340 internal balance or not?

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Gib Warnick

'65 Valiant signet
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I am putting together a 71 340 with a later cast crank. Is it necessary to have internally balanced or can I use the cask crank harmonic balancer and use the B+M flex plate for the 904/340 combo?
I understand there is an adapter that needs to be used with the early 904 and 340 combo. I remember reading about that earlier, but cant' find it now.
What do you think?
Thanks for your help. FABO folks are the best!
 
Yes, you can use the cast crank balancer and appropriate B&M flexplate, but it all depends on the weight of the rods & pistons. If you are using aftermarket or the original '71 pistons, you should get the overall balance checked anyway. Different pistons can have a different weight than the original low compression versions used with the cast crank and can throw the balance off.

An adapter ring is only needed if you try to mate an early 904/converter (up through '66 or '67?) to the later cranks because the converter snout is smaller than the corresponding hole in the crank.
 
I have a cast crank with the harmonic balancer still attached- Machine shop check out- acid dipped it and polished- original not cut.
I will be using the stock '65 torque converter with 904- I understand converter choices are limited?
 
Gib Warnick said:
I will be using the stock '65 torque converter with 904- I understand converter choices are limited?
Click to expand...

Yes, because not only was the snout smaller, the input shaft spline count was also different. Some converter companies could put together a combo with a bigger snout for the newer crank and correct spline count for the old trans/converter. I understand that your budget may be a factor, so best bet would be the adapter ring if your old converter is in good shape.
 
Since I just put a 340 in a 66 barracuda that had a 318 in it, why did it fit the 904 transmission? The timing marker and damper are the 273 oem units to fit. And yes I did reuse the flex plate with oem converter.
 
amx364 said:
Since I just put a 340 in a 66 barracuda that had a 318 in it, why did it fit the 904 transmission? The timing marker and damper are the 273 oem units to fit. And yes I did reuse the flex plate with oem converter.
Click to expand...

A 318 was never available in a '66 Barracuda, so whoever did the swap before could have used a later 904 and converter whose snout properly fit the 318 as well as the 340 crank.

Someone did say that they've used the small snout converter in a later crank without an adapter ring, but it's just not the right way of doing it. Chrysler engineered it for a snug fit for a reason.

As long as the balancer and timing mark are used together, they will line up properly. I believe the early set-up was on the passenger side while the later set-up was on the driver's side.
 
Locomotion said:
Yes, because not only was the snout smaller, the input shaft spline count was also different. Some converter companies could put together a combo with a bigger snout for the newer crank and correct spline count for the old trans/converter. I understand that your budget may be a factor, so best bet would be the adapter ring if your old converter is in good shape.
Click to expand...

Anyone know where to find the adapter ring?

thanks again for your help
 
ebay...torqueflite patty has them

Pat Blais Transmissions 206-365-1966
 
Sounds like you're mixing and matching parts for this engine build. That's OK. Here's some things you may know. or maybe not, about balance, cranks, and rods. 340s with a forged crank were internally balanced, 340s with a cast crank were externally balanced. (made as running change during 1972 model year). All 340 motors used HD rods. The 318 from 67 on used a cast crank and the motors were internally balanced using non-HD rods.

Easy ID for cast crank is a ¼" wide casting mark in non-machined areas. On a forged crank, the mark will vary from 3/8 to 1/2 inch wide. Hope this is helpful.
 
CUDACOX said:
Also if using a cast crank do you not need to have weights added to the convertor?
Click to expand...

Yes, proper weight on the converter or use a B&M flexplate for the application.

It is possible to eliminate the need for extra external weight on such applications if the rods and pistons are light enough. But anytime one starts mixing parts, the whole assembly needs to be checked and adjusted through balancing.
 
2 Darts said:
Sounds like you're mixing and matching parts for this engine build. That's OK. Here's some things you may know. or maybe not, about balance, cranks, and rods. 340s with a forged crank were internally balanced, 340s with a cast crank were externally balanced. (made as running change during 1972 model year). All 340 motors used HD rods. The 318 from 67 on used a cast crank and the motors were internally balanced using non-HD rods.

Easy ID for cast crank is a ¼" wide casting mark in non-machined areas. On a forged crank, the mark will vary from 3/8 to 1/2 inch wide. Hope this is helpful.
Click to expand...


09-07-72C20page1CastcrankTSB_zpsbf9210df.jpg
 
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