Guys, teach me about 340 blocks

Hello everybody. I'm buying a 340 block in Brazil but to be honest I don't know to much about this blocks. What kind of details I need to pay attention?

I'm planning to build a Pump Gas N/A engine, 12.0CR, solid flat tapped cam, Victor Heads, Tunnel Ram, Steel main caps... Can I drill to use 4 bolt main caps or this block like 318 no has enough material to add more 2 splayed bolts?

How is safe to bore for a N/A Street engine?

Here is a picture of the cast number.

Thanks

I am told that drilling for four bolt mains will actually weaken the block.
I have a set of Milodon caps and am considering just line boring and using the two bolt configuration on my next deal.
Casting number says 71-73 340 block.
I would have the block sonic tested before I decided how large to bore it.......especially if you want to really open it up.
I wouldn't sweat .030 over , but if you want to open it up , better to know how much meat you have there.
Stroking is the best way to add inches to my way of thinking.

12.0 CR is pump gas? Lots of alcohol in the gas down there? Me personally, I'd be building a 360 - probably not as rare or $$, cheaper, bigger bore, lot's of stroker kits available, won't have to build it as radical as the 340. But..most should go .060 but get it sonic checked. 12.0 will need head studs and really good gaskets - 4 bolts per cylinder can lead to sealing issues. Years ago guys o-ringed those blocks, nowadays Cometic might make a head gasket to handle it. May need oiling mods too, if you are looking to turn some RPM. The MP/DC Engine manuals are your friend.

I have a 340 block that has 4 bolt mains but understand that the outer
main bolt are 3/8" and they go straight down.. been a great block for
years now.. I bought it from a dirt roundy round racer.. all my blocks
are 340 based.. even my 2 bolt blocks I turn to 8000+ rpms.. sonic
check ALL blocks

Is your pump fuel straight Ethanol or Methanol now?
As far as main caps I wouldn't, and don't', use steel caps on any cast iron block, ductile iron or maybe aluminum but not steel
As far as bore size I would bore it just big enough to get the bores straight now and leave some meat for later development
I have built several 340 NHRA legal stocker 340 motors that made 430 HP at 5900 RPM with stock 1971 heads, intake, carb and NHRA legal hydraulic camshafts and lifters That was a long time ago, early 1990s, much better parts available now
I built one pump gas 340 4.0 inch long stroker motor with a set of race mildly ported race W2 heads that made 514 HP with one 850 CFM carb. on a cast single plane W2 intake using CA 91 octane pump swill
Stroke that rascal and put some good heads on it and let it rock

Looks like you are going to make some power. I would pass on a stock block and pay the money for a R3 or a Ritter now, but thats me.

Great advice.
If you want to make serious steam , you will never regret good parts in the basement.

He will have a really hard time finding a R3 block (=extinct) and having it shipped to Brazil.
And the Ritter block will cost him 6 - 7K before he can bolt his parts on.... the 340 block he already has.

Thanks for all advice Guys. R3 or Ritter is not an option for me unfortunately. Price + shipping + taxes to ship to Brazil is unbelievable expensive. Best pump gas in Brazil has 96 oct and 30% of ethanol. Currently I`m running with 12.0CR on this Pump Gas, Cometic head gasket and ARP bolts. No Issues so far. This engine has a 318 block, 3.58 Scat Series 9000 and overbored to 4.000". After all machine works I did a sonic check at the cylinders bore and the thin wall measure that I found was 0.148".

I'm looking forward to use Victor Heads but this heads has 2.15 intake valves and I believe that 4.000" bore is not enough for a good valve shroud. This is the main reason that I'm thinking change for a 340 block, maybe use 4.080" or 4.100"

If I'm correct, isn't that block by virtue of the casting numbers, a 6ix pack 340?

I'd also agree that a 360 is the cheaper more viable way to go and make gobs of torque without the strain.

I would stick with the mains as is. I don't like to drill into stock webs. not much there to begin with. the only reason to change caps would be to keep the cap material from splitting. I have never seen a mopar cap split....just web cracks.

That's not a T/A block.

Nope.

T/A blocks have their own casting number & the letters TA actually cast in as well.

One of my blocks is a TA block.. its the for runner to the X block..
thicker main webbing
EDIT
that 6 at the end of the number is just the casting box number.. that
just another check to see if that casting box is having any issues

Save the big bores for the X, R, & Ritter blocks. OE 340 blocks are thin. .030 is considered safe. Any bigger should be checked. Personally, I wouldn't go any larger than needed, unless there was a real reason. They aren't making anymore and I don't want to use it up with one overbore.

2 bolt mains and stock caps aren't the weak point. 550 to 600 is about the safe limit. Early 360s can go to 340 size bores.

I agree with everybody, I ran 340s for years hard to find a good block that sonics well, switched to a 360 ( Magnum ) block awhile back I like it better more torque and top end. Now running an R3 block with a 4 inch stroke, Indy 360-1 heads

. In my opinion there's no need to butcher the block the head will make plenty of power

Thank you guys! Block is over 0.30 and was sonic tested and magnafluxed. No issues and are ready to build a 4.070".

Now I need to decide what stroke I should use. 3.790 x 4.000 or 4.125?

I have a couple of 3.79 and a 4.00 strokes.. I think you would be happier
with a 4.00 stroke.. plenty of them around and if you wanted a cast crank
they are pretty cheap.. all my stuff is billet or forged.. the 3.79 are billet
ones

If I were doing it, stroke would be determined by whether or not I was going to use stock or custom length rods and custom pistons. I would want to take advantage of the LA deck height to keep the rod ratio at least close to stock.

I was thinking to keep everything simple and buy a Scat 418 Forged crank, H-Bean rods and forged flat top Icon pistons to give around 11.5 / 12.0 of CR. My guess is that I can make around 650 - 680 HP on this engine since Victor Heads has a very good flow numbers.

You had it sonic checked.. what was the wall thickness after it was
machined.. I had a wall crack even at .030 over and they sonic checked
good.. it just had 1 thin area on #6cyl and it was just a tiny spot

Yes, was sonic checked and the thinner spot that we found is .158" on rear of cylinder 5 .