores of the block precisely to center the boring bar relative to said bores; bar support means releasably mounted on said block in spaced relation to said bores to hold said boring bar in said centered position during powered rotation and axial feeding movement of the boring bar in the block, said bar support means including a plurality of bridging members individually having an upper bracket adapted to be bolted across said block, an arm adjustably extended through said bracket into the block, and a bearing on the distal end of said arm rotatably and axially slidably receiving said bar therethrough; said arm of the bridging member having an upper end, a pair of diametrically opposed pins extended from said upper end of said arm, and slots in said bracket for receiving said pins therethrough to permit downward sliding of said bracket over the arm with said pins being angularly related to the slots upon rotation of the arm to align said bearing with said boring bar to provide a positive stop to preclude inadvertent disassembly by gravitational descent of the arm and bearing from the bracket should the boring bar be prematurely removed from said bearing; and pre-set bore cutting means adapted to be releasably mounted in selected location along said boring bar adjacent to said bores of the block.

2. The line boring bar assembly of claim 1 in which said cutting means includes a separable tool bit holder adapted to be releasably clamped around said boring bar in any desired axial position therealong, a tool bit adjustably slidably mounted in the holder, and lock means releasably to hold the bit in predetermined radial extension from the holder.

3. The line boring bar assembly of claim 2 in which said boring bar includes opposite ends, powered rotary drive means releasably connectable to one end of the bar, and axial drive means releasably connectable to the opposite end of said bar from the rotary drive means with each of said opposite ends of the bar being adapted selectively alternatively to receive either of said drive means.

4. The line boring bar assembly of claim 3 in which said rotary drive means is an electrically powered hand drill, a bore in each of the opposite ends of said boring bar, and adapter means releasably mounted in a selected one of said bores providing an elongated chucking spindle for the drill.

5. The line boring bar assembly of claim 4 in which said axial drive means is an hydraulically powered drive unit adapted to feed in either axial direction having a coupling end releasably selectively mountable in said bores of said ends of the boring bars.

6. A line boring bar assembly for in-line boring of bearing bores of engine blocks having more than two aligned bearing bores for establishing their cylindricality, axial alignment, and desired diameter comprising an elongated boring bar; a plurality of rings fitted to at least two of the bearing bores and adapted to be clamped therein releasably rotatably mounting the bar for rotation and longitudinal advancement concentrically of said bores while extended through another of the bores; drive means connected to the bar for rotating the bar; further drive means connected to the bar for advancing the bar axially in the bearing bores during rotation; and cutting means releasably mounted on the bar adjacent to said other bore and caused to traverse the same while the bar is rotated and advanced including a separable tool bit holder adapted to be releasably clamped around said boring bar in any desired axial position therealong, a tool bit adjustably slidably mounted in the holder, and lock means releasably to hold the bit in predetermined radial extension from the holder.

7. A portable line boring bar assembly, for in-line boring the main bearing bores of engine blocks for re-establishing their precise cylindricality and axial alignment longitudinally of such a block, comprising an elongated boring bar for extension longitudinally through the bores of said block; centering means slidably mounted on said boring bar for temporary fitted engagement in selected bores of the block precisely to center the boring bar relative to said bores; bar support means releasably mounted on said block in spaced relation to the said bores to hold said boring bar in said centered position during powered rotation and axial feeding movement of the boring bar in the block; said bar support means including a plurality of bridging members having brackets adapted to be mounted on the block individually having an arm extended through the bracket into the block, and a bearing on the distal end of said arm rotatably and axially slidably receiving the bar therethrough, said arm of the bridging member having an upper end, stop means on the upper end of said arm, and a slot in the bracket for receiving the stop means therethrough to permit downward sliding of said bracket over the arm with said stop means being angularly displaced from its slot upon rotation of the arm to align said bearing with said boring bar to provide a stop to preclude inadvertent disassembly by gravitational descent of the arm and bearing from the bracket when the boring bar is removed from the bearing.