I think you want to supercharge this engine, and with the readings as low as .140, you should probably not bore more than .040 over.  I also think you want to bore bigger than that, so it's probably not a good idea.  With the blower, I would strive for .120 wall thickness after boring.  As stated above, you could sleeve the thin ones, but 292 blocks are not too hard to find.  They didn't all have core shift.

John

Chuck.  Granted that the block has what appears to be significant core shift on the drivers side bank but it still looks to be able to go as much as 0.080” oversize even if not offset boring the cylinders.  But depending upon how much oversize you’re considering and the accuracy of the sonic test, then the block in question can potentially be bored even more.  If considering a reasonably heavy overbore, then I’d recommend having the machine shop offset bore into the heavy sides in order to equalize cylinder wall thickness at which rate you can consider overboring 0.110” or more.  Double check the sonic check sheet to insure that the #1 cylinder was numbered for the Ford and not for the scrub lineup as being reversed could prove to be disastrous if pursuing offset boring.

I do have a question regarding the wording on your sonic check sheet though.  It lists ‘thrust’ for each bank but it’s not clear (to me anyhow) which side of the cylinder is really being referred to as the opposite side of the cylinders in both cases are listed as ‘up’.  It would make more sense if one bank was listed as ‘down’.  But if ‘thrust’ just refers to the side of the cylinders closest to the outside of the block, then that would explain it.  What’s driving this thought process is that when looking at a block from its top, the major thrust within the cylinders on the drivers side would be the side of the cylinders closest to the lifter valley while the major thrust on the passenger side would be the side of the cylinders closest to the outside of the block.  If offset boring a block, it’s imperative that the sonic check sheet be absolutely clear as to the orientation of the numbers within the cylinder.

               Sonic tests tend to average the area under the probe. there can be more or less in any particular spot. If these are the minimums you are likely OK for Normally Aspirated. If it's for only one spot on each section, you don't know much more than you did. Listen to John on the blown recomendations. Too little wall will get you wet pistons.

                                                  Frank   

I was thinking towards the blower motor to be done right once! The machine shop seemed to think that offsett boring would be too expensive because of the many passes required. I do have another block to tear down plus the one in the Vicky. One has to be good.

PS: I put a closed oil fill breather on and attached it to the air cleaner (without the blower). Result was the dip stick was getting pushed out by oil pressure. I put the crankcase breather back on and found some improvement at the top BUT it now pushes oil out the breather as I drive. Think I might have some blowby issues? (and thats with a PCV valve!)

As you know, I bore 292 blocks to .030 over 312, put in 312 cranks, and supercharge them.  Never sonic checked them, never a wall failure.  Guess I've just been lucky.  Never had a Canadian block either.

John

            You are correct about the amount from each wall being half the total bore increase. You really want to keep a .125 wall on the thrust side. With the blower, depending on the amount of boost, at leasr .100" wall is needed. Remeber, especially with the blower, you'll lose more to blowby than you'll gain from the displacement increase. As the walls get thinner they start to flex more. More cylnder pressure equals more flex equals more blowby. Just like other sports, stiff is GOOD!!

I can bend a flat piece of cast iron .100 thick with my bare hands, not much, but a measureable amount. the cylindrical shape helps, but the pressure in a cylinder in a running engine is a lot more than I can put out. I've seen spikes at over a 1000 psi.

You are correct, I'm boring 292s .080 over.  With the stock stroke 312 crank, I get 317 cubic inches.

John

         I would doubt that adding cubic inches would make horsepower as fast as adding boost. Here's the deal, lets assume that your 312 is 100% efficient at max hp. It would make 312 ft lbs at that point. (theoretical, won't happen unless you're Wally or Ted Eaton). If you could add a blower that blew 7.35 lbs and consumed zero hp to do it, you would now have an engine that acted like a 312 x 1.5 or 468 cubic inches. Kick that to 9 lbs and it's now a 502. This all assumes the heads can carry that much air, the blower doesn't take any horsepower away and the heat of compression in the blower doesn't thin the air out (it does). But you get the point, you can't make a 312 that big with boring and stroking. Best to make the walls stiff, the rings seal, and the heads flow all they can. Then run a cam with limited overlap so the blower stuffs the cylinder, and does not blow half of the charge out the exhaust pipes. Retard the timing, run 106 octane, and keep the carb a little rich, Make sure you have fuel pressure and Moon Shadow will destroy tires and the '56 rear end.  

These are all the minimum values when sweeping the sonic check probe the length of the cylinder wall.

After sonic checking a variety of Y block castings, there is no set rule for one block casting number being better than another.  I've now seen 272's that would bore out to 312 plus while others are iffy at 0.060" over.  The C2 blocks seem to be the most predictable but core shift is still an issue thus forcing offset boring if going for the larger bore sizes.  Sonic checking is a prerequisite if wanting to be sure of what you have.

        Different areas of strength. A normally aspirated motor won't see the cylinder pressures that a blown motor can generate. BUT it will usually need more RPM to achieve the hp numbers, So the bottom ends and reciprocating parts will take beating. Remember from you high school physics that the amount of force goes up with the square of the velocity. Rods, pistons, and the bottom of the block (main bearing webs) as well as the crank must be much stronger and lighter reciprocating pieces are a major benefit. Blown motors, where the mixture is forced into the cylinders don't have to spin as hard to see big HP numbers. But boy do they make torque, On ther dyno at ESI my F code made over 425 ft lbs at 3200 or so rpm. The torque curve starts to drop after 4000 /4500 because with stock valves I run out of cylinder head at that point. I've made changes to the motor since the dyno runs that have extended the range, but it's still a low rpm motor. But, the walls on my motor are fairly thick and stiff, because we wanted to be sure we didn't lose ring seal and pressurize the crankcase, no horsepower there.

     Ted or Wally or Dave on the other hand, don't make the torque numbers I do, But they all bury me on the Horsepower end from 4 grand up. Make sense?