Balance Issue / Vibration

-
340-4spd

340-4spd

Well-Known Member
Joined
Oct 28, 2010
Messages
305
Reaction score
21
Location
Out West
Is it possible for a forged crank 340 to get rebuilt with the wrong crank/damper combination? I know stupid stuff happens all the time, but I am afraid some sort of FUBAR like this may now be what I am faced with. The engine in this car was rebuilt before I bought it, at least that is what I was told, and there is a re-paint showing. Runs great. But (big but) It seems to have developed a vibration issue now with the new automatic.

Block casting is a 68-73 340, by the driver side number. I understand the later blocks were cast crank/external balance. I can see from the paint it was painted orange over the original blue. Blue would be a later cast crank, right? It has no weights on the vibration damper, just some dimples I assume relate to the stock internal balancing process for a forged crank.

This comes up because I was doing some function/leak testing after getting the new 727 installed and it develops a very noticeable vibration starting around 1900rpm. I'll know for sure next week once I get the driveshaft in and get it on the road. But it should not do that, even just running free, should it? It has a neutral flexplate and 2400 stall 11" converter with no weights.

TIA for any thoughts or advice.
 
Yes, it's very possible. I've seen it may times. Unfortunately, I don't know any way to tell if it cast or forged short of pulling the pan. Maybe if you had a camera that fit down the dipstick tube or through the lifter valley down into the pan you could see but I've never had the equipment to try that. You can determine the year of the block by looking at the date code but that's no guarantee it has the original crank.
 
Presently, I have to theorize (risky, I know) that it is a correctly matched forged crank combo, simply because it ran smoothly all the way to 6k rpm with the 4-speed behind it before. The options I see are, 1) I have a bad torque converter, or 2) Maybe something about running it without the yoke/driveshaft installed is causing it?

I'm going to get the date code off the block and see if it jives with my hunch about the paint. That would be weird. What year started the cast crank 340s?
 
What brand is the converter, I have seen some that were not properly balanced from the manufacturer.
 
Don't go off of the year of the block, look at the harmonic balancer. You should be able to tell the difference unless it's aftermarket and then it gets tricky. Cast crank should have a balancer with a slot cut around part of it to take the weight away, or weight added to one side.

BTW my 68 340 block was fitted with a cast crank by a previous owner.
 
340-4spd said:
Presently, I have to theorize (risky, I know) that it is a correctly matched forged crank combo, simply because it ran smoothly all the way to 6k rpm with the 4-speed behind it before. The options I see are, 1) I have a bad torque converter, or 2) Maybe something about running it without the yoke/driveshaft installed is causing it?

I'm going to get the date code off the block and see if it jives with my hunch about the paint. That would be weird. What year started the cast crank 340s?
Click to expand...
If all was good until you replaced the trans then I'd look there. I just don't believe in coincidence too much.
Just curious, Why ditch the 4-speed????:violent1: I'd die for a 4-speed in my Dart.
 
OwdKasd said:
Just curious, Why ditch the 4-speed???
Click to expand...
It was too much fun winding it out and banging the gears all the damn time. :banghead:

It'll go back someday after I rebuild it and get tired of the automatic. :D
 
BrianT said:
BTW my 68 340 block was fitted with a cast crank by a previous owner.
Click to expand...
I'm thinking mine is a forged crank in a later cast-crank block, based on the paint scheme clue. The VD appears to have only three dimples or shallow holes machined into the face.

Would those round dimples maybe correspond to the slot or weights for a cast crank/externally balanced engine?
 
340-4spd said:
It was too much fun winding it out and banging the gears all the damn time. :banghead:

It'll go back someday after I rebuild it and get tired of the automatic. :D
Click to expand...
Good luck on the vibration issue. You can borrow my 727 to test for vibration if you want. Just swap in your 4-speed for it Lol.
 
I would go back to where the problem started- if it didn't vibrate with a 4 speed, good chance trouble is with the automatic. Check the flexplate and convertor for balnace weights or signs of missing weights (broken welds). Just hope you can track this down quick- vibrations from convertor eats front pumps quick. Good Luck!
 
Big Jim said:
Check the flexplate and convertor for balnace weights or signs of missing weights (broken welds).
Click to expand...
I was very careful about all that - new B&M flexplate, converter looked good - no weights, never had 'em, and made sure everything was to match for a forged crank engine. Now I'm just not sure if I have a bad torque converter or a cast crank engine. And I'm probably confused about the internal.external balance vs. forged/cast crank issues. I always thought the forged cranks were internal/neutral balanced and the cast cranks are external balance.
 
OK, Here's the harmonic balancer:

http://stuff.is-a-geek.net/PhotoAlbum/Cars/Pics/VD1.jpg
http://stuff.is-a-geek.net/PhotoAlbum/Cars/Pics/VD2.jpg

The 2nd one is a little blurry, but near the top you can see the little round dimple I mentioned. The casting number is literally impossible to see due to the headers, but it ends in ..........27 71. Supposedly the cast cranks started in mid '72 with engine number 39118000, but not seeing the first part of the number doesn't really tell me anything, or does it? I'm still saying forged crank. At least it looks like it to me (except for the blue paint!), but what do I know?

What say you, Mopar gurus?

I'm probably just going to get another torque converter - but not until I know which one I need....
 
I'm gonna go out on a limb and say that looks like a forged balancer to me.

I agree with the others above. If the vibration didn't start til you added the new flex plate, convertor and transmission, that's where I'd start looking.
 
Yes, it's obviously some issue with the torque converter as far as I can tell. I just started thinking about it trying to connect the dots after it acts up on the first run with the new trans and see that blue paint and think gawddamn, what have I got here, when I thought it was supposed to be all a perfectly matched forged crank setup?
 
340-4spd said:
I was very careful about all that - new B&M flexplate, converter looked good - no weights, never had 'em, and made sure everything was to match for a forged crank engine. Now I'm just not sure if I have a bad torque converter or a cast crank engine. And I'm probably confused about the internal.external balance vs. forged/cast crank issues. I always thought the forged cranks were internal/neutral balanced and the cast cranks are external balance.
Click to expand...
Ok, I think I have this correct in my mind. It's a forged crank balancer first of all. If the problem started ONLY after the transmission change, that's where the problem is. More than likely it's because you don't have the transmission full of fluid yet. If the torque converter isn't full, it could be out of balance. I'm also assuming you didn't accidentally install a 360 flex plate, right?
 
340-4spd said:
OK, Here's the harmonic balancer:

http://stuff.is-a-geek.net/PhotoAlbum/Cars/Pics/VD1.jpg
http://stuff.is-a-geek.net/PhotoAlbum/Cars/Pics/VD2.jpg

The 2nd one is a little blurry, but near the top you can see the little round dimple I mentioned. The casting number is literally impossible to see due to the headers, but it ends in ..........27 71. Supposedly the cast cranks started in mid '72 with engine number 39118000, but not seeing the first part of the number doesn't really tell me anything, or does it? I'm still saying forged crank. At least it looks like it to me (except for the blue paint!), but what do I know?

What say you, Mopar gurus?

I'm probably just going to get another torque converter - but not until I know which one I need....
Click to expand...
The "dimple" was made to zero balance the damper. The cast crank started in April of '72 but the block could have been cast months before that date. Take a look at your 4 speed flywheel. It should have some BIG holes drilled in it if it's for a cast crank. Otherwise, it will have at the most, small ones like the harmonic balancer.
 
cudamark said:
More than likely it's because you don't have the transmission full of fluid yet.
Click to expand...
That was my first thought as well, so I let it idle in neutral until fully warmed up, mostly to check for leaks with all the new plumbing in my custom coolers. Shut off, pulled the dipstick, added fluid and restarted a couple times. Had warm fluid flowing all through the system. No leaks, but only got 2 gallons in it. Seems like it should have taken more. I will try it one more time with the new driveshaft installed later this week after I get it from the shop. It definitely has a neutral balance flexplate, B&M-style SFI.

Do you think maybe it will take longer to fill the torque converter?
 
cudamark said:
It should have some BIG holes drilled in it if it's for a cast crank. Otherwise, it will have at the most, small ones like the harmonic balancer.
Click to expand...
OK, now I'm absolutely sure it's a forged crank engine and should have all the right parts installed - no holes in the flywheel. That sucker is heavy. almost took my foot off when I dropped it coming out. :banghead:

But where in tarnation did the blue paint come from? Early '72 build - last of the forged cranks? Forged crank swapped into a later block and painted to match? Mopar CSI.
 
How do you get it full and working with no driveshaft in the end? In some years, even '69, they had some turquoise 340's depending in which plant the car was built.
 
cudamark said:
How do you get it full and working with no driveshaft in the end?
Click to expand...
Transmission plug:

!B9!4gd!B2k~$(KGrHqEOKn!EywoiFsG-BM4ndVJ13g~~_12.JPG
 
cudamark said:
In some years, even '69, they had some turquoise 340's depending in which plant the car was built.
Click to expand...
Yes, I've heard similar paint crossover stories. Nothing was a hard and fast year model rule when changes were happening. But I also thought they never used blue on anything anywhere before the '72 model year. This is corporate blue we're talking about here, not turquoise. There were three different colors sprayed across the old V8 years back in the day: turquoise, orange and blue.
 
You must log in or register to reply here.
-

Back
Top