I'm not sure I agree, but that is based on 1) how much do you know about this engine, 2) is it a high compression engine (11 : 1 compression or better) 3) were there any problems before the tear down, 4) how many total miles are on the engine block, and 5) how far is it torn down. Simply looking at the top of the cylinder does not take into consideration any cylinder wall taper that may have occurred.
If it is already bored .030", it has already been rebuilt once and had problems, or it would have been bored .030".
When the piston goes up and down, the connecting rod is moving up and down at the top of the stroke but it is moving in a circular direction at the bottom of the stroke due to the rotation of the crankshaft. This force is transferred to the piston at the wrist pin and places more side to side wear on the cylinder walls at the bottom of the stroke than at the top of the stroke and, over time, causes the cylinder wall to wear round at the top and to wear more oval shaped at the bottom. This is the taper I was talking about. The harder the engine is loaded, the more prominent this wear pattern is. If your block has enough taper, the engine will work after you rebuild it, but the new rings will never seat.
Mine was the 350/370 hp LT-1 with 11.5:1 compression. My '68 engine block didn't have much of a lip on the top of the cylinders when I pull it down, but it needed bored because it had too much cylinder wall taper.
I drove it fairly mild in town, but I really pounded it driving autocrosses and other interclub competition events. Over time, as gas got crappier, I had to run it on pumped gas (93 octane). I didn't push it anymore due to the bad gas, but I still had detonation (spark knock) problems. When I tore the engine down to rebuild it for my current restoration, I pulled the pistons and found all eight cylinders had broken compression rings from the detonation.
If it is out of the car and the pistons, heads and crank are all removed, I would take it to a machine shop, have them mike the cylinder walls for taper, check the crankshaft and if necessary turn or polish the crank, tank clean the engine, especially the cooling system and most of all, do a magnaflux on the block to see if it has any cracks.
1970 small blocks range anywhere from 300 hp to 370 hp, depending on the cam and the compression ratio. If it had a high compression set of pistons and had unleaded low octane (less than 98 octane, it probably had a lot of detonation going on.
Remember, unlike yours and mine, today's Corvette engines have 10.5 : 1 to 11 : 1 compression and can run pumped gas, but only because they have 1) spark knock detectors in the block that retard the timing if detonation is detected, 2) they have aluminum blocks and heads to run cooler, 3) they are fuel injected, 4) they have reverse cooling flow where the colder water goes to the heads first, not like the older engines where the colder water enter the block first and exits through the heads and last, 5) the engine is monitored and it's critical parameters are controlled by a computer.
Were ANY of the compression rings cracked or broken when you pulled the pistons. If they were, it was most likely due to detonation, which means the crank could have been beaten up a bit.
Re-building the engine with an existing flaw that you can discover right now is a major bummer and double expense. Remember, there is never enough time and money to do it right, but there is always enough time and money to do it over. Checking it out will not cost that much. Building an engine that has problems from the start will wind up costing you a fortune in comparison.
If you are replacing the cam, replace the cam bearings while it is out.
That's my 2 cents!
Last edited by rbryce1
on Nov 9th, 2014 9:02 pm, edited 8 times in total.