How can you tell a 3400 block from a 3100 block?

[sd_iconoclast]

How can you tell a 3400 block from a 3100 block without measuring the bores?
In the junkyard, I frequently come across 3100/3400 engines that do not have the upper plenum attached, but still have the haeds on them. I thought that all 3400 blocks had a big "B" cast into the side. I found out that I was wrong when I brought one home and found it was a 3100.
How can I recognize a 3400 block without the heads off?

[SappySE107]

3100 blocks have the B, 3400 blocks have the A. Where does it say the other way around? It may be possible that it doesn't mean anything but I have 98 3100 and 3400 blocks here and thats how they are.

[Guest]

3400s have a letter A cast into the block. ALL of them.

[sd_iconoclast]

3100 blocks have the B, 3400 blocks have the A. Where does it say the other way around? It may be possible that it doesn't mean anything but I have 98 3100 and 3400 blocks here and thats how they are.

I never read it anywhere, I thought that I had just observed that 3400s had the "B" and 3100s had the "A". It is very possible that I just had a senior moment and got it mixed up.

[sd_iconoclast]

3100 blocks have the B, 3400 blocks have the A. Where does it say the other way around? It may be possible that it doesn't mean anything but I have 98 3100 and 3400 blocks here and thats how they are.

Hmmmmm. I just went in the garage and looked at my blocks.
I have two 3100 blocks. One has the "B" cast into it. This is the one that I mistakenly thought was a 3400 block.
The other has neither an "A" or a "B" cast into it. It came out of a '98 Olds avhieva.
My 3400 block is still at the machine shop. It turned out to be cracked.
Ben, would you like to sell me one of your 3400 blocks?

[bszopi]

I don't think the 3100 started getting the "B" until 2000, but the 3400s have always had the "A".

[sd_iconoclast]

I payed attention on my last trip to the junkyard. I saw another 3100 with no letter cast into it. I also saw a 3100 with a "G" cast into it.

[Doubt_Incarnate]

learn the vin code, that way you can peek and run

[Guest]

3400s have a letter A cast into the block. ALL of them.

Listen to this guy, he knows what he's talking about.

[bszopi]

Just to add to the letters, the 3500s have a "C".