Yet another overbore question

[Superdave]

Just use a 3100 block.. and keep it a 3100. besides, the '00+ 3100's had almost the same power as the 3400's. If you want more power very cheaply just swap in a set of iron head 3.1 pistons.. the raise in compression will offset the loss in bore size.

[RacerX11]

You could become the guinea pig for us. Then when it fails, we can all just point and say "I told you so..."

Or you could bore a 3.1 to a 3.4, run the crap out of it for several years, and not have a failure. Just a guess, anyway. Boost will be the ultimate test.

Marty

[SappySE107]

Ive heard of people going .120 over, saying it works great for their NA build. I would run the 3100 as a 3100. The only advantage the 3400 has aside from a small amount of displacement, is the ease of using DOHC pistons in it to raise the compression. Otherwise its not really going to make a hell of a difference between either block that you use.

You can use sonic testing to get cylinder wall thickness, as that is what it is mostly used for from what I can tell. This is how you determine core shift and the real thickness of your walls. From there you can decide what bore you wanna go with.

[EspanolaGuitarista]

I remember reading this so I looked it up, hope it helps.

Regarding cylinder wall thickness:

http://www.popularhotrodding.com/enginemasters/articles/chevrolet/smallblock/0404phr_sonic/index.html

[EspanolaGuitarista]

So a little figuring...

Max wall thickness 4.5 mm
min thickness = 3.556 mm
________________________
Safe bore without 1.044 mm
block filler

So given the 4.5 mm cylinder thickness a bore of .060 or 1.5 mm is way over.

[EspanolaGuitarista]

If we went with 8.9 mm....

Wall thickness 8.9mm
Minimum thickness 3.556mm
Safe bore 5.344 mm or .200 overbore?

The numbers don't add up here.

5.588 mm = 2.032 safe minimum or .080 max overbore. Which, measured halfway down, is most likely a real world number. .100 could be possible with block filler support.

[sd_iconoclast]

Re: If we went with 8.9 mm....

Wall thickness 8.9mm
Minimum thickness 3.556mm
Safe bore 5.344 mm or .200 overbore?

The numbers don't add up here.

5.588 mm = 2.032 safe minimum or .080 max overbore. Which, measured halfway down, is most likely a real world number. .100 could be possible with block filler support.

Finally, some real numbers. Thanks for the article, and the calculations.

I notice that you are taking your minimum wall thickness as the spec mentioned in the article for SBCs. But we should also note that the article seems to indicate that SBCs have a high minimum because of their rather long unsupported cylinder length.

I asked my machinest what the minimum wall thickness is and he said that the rule of thumb is .090". I am under the impression that his experience comed from 302 fords, so this seems to corroborate the article's assertion that shorter unsupported lengths can get away with thinner walls.

Today, I also measured the diameter of the cylinder casting (measured at the deck, edge of water jacket to edge of water jacket) on my 3.4 TDC block. I foound that measurement to be only .010" greater than that on my 3100 block. I admit that this dimension would be more meaningful if it were done halfway down the bore. Based on this, I think we should start questioning the idea that using 3.4 or 3400 blocks result in thicker cylinder walls than boring a 3100 block .120 over. It appears that the diameter of the cylinder barrel on these blocks is essentially the same.
Now it is very possible that I am notdoing the greatest job of measuring. I really do not have the best caliper for the job. What do you guys come up with when you take the same measurement?

[prodigy_16]

the problem with over boring is that the thinner the walls the more prone they are to distortion or deflection under high rpms or heavy load. and since the 3x00 engines make their power higher in the rpm range to begin with it stands to reason you would want as much cylinder wall thickness as possible. not to mention anything like extra cam, larger intake ect. will push the power band higher in the rpm range making any problems that might occor worse.

find a 3400 if thats what you want. you might have to spend a few extra bucks but thats the best way to get the displacement you want. take your time and do your project right, and you will have less problems and be happier with the results later. remember shortcuts lead to short engine life and poor performance.

[RacerX11]

Re: If we went with 8.9 mm....

Today, I also measured the diameter of the cylinder casting (measured at the deck, edge of water jacket to edge of water jacket) on my 3.4 TDC block. I foound that measurement to be only .010" greater than that on my 3100 block. I admit that this dimension would be more meaningful if it were done halfway down the bore. Based on this, I think we should start questioning the idea that using 3.4 or 3400 blocks result in thicker cylinder walls than boring a 3100 block .120 over. It appears that the diameter of the cylinder barrel on these blocks is essentially the same.
Now it is very possible that I am notdoing the greatest job of measuring. I really do not have the best caliper for the job. What do you guys come up with when you take the same measurement?

Go take a closer look at the block, and you will notice the top 1/2" or so of the cylinder bore is slightly thicker than the rest. Any measurement taken at the deck surface does not indicate the true wall thickness.

[Guest]

Re: If we went with 8.9 mm....

Today, I also measured the diameter of the cylinder casting (measured at the deck, edge of water jacket to edge of water jacket) on my 3.4 TDC block. I foound that measurement to be only .010" greater than that on my 3100 block. I admit that this dimension would be more meaningful if it were done halfway down the bore. Based on this, I think we should start questioning the idea that using 3.4 or 3400 blocks result in thicker cylinder walls than boring a 3100 block .120 over. It appears that the diameter of the cylinder barrel on these blocks is essentially the same.
Now it is very possible that I am notdoing the greatest job of measuring. I really do not have the best caliper for the job. What do you guys come up with when you take the same measurement?

Go take a closer look at the block, and you will notice the top 1/2" or so of the cylinder bore is slightly thicker than the rest. Any measurement taken at the deck surface does not indicate the true wall thickness.

QFT! not even close.

[sd_iconoclast]

Guys,
I said in my posts that the measurement at the deck is not meaningful.
I was just trying to point out that it seems a bit suspicious that the measurements for the 3400 and 3100 do not differ by .060" (half of the difference in their bores).
I keep hearing people say that I should just get a 3400 block, but everyone is simply assuming this will result in thicker cylinder walls. To date, it appears that I am the only one presenting any actual measurements to prove or disprove that assumption. That being said, here are my latest measurements:

3100 Wall thickness : .210"
3400 Wall thickness : .190"
3.4 TDC Wall thickness : .155"

I took my measurements at the rear of the #6 cylinder and on the inner (cam side) side of the #4 cylinder. I chose these locations because my caliper would best fit into these locations. The cylinders in all blocks are definitely thicker near the deck. I made these measurements about 1.5" below the deck. Visually, this appears to be below the thicker areas.
Admittedly, I do not really have a good caliper for this job, but I did find my results to be pretty repeatable. I consider my measurements to be accurate to within about .005". I am hoping that one of you guys can try the same thing and either confirm or refute what I have found.

If we can believe me measurements, then we can conclude the following:

1] Using a 3400 block does result in the thickest cylinder walls, but not .060" thicker as we might assume.

2] 3x00 blocks have substantially thicker cylinder walls than the 3.4 TDC.

3] Boring a 3100 block .120" will result in thinner cylinder walls than using a 3400 block, but the thickness should be well within th safe range as it would be the same as a .010" over TDC.

I think I am ready to take my 3100 block to the machinist, have him bore it .120 over and sonic test it. I will let you know what I find.

[prodigy_16]

did you chesk all the cylindrs? it is possible that other cylinder walls are thinner than the opne you measured.

[blackbombshell95]

I'm getting the sonic test results of my 3400 here shortly. I'll post the results

[blackbombshell95]

Ok. I got the sonic test results via the phone. They guy said that he wants go to 30 over on the 3400 block. He said that 30 over leaves plenty of room to be safe. He said we could of gone over more if we wanted to. But he said the walls were a little thin in certain areas and once again that a 30 over is very safe.

[sd_iconoclast]

did you chesk all the cylindrs? it is possible that other cylinder walls are thinner than the opne you measured.

I did not check all the cylinders. Like I said, I am using a kind of funky method to measure the thickness and I can only do it in certain areas where the holes in the deck are large. To the naked eye, the cylinders do not appear off center.
I keep meaning to order a more appropriate measuring device, and then I forget.
I will do it tonight.