The Stovebolt Site Forums The Shop Area Engine Shop Engine Machining Question

Something I have been wondering about is : When is line boring required? It would seem to me that this is something that is not often required. What do you guys think?

Hy MikeE, Merry Christmas, I use the crankshaft turning test, if all the components were from the same engine with the crankshaft machined and new bearings in place, no rear seal and some assembly lube on the bearings, place the crankshaft in the block and check the ability to spin the crankshaft. If it passes the test you can install the two center main caps ( if we are talking the six cylinder 216 - 261 engine) and do the turning test, if it still passes install the remaining main caps and test for turnability. If it is turnable with all the main caps in place it should be fine. The 216 - 261 engines have a different main bearing diameters each main bearing diameter is different from the others, this makes the line boring job more labourious, also with a gear driven cam setup you can't remove much if any material from the block side of the main bearing bores, that would remove the clearance between the cam and crankshaft gears, hope that helps.

Ditto to all the above. The most common reason for line boring is if one of the main bearings fails and "spins" in the block during a catastrophic engine failure. If the scoring of the block (not the cap) can be smoothed out by removing a couple of thousandths of material, the block can be salvaged. If it requires a deeper cut than that, it's probably junk. I begin a line bore by getting the boring bar exactly centered in the bores in the block, up and down and side to side, and then carefully move the bar toward the top of the block about .002" (the thickness of a strand of hair). Then a .010" cut is made at the parting line of the main bearing caps, shortening the top bottom dimension of the bore. The line bore cutter is set to cut the hole back to the original diameter, and all the main bearing bores are cut before the bar is moved. This takes .002" out of the block, and .008" out of the caps. This results in the holes being in a straight line and the correct diameter, with the crankshaft centerline .002" closer to the top of the block. This induces a very slight bit of slack in a timing chain, or it moves the teeth of a gear-driven cam and crankshaft closer together. As long as the teeth of the timing gears don't bottom out with each other, no problem! On a racing engine, the engine buildup begins with a line bore, and all the other dimensions like cylinder boring and block decking, etc. are indexed off the crankshaft centerline.
Jerry

3B and HRL, Thanks for the education. This confirms my thoughts on the need to do this and explains a process that should not be left to the new guy in the shop. The timing chain gear and chain, is perfectly logical, but not something I would have thought of. I had this done on the 216 from hell and it was probably not needed. And most likely contributed to the trouble I have with that engine. Thanks again.

On engines with all the main bearing bores the same size, like a small block Chevy V8, there's another procedure that's a lot more common, called "align honing". A long expandable abrasive mandrel similar to a connecting rod hone that can span all the main bearing bores is used to correct minor misalignment between them after taking a slight skim cut off the mating surfaces of the bearing caps. The downside of this process is that the same amount of material is removed from the block and the caps. On inline engines like the stovebolt Chevy and other GM inlines such as Buick, Pontiac, Oldsmobile, etc. it would be necessary to use a stepped-diameter hone that would be difficult to manufacture and almost impossible to keep in alignment as the honing stones got a little wear on them.
Jerry

I can visualize such a thing, and I would think it would be a nightmare to keep accurate.