So... What did I buy?

[crackedback]

Might take some doing to get the crank balanced using the 340 stuff. Get some really light pistons if using those chunky rods. I bet an aftermarket set of rods and decent pistons would get the bob weight pretty close to the OEM 318 bobweight.

Stock 340 rods are about 760 grams IIRC. Don't recall what 318 rods weigh. Aftermarkets like Scat I beams are in the 600-620 area IIRC. Other may be even lighter.

 

[NC Engine Builder]

Here is a old school TRW forged piston, pin, and factory 340 rod compared to a icon forged piston, pin, and scat rod. This difference is staggering

 

[Oldmanmopar]

1967 - 1972 273 and 318 had a narrower connecting rod beam width and lighter then 340 , 360 rods. 1973 and later all had the same beam width rods.

But no forged cranks after 72. The forged crank you pictured is a pre 1972, 273. 318 That is why the journals are not drilled. Because it was from a narrow beam connecting rod engine.

 

[str12-340]

But they should have a round medallion (IIRC) glued / attached to one side of the block

Are you talking about a rebuilder's medallion? The only time I have seen something like that is a medallion that rebuilders use. Whatever they are stuck on with comes undone at a specific temperature so they know if you managed to overheat the block - no medallion, no warranty.

 

[Oldmanmopar]

Smaller rod's? I've always read that all the small blocks have the same length rod and take the same rod bearing. Please explain this

Narrower beams which made them lighter. All the rods were the same in length and the big and small ends were machined the same. So they would all fit. Just a weight and strength difference. Forged cranks without the journals drilled were for the narrow beam rods.

 

[Swinger 340]

It has 340-6 on the block after the casting number

If you think you have a 6-pac block, it is not. It would have a "TA" stamped. I'm confused with the front 318 stamp, instead of a 340.

 

[Oldmanmopar]

Looks like forged crank. Where's the weird block #'s? In 5th pic I see "JM 340P". Nothing on vin rail?
340-6 stamping is normal. The numbers following the dash usually range from 1-8. I've never seen a dbl digit there.

Most all blocks higher then 340-8 were garbage but there were some that passed QC and made it out the door. I have seen as high as 13 but over 10 is very rare. Race blocks went higher due to high nickle content making the end of the ladle more stable and hot.

The block the OP has pictured is not a T/A block the main webbing is to thin at the rails . Here is what the webbing of a T/A looks like which accepts 4 bolt mains Look at the width at the rail compared

 

[demonjim]

Looks like the"340-3 " is there above where the starter would be and the crank is forged...id run it for 500 bucks ya did ok.

I missed that when I first looked at the pictures, but when I read your post, I went back and looked again, turned my computer sideways, and sure enough the last numbers are 340-3. That was a steal.

 

[Dana67Dart]

Are you talking about a rebuilder's medallion? The only time I have seen something like that is a medallion that rebuilders use. Whatever they are stuck on with comes undone at a specific temperature so they know if you managed to overheat the block - no medallion, no warranty

 

[Hellrats]

If you think you have a 6-pac block, it is not. It would have a "TA" stamped. I'm confused with the front 318 stamp, instead of a 340.

I know it's not a 6-pack block. It does not have 318 on it has 340 after the casting number. What I thought was strange is there is no VIN anywhere on it. Plus the numbers on the passenger side below where the cylinder heads goes. They are in picture 3 and are all a mixed up mess. I was just wondering what it was. I have never seen those goofy numbers and one with no VIN. IDK, it was a bare block and the cylinders were standard bore when I got it. It mag and sonic checked good and it's .040 over bore now. It's getting that forged crank and built for something. I don't what yet but I'm glad I bought it!

 

[toolmanmike]

That is the only thing on the front of the engine. There is no VIN after that or on the side by the oil pan. It's cast in 72 but the "JM" I thought meant it was a 73? IDK?

Yes, cast 5-26-1972. That's close enough to the 73 start of production to be a 73 340. If there are any VIN stamps they will be on the machined head surface on the drivers side front.

 

[oldkimmer]

Only thing I can add is it may be a wide block crank. It’s not a good candidate for a 340. unless u get super light pistons and rods. U could notch the rear of the crank so the rear throw could have a 1 inch hole drilled thru it as well as the front throw. Kim

 

[Hellrats]

Yeah that's what made me start this thread. The only thing on there is "JM 340P"

 

[toolmanmike]

Yeah that's what made me start this thread. The only thing on there is "JM 340P"

The #2780930-340 is a 68-73 340 block casting number. I wouldn't be concerned about the JM 340P. I don't see anything in the book that talks about that.

 

[Hellrats]

I wasn't really concerned I was just wondering. If it was something different or special someone could use I'd sell it to them for their build. I just had never seen one without a VIN. I was really wondering about the crankshaft too. But it seams we have figured that out as well. I'm going to send the crankshaft with the rods and pistons I have and hopefully it will balance out without a big price tag. The pistons and rods have been balanced already so if the bob weight can be done on the cheap I'd be happy. I do have a cast crank I could use but it's kind of ugly and I'd rather use the forged one. Plus then there is more money and parts. We will see. I still have the other block to sell but it will need sleeves but would be a great candidate for a stroker motor. I surely can't afford to build two of them!