So... What did I buy?

-
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.
 
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

20241226_165119.jpg


20241226_165103.jpg
 
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.
 
Dana67Dart said:
But they should have a round medallion (IIRC) glued / attached to one side of the block
Click to expand...
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.
 
Dan the man said:
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
Click to expand...
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.
 
4spdragtop said:
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.
Click to expand...
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
 
Last edited:
Dartswinger70 said:
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.
Click to expand...
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.
 
str12-340 said:
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
Click to expand...
Screenshot_20241228-143258.png
 
Swinger 340 said:
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.
Click to expand...
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! :)
 
Hellrats said:
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?
Click to expand...
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.
 
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
 
Last edited:
Yeah that's what made me start this thread. The only thing on there is "JM 340P"
 
Hellrats said:
Yeah that's what made me start this thread. The only thing on there is "JM 340P"
Click to expand...
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.
 
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!
 
Oldmanmopar said:
The ( -6 ) is the Pour number from that ladle. I believe they at sometimes they would get (13 ) blocks from one ladle. I have already seen a (-13) .

The (340 - 1) would be the hottest with the most bi-metal The higher numbers the pour gets colder and the flow is not consistent plus some slag. I have seen a -11 and the block was not that complete around the lifter bores who knows what it would have sonic tested at.

I don't think it is core shift as much as it is incomplete flow around the bores Also at the end of the shift QC is gone. They usually go home before the hourly workers.
Click to expand...
Funny, I just read this thread and others and headed over to marketplace and found this;

Screenshot_20250101_181412_Chrome.jpg
 
4eighteener said:
Funny, I just read this thread and others and headed over to marketplace and found this;

View attachment 1716346692
Click to expand...
Yep, the ladle story is just that. I would like to see the casting date on the teen in your photo. I bet it was a late model 318 towards the end of production. The -xx numbers are revision numbers to the cores and molds.
 
4eighteener said:
Funny, I just read this thread and others and headed over to marketplace and found this;

View attachment 1716346692
Click to expand...
I stated I have see as high as 13. 14 beats that. Look at the porosity in the metal. Numbers not as defined as the lower numbered block and the block not as smooth. Often wondered how high they went . 14 is the highest I have seen now so far. Maybe someone will come up with a 15

1735789969409.png


100_0521.JPG
 
demonjim said:
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.
Click to expand...

I think it was ok bought. I don’t think it was a total steal going on the info present at purchase time.

The block needing sleeves is almost a looser after sleeve costs

The block that what 40 over could have neeeded sleeves. Rust pits depths are really tough to predict.

I think $500 for 2 blocks needing sleeves and one 340 Crank needing turning, is fair to buyer and seller. IF that had happened…

Gamble paid off when one went 40 and you got a truck crank!
 
Last edited:
Hellrats said:
Well I can tell you this... it DID happen and for $500 like I said. I'll sell the rusty block or make a table out of it if I can't.
Click to expand...

Just saying I thought that was a fair offer/paid for what was presented by seller.

I see people asking $750+ for rusted bore 340 blocks that are already 40 over. That have a good chance of being a full sleeve situation and I don’t think worth that kind of money.
 
Any 340 block found today under $1000 is a good find. They are 50 years old and very few left out here. I have sold complete clean standard bore short blocks for as high as $2000. blocks .030 over bare for as high as $1000. HP 383's are even harder to find. Many threw them away and installed 440's. We get calls for early production 70-73 casted 360's to build LA strokers . Very thick cylinder walls .
 
Oldmanmopar said:
Any 340 block found today under $1000 is a good find. They are 50 years old and very few left out here. I have sold complete clean standard bore short blocks for as high as $2000. blocks .030 over bare for as high as $1000. HP 383's are even harder to find. Many threw them away and installed 440's. We get calls for early production 70-73 casted 360's to build LA strokers . Very thick cylinder walls .
Click to expand...
I was just browsing the thread and thought the same they are very old castings. I do have a .030 o er 340 and a sightly worn std bore block... the thing with 340 is, people want them they dont want to pay for them. Anyone would take a 340 for the price of a 318:) ( and i remember getting 318s for free)
 
toolmanmike said:
Yep, the ladle story is just that. I would like to see the casting date on the teen in your photo. I bet it was a late model 318 towards the end of production. The -xx numbers are revision numbers to the cores and molds.
Click to expand...
It was still there today, although I can't really make the year out

Screenshot_20250102_160818_Chrome.jpg
 
You must log in or register to reply here.
-

Back
Top