At what point do i overbore a 401 block

I took a straight stone hone in my drill for just a couple strokes to get the bores free of any cooked on gunk.  Next I used a piston ring that came from the motor and showed a light from the bottom to see how the "round" ring sealed against the block. Any light coming through the ring against the block showed that I should use a shim to measure how bad it was. That gave a decent indication if the walls had localized low spots.  

The next trick is to use the same ring and measure the gap at the top, middle and bottom to approximate the taper (circumference = pi*diameter) by measuring the ring gap you get a pretty decent resolution of how much the bore is tapering without having to spend a bunch of money on a bore gauge.

I realize that this is a bit of a backyard redneck solution, but you can then make a decent decision about the health of that part of your block. Assuming the taper is not excessive the rings will probably seal, especially at lower rpm.  At higher rpm, the rings may not be able to keep up with the irregularity of the bore (think like valve float) and there may be excessive blowby.

For what it's worth, the little 4.6 with all the stock junk and new stock replacement pistons and rings burns no oil and does not smoke.  After break in I had 200psi on all 6.  

That's just a small example of doing something on a different thing that may justify not cutting that sweet virgin 401.  

Yes! forgot to clarify that.  

Use a piston upside down to press the ring into the place in the bore evenly and straight. then measure the change in ring gap in all places and note the difference. A change of .003 on the end gap is roughly equivalent to .001 on the bode diameter because C=pi*D.  Therefore D= C/pi.

That formula translates to changes too.  Change in Diameter = change in circumference/pi.

change in diameter = .003 end gap change / 3.14

change in diameter  roughly equals .001 

This gives a nice resolution that's easy to approximate without buying a bore gauge.

merr6267,

That there is some perty fancy redneck calculatin.  Im'a guessing shadetree (me) is a shade below redneck.

But really, its amazing what one can learn here.  My problem now is knowledge retention.  Love this place.

I finally had a chance to get back to this looking at this.  Since I created this thread, I had a chance to pick up a dial bore gauge and when I get some extra time will check the bores with it and then cross check with the piston/ring/feeler gauge method as well.