Yet another overbore question

[sd_iconoclast]

I have been reading past posts trying to figure out what the maximum overbore potential of a 3100 block. It seems to be widely believd that anything over .060 is dangerous, but I cannot find the source for this belief.
Does anyone know where this idea came from?
I have also read that starting cylinder wall thickness (on the 3400 I think) is 4.5 mm (.177"). I measure the wall thickness on my 3100 block to be 8.89mm (.350") at the top of the cylinder (probably does not count) and about (It was hard to measure) 5.588 mm (.220") halfway down the bore.
If we can believe that we can safely do a .060" overbore on a 3400, doesn't this mean we can do a .146" (((.220-.177)*2+.060))overbore on a 3100?
If this is not true, which of these numbers is incorrect? Or, stated another way, what is the minimum wall thickness?

[geoffinbc]

Yes the walls become too thin to be overbored much beyond .060. Thing like core shift and casting flaws make it even worse. So don't push your luck. Your not going to gain much by going an extra .060 anyway. If you want to go bigger be safe and start with a 3400 block.

[Guest]

Yes the walls become too thin to be overbored much beyond .060. Thing like core shift and casting flaws make it even worse. So don't push your luck. Your not going to gain much by going an extra .060 anyway. If you want to go bigger be safe and start with a 3400 block.

QFT!

The info comes from GM, a long time ago, and has been followed for many years, even with the changines in castings through the years. People, machine shops, etc have checked many different ways and wnything beyond .060" is a potential for a split cylinder wall.

Again, not much will be gained if anything by going to a larger bore, except possible unshrouding of the valves, but I woudl start with a 3400 block if that's the concern.

[sd_iconoclast]

Thanks for the reply's guys, but those reply's do not answer any of the questions I am asking.

The information that I have heard concerning maximum overbore appears to be contradictory. If we can believe the .060 overbore maximum for any block, than the 4.5mm starting cylinder wall thickness must be incorrect.
If the 4.5mm starting cylinder wall thickness is correct, then the .060 maximum overbore must be incorrect. The fact is, I cannot verify any of this info that I have heard "word of mouth" and I am looking for the source.

Does anyone have a GM manual or something similar that states any of the following:
1] .060 maximum overbore for a 3100
2] .060 maximum overbore for a 3400
3] From the factory, 3400 cylinder walls are just as thick as 3100 cylinder walls.
4] What the minimum safe cylinder wall thickness is for a 3100 or 3400.

[Guest]

If you're not going to accept the fact that .060" over bore is the safe allowable max overbore for any of the 660s, stated by GM in thier power build books, IIRC, also in any engine rebuild manual that specifically lists the 660, then I don't know what else to tell you.

What is it you're trying to accomplish?

Any engine builder will tell you to only bore enough to clean up the cylinder walls, anything beyond that is just a waste and will reduce any posibility of future rebuilds.

[geoffinbc]

The nominal thickness for a 2.8L or 3.1L is 4.5MM. We could asume a 3400 is aswell. But the 3400 is a different casting to allow for an acceptable wall thickness with the factory bore.

[tejohnson]

the 3400 wall "looks" a lot thicker than 4.5mm. Taking .030 out of it, if it is really only that thick, would seem insane...

Not to mention, the resulting rise in the CR. Unless that is the intent...

[sd_iconoclast]

If you're not going to accept the fact that .060" over bore is the safe allowable max overbore for any of the 660s, stated by GM in thier power build books, IIRC, also in any engine rebuild manual that specifically lists the 660, then I don't know what else to tell you.

What is it you're trying to accomplish?

I am just looking for the source of that information.
How do I find one of these books that you are speaking of?
What do you mean by "GM power build books"? I have the "Chevrolet Power" book and it does not address cylinder wall thickness in the 3x00 blocks. It was published before the 3x00 came out.
However, I have just noticed that it DOES say "The nominal cylinder wall thickness for production V6/60 blocks is 4.5mm". Let me point out though, that this cannot be referring to the 3x00 blocks because (once again) the Chevrolet Power book was published before those engines came out.
I have personally measured the cylinder walls of my 3100 block to be substantially thicker than 4.5mm. This suggests that we may all have been proceeding on information from earlier blocks, that doesnot apply to the 3x00.
I do not know this for sure, I am just trying to find out by locating the sources of our beliefs.

[sd_iconoclast]

The nominal thickness for a 2.8L or 3.1L is 4.5MM. We could asume a 3400 is aswell.

That assumption is exactly what I am trying to verify or disprove.

I do not have an earlier block to look at to see if its cylinder walls are the same thickness as the 3x00 blocks. If somebody does have one, that informatin could be useful.

[bszopi]

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

[Guest]

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

QFT!

You still have not answered why you want to bore the block more than .060" anyway.

Yes same books, but as I have said, in all cases that I have been involved with, the same thing applied to even the genIII blocks, maybe you have a casting that has thicker walls then the ones I had had my hands on as well as other engine builders.

That and I only bore as much as I need to to clean up the bores, since there really is no benifit to going anymore than that.

[sd_iconoclast]

You still have not answered why you want to bore the block more than .060" anyway.

Oh yeah, I forgot to answer that one.
My most immediate reason would be to allow boring a 3100 to 3400 dimensions. 3400 blocks have been pretty hard to come by here in San Diego. I can only find two in the junkyards locally. In both cases, the junkyards will only sell them as complete engines and this does not suit my purposes. On the other hand, you can't swing a dead cat without hitting a 3100.
Raven, it sounds like you are a machinist. How do you decide generally decide what the maximum overbore is for ANY engine?
Is there some accepted minimum wall thickness that applies to all engines?
Is there a book that lists it for each engine?
How do you know?

[donk_316]

buy the block you want and sonic test it. that will tell you.

[Guest]

You still have not answered why you want to bore the block more than .060" anyway.

Oh yeah, I forgot to answer that one.
My most immediate reason would be to allow boring a 3100 to 3400 dimensions. 3400 blocks have been pretty hard to come by here in San Diego. I can only find two in the junkyards locally. In both cases, the junkyards will only sell them as complete engines and this does not suit my purposes. On the other hand, you can't swing a dead cat without hitting a 3100.
Raven, it sounds like you are a machinist. How do you decide generally decide what the maximum overbore is for ANY engine?
Is there some accepted minimum wall thickness that applies to all engines?
Is there a book that lists it for each engine?
How do you know?

I see what you are trying to do, and don't. There is a reason GM made a whole new block for the 3400.

It's odd that you can't find any or rather many 3400s they're pretty easy to come by up here. In fact some people have resorted to giving 3400 blocks away because everybody wants complete 3400s.

No I am not a machist, but my grandfather is, and I have picked up a few things from him.
I also listen to my engine machine shop, seeing as they've been in the buisiness for about 40 years, maybe longer.

[sd_iconoclast]

buy the block you want and sonic test it. that will tell you.

No, that will tellme how thick the walls of the block are.
It will not tell me the minimum wall thicknes, which is what I am seeking.