Are All 340 Blocks the Same?

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Franko

Franko

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I am looking into buying a 340 short block as a backup to my 71 340. The guy is going to get back to me on the numbers on the block, but he thought it was a 72 or 73. I believe the compression for the 71 is 10 1/4 and it dropped to 8 1/2 in 72. He didn't think it had the stock pistions, anyway. If I ever had to would all my parts fit right in or bolt on the 72 or 73? Would the crank and rods be the same and the 71 heads bolt right on?

Thanks.
 
73 went to cast crank IIRC
your parts will bolt on with proper blueprinting.
 
Franko said:
I am looking into buying a 340 short block as a backup to my 71 340. The guy is going to get back to me on the numbers on the block, but he thought it was a 72 or 73. I believe the compression for the 71 is 10 1/4 and it dropped to 8 1/2 in 72. He didn't think it had the stock pistions, anyway. If I ever had to would all my parts fit right in or bolt on the 72 or 73? Would the crank and rods be the same and the 71 heads bolt right on?

Thanks.
Click to expand...

I had to read your post 3 times. Please hit enter at the end of a sentance.
Any ways a 71 is internally balanced.
Some 72 are external balanced.
All 73 are, which are easy to see it says cast crank on the balancer.
 
I believe that all the blocks are the same, and that all of your reciprocating parts will bolt right into any 340 block. There may be a difference in piston configuration between the early pistons and later low compression models, but the standard bore is the same between all years. Geof
 
As stated most 72 and all 73 340 had cast crankshafts. 68-71 were steel cranks. One of our vendors MRL performance from his experience, has stated that the later blocks were better than the early blocks as far as core shift. To answer the rest of the question everything is bolt on same save maybe the 72-3 crank, balancer and flywheel/converter since they are externally balanced.
 
Rocky_JS said:
One of our vendors MRL performance from his experience, has stated that the later blocks were better than the early blocks as far as core shift.
Click to expand...

That only holds true for later blocks like Magnum blocks. All 340 blocks are junk as far as Im concerned. I have had to sleeve so many of these blocks its not even funny, many with stock bores. the core shift in 340 blocks is the worst. A lot of 360 are pretty bad too.
 
MRL Performance said:
That only holds true for later blocks like Magnum blocks. All 340 blocks are junk as far as Im concerned. I have had to sleeve so many of these blocks its not even funny, many with stock bores. the core shift in 340 blocks is the worst. A lot of 360 are pretty bad too.
Click to expand...
I've found the same thing. All of the 340s I've had were crap as far as cylinder wall thickness/core shift were concerned.
 
Mike, thanks for clearing that up.

John


MRL Performance said:
That only holds true for later blocks like Magnum blocks. All 340 blocks are junk as far as Im concerned. I have had to sleeve so many of these blocks its not even funny, many with stock bores. the core shift in 340 blocks is the worst. A lot of 360 are pretty bad too.
Click to expand...
 
I have 2 340's , a 68 and a 70 , and both are pretty bad as far as the shift . The 68 wall thickness is terrible . The 70 is an AAR block , but you can see it has problems .
 
Enlighten me please - what is core shift ?
 
What is the maximum a 340 block can be safely bored? Approximately how expensive is getting a block sleeved?
 
I would rather yield to someone who is more in the know.

To find out how far you can bore you can do a sonic check of the cyls. A sonic check is where the cyl is checked - bottom, middle, and top - also left and right. At that point you can safely know your min/max thickness at any given point.
 
Core shift happened when the block was cast at the factory. It refers to a variance in the thickness of the cylinder wall. If the casting core shifted at the foundry, it results in cylinder walls that are thinner or thicker. Only way to tell is have the block sonic checked before boring it.
 
Like 66 said - any iron casting is made by pouring molten iron into a mold. The mold is full of cores. Cores are basically where the iron will not be - so the cylinders each have a core, the crankcase has a core, etc. If at some point in the casting process one or more cores move before or during the molten iron pour, the resulting part will "exhibit core shift". In blocks you see it when it's sonic tested as thinner cylinder walls and an odd seam where the top and bottom cores meet right under the distributor in the valley area, or a larger "ear" of iron on one side of a lifter bore (so the hole looks like it was bores off center). On heads it's port alingment...
Core shift in blocks is what makes all the stories about "thick" and "cold weather" mopar blocks the stuff of legend. Mopars have a lot of core shift - especailly the 340s and I'll expand on Mike's and say the 72/73 340s seem to be the least shifted. At least the ones I've tested.
 
Just thought I get back and say that I passed on the 340 block because it was already bored .060. Thanks for all the advice. I learned a lot about core shift.
 
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