WTB a 1970 340 Block Bored to 4.125"

[cudaracer]

I found a 340 block that was bored rough to 4.125" and showed a leak at hot test in 2 spots near drain core plug, revealed through magnafluxing I believe.

Looking for opinions on if it's worth a few hundred $$ to purchase and fix. It was sonic tested prior to semi rough boring. Needs final finish, so does that mean I would take it to 4.130" or so? I imagine piston selection is barely there at that size.

Opinions and thoughts welcome.

 

[dano]

Honestly I'd pass. It needs repair and pistons will be custom. A 360 block would be a better choice if budget was a consideration.

 

[Captainkirk]

I would pass. Plenty of good 340 blocks out there and even more 360 blocks.

 

[cudaracer]

I have a 318 short block currently. I am struggling with the fact that everything swaps right out. A 360 block will need a 360 short, engine mounts, oil pan, flex plate, etc.

 

[madscientist]

I have a 318 short block currently. I am struggling with the fact that everything swaps right out. A 360 block will need a 360 short, engine mounts, oil pan, flex plate, etc.

If you use the 340 you still need to change the mounts. The 273/318 drivers side mounts are the same and the 340/360 drivers side mounts are the same.

4.125 is WAY too far over. Unless it is an X block. I have had 3 and sonic tested 2 of them. The 2 I tested would easily go 4.125 in spite of the MP books saying it wouldn't.

Unless you can verify exactly what it is...PASS on it.

 

[cudaracer]

Awesome.
What is the cast markings for an X block?

 

[dano]

My 1972 block is at 4.1 and will go more, sonic checked. So some stock blocks can take a larger bore. But you would need to verify. To me, it would be cheaper to find a 360 short block, preferable a roller cam one.

 

[madscientist]

Awesome.
What is the cast markings for an X block?

There is a big "X" cast into the block on the front of the #2 cylinder. There is also an "X" on the back of the block but I don't remeber where it is at the moment and my only "X" block is at the machine shop.

It also MUST say 318 in the casting number (damn near got into a fist fight with a liar at a swap meet trying to sell a 360 block as an X block by saying he ground and polished the X off...the dead give away he was a liar was the fact it had a 360 casting number and 360 drivers side motor mount) and have the 318 drivers side motor mount.

I would never take a 340 past 4.100 unless it was sonic checked...and then I would use some block filler. The Chryslers have RELATIVELY long cylinders and even though there may be enough wall thickness (as per chevy rules of thumb) I would want more support as the piston goes around BDC. A stroker crank would magnify the issue.