So I have dug into my 340!

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the_fatman

the_fatman

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so I have opened up the 340 and have a few questions. first off i measered the intake valve on my j heads it measures at 1.88 what kind of power can i expect out of the heads this way. I know you can put 2.02 valves in is it a noticable change in performance?

also where on the crank would I find a number to help me identify what it is. I am trying to find out what all parts are 340 and what parts are 360. so i know what parts to get if I need to replace anything. rod length would help as well if anyone knows what the stock rod length/s would be for a 340/360.
 
rod: 6.123
crank: cast=external balance(flywheel/converter/harmonic=weighted)

1968-1971: forged crank, part number 2532457, 2128869, or 2843868; service part number, 2843868

1972-73: cast crank, 3462387, 3658393, or 3751841 (some 1974-86 cast cranks use the same 3462387 casting number as the 1973 340 crank); service part number 3751162

http://www.allpar.com/mopar/mopar340.html
 
'73red-duster said:
What date code is your engine. Early '72's were still forged cranks.
Click to expand...

Like 73 RD said, check the date code for the block:

"The crankshaft was switched from forged to cast somewhere in the production cycle, believed to be in early April 1972, with engine 39118000 (thanks, Karl Thomas); a milder camshaft was used."


Still does not mean at some point a P.O. didnt change it out for some reason.
 
Look at the parting line on the crank throws.

Skinny, like a small ridge = Cast

Wide, looks ground down = Forged.

Have the balancer? That will say for cast crank only if it's a cast unit.
 
crackedback said:
Look at the parting line on the crank throws.

Skinny, like a small ridge = Cast

Wide, looks ground down = Forged.

Have the balancer? That will say for cast crank only if it's a cast unit.
Click to expand...
thin line, balancer says cast crank only...... whats my power limitations for a cast crank???

date code on block is 2*10*72 so shouldnt it have a forged crank?????
 
The milder you build the engine, the less 2.02" intake valves will help. They work better at higher rpm's. The biggest advantage will be if the existing 1.88" valves have worn seats and/or have had several valve jobs. The new seat location for 2.02's will be "higher" in the head and flow better. Either way, use stainless nail-head style valves as opposed to the OEM "tulip" style.

Check out your harmonic/vibration damper. OEM 340 forged cranks had a flat-faced damper. OEM cast crank units had a raised counterweight cast into it around where the pulleys mount and say "For 340 cast crank only". Forged cranks have a wide, machined "parting line". Cast have a thin parting line on them.

Well, the forged crank is polished smooth, but the cast crank is on the lft.

In '72 and '73, 340's and 360's shared the same heads. Valve springs were different.

340 rods were bushed at the small end. 360 rods were press-fit. I believe the '74 hi-po 360's used bushed rods. But they are basically the same, besides the bushing.



Darn, took me too long to post!
I wouldn't worry about cast crank strength under 500hp.
 

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  • cast crank.jpg
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FYI I also have a cast crank 340 with a 2*8*72 build date.
 
Lots of debate, on when the cranks went from steel to cast. Sometime during the '72 model year. It is what it is. Unless you're planning on building a monster, the cast crank will be fine.
 
Locomotion said:
The milder you build the engine, the less 2.02" intake valves will help. They work better at higher rpm's. The biggest advantage will be if the existing 1.88" valves have worn seats and/or have had several valve jobs. The new seat location for 2.02's will be "higher" in the head and flow better. Either way, use stainless nail-head style valves as opposed to the OEM "tulip" style.

Check out your harmonic/vibration damper. OEM 340 forged cranks had a flat-faced damper. OEM cast crank units had a raised counterweight cast into it around where the pulleys mount and say "For 340 cast crank only". Forged cranks have a wide, machined "parting line". Cast have a thin parting line on them.

Well, the forged crank is polished smooth, but the cast crank is on the lft.

In '72 and '73, 340's and 360's shared the same heads. Valve springs were different.

340 rods were bushed at the small end. 360 rods were press-fit. I believe the '74 hi-po 360's used bushed rods. But they are basically the same, besides the bushing.



Darn, took me too long to post!
I wouldn't worry about cast crank strength under 500hp.
Click to expand...


the weird thing is that it looks like a cast crank to me but has characteristics of the forged crank in the picture.......

20120801174343.jpg


20120801174322.jpg


20120801174308.jpg
 
Unless somebody did some creative balancing, which would be obvious, the damper shows it is a cast crank. The best spot to check the "line" is at the front of the #1 & 2 rod journals, like the crank pic I posted shows.

If you build that engine and keep it externally balanced with the damper that came with it and use a "neutral" balanced converter, you'll need a B&M flexplate cut specifically for the cast crank 340. The OEM converter had extra weights welded on it and the correct B&M flexplate will duplicate that offset weight.
 
Cast cranks can flex more and wear out the main bearings quicker. They might not break but doesnt mean they are as good as forged

340's can be rev, 7,000 - 7,500. This is were they outshine stroker engines. Sure the extra 50 - 60 CI is nice to have but if you could increase the 340 bore to give you 416 CI, the shorter stroke engine would safety be rev higher and make more power. Higher the stroke the higher the friction. The piston with a 4 inch stroke crank pushes on the cylinder walls harder then the 3.31 stroke does.
 
so is there any numbers on the crank anywhere or is it all visual characteristics?
 
'73red-duster said:
Lots of debate, on when the cranks went from steel to cast. Sometime during the '72 model year. It is what it is. Unless you're planning on building a monster, the cast crank will be fine.
Click to expand...

April 1st, 1972 was the official use/start date of using cast cranks in 340s. So if a car was built any date prior it should have a forged crank. That said it would be surprised is I found a 340 car (within 2 weeks MAX before April 1st) to have a cast crank 340 in it if the factory ran out of forged crank engine before hand.
 
1970Duster said:
April 1st, 1972 was the official use/start date of using cast cranks in 340s. So if a car was built any date prior it should have a forged crank. That said it would be surprised is I found a 340 car (within 2 weeks MAX before April 1st) to have a cast crank 340 in it if the factory ran out of forged crank engine before hand.
Click to expand...


I don't disagree with the "official use/start date, but I will gurantee you that if mopar ran out of forged cranks, they threw in the cast ones to keep the production line running, or a shipment of cast cranks came in and were used, then another shipment of forged cranks were found in the supply chain.

The 2 motors within 2 engine "cast days" of each other, does not dictate when the engine was actually assembled.
This could have happened months later.
The factory records are not what they are today, so unless you bought the car new it would be hard to prove anything.
 
From Allpar

1972 - The 340 four-barrel dropped to 240 horsepower:◦Compression dropped to 8.5:1; the compression height of the piston via wrist pin location changed by 0.10”
◦The crankshaft was switched from forged to cast somewhere in the production cycle, believed to be in early April 1972, with engine 39118000 (thanks, Karl Thomas); a milder camshaft was used
◦ 340/360 were moved to “J” casting heads with 1.92” intake valves; exhaust valves remained the same

Not to argue, but you can look all over the internet, and talk to differant experts, and there is a lot of debate on when this actually happened. My '72 motor is early production (late '71 date code) and is a forged crank. Cast or forged, you still have a great motor to build.
 
Kind of interesting how the forged cranks look like they were made more sloppily than the cast ones. A friend of mine had to show me the difference.
 
hi, lets clear up misinformation. the 72 , 73 340, are a 1.88 intake valve , not 1.92, also the hp 360 and 340 use same camshaft specs, springs were same specs also. except the two bbl 360. 360 rods were all press fit!! the 1.88 valve flows very good, it has more port velocity over the 2.02 , velocity fills the cylinder faster. just food for thought!!
 
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