Balancing cast crank 340

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ray79rt

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I hope you guys and gals can help me. I am rebuilding a 72 cast crank 340 in a numbers matching car. It is a late 72 model so it has a cast crank as the 73s do. I want to earlier 340 pistons to regain the compression that is lost in the 72 and 73 models. Does anyone know the correct weight piston for the cast crank motors, are they the same weight as the steel crank motor? I checked the weight of my used pistons to the weight of "replacement 340 10:1 pistons. My used pistons were 90 grams lighter (the motor was previously rebuilt with .040 pistons so I do not know if they are correct). I plan on balancing the rotating assy but would like keep from having any miss matched parts problem. Also the weight on the torque converter is not the same as a 360 is it? Any spec on the amount on weight that should be on the tc? I bought this car as a basket case. The trans had been replaced with a 904 that did not have any weight on the torque converter, surely the motor was vibrating. I know a 360 shakes bad without a weight on the torque converter. Any help or suggestions would be great, thanks Ray
 
According to my Mopar engine book all 340's pistons weighed the same, about 719 grams. The only difference was the compression height. With this in mind you should be able to put pre '72 pistons in your motor and still have the factory balance.


Chuck
 
I would go with lighter aftermarket pistons for that build. Engine will rev faster and be more durable due to less weight stressing the rods and crank. And keep in mind you could go with an internal balance and then use neutral balance vibration damper and torque converter like the earlier 340s did.
 
318 crank would fit then? I know they are internal balance and most are cast. Cast would be ok for this street build. If i use an external balanced shaft, how do I keep the stator in the torque converter from moving around while it is bolted to the crank during the balancing process?
 
You don't bolt the torque converter to the flex plate for balancing. The weights are just welded on using a simple pattern paper. If you have the extra coin just buy a B&M flex plate for an external balanced 340 and be done with it. Then later on you can swap converters without worrying about the weights.
 
Chuck, is that 719 grams without the wrist pins? If so than the new pistons I have would be very close. I wonder how the machine shops balance a crankshaft without having the torque converter bolted on when working with an external balance. My machinist told me he has only done internal shafts and if I came up with a way to neutralize the stator inside of the converter then we could balance mine with it bolted up.
 
If the rotating assembly is going to be rebalanced, it doesn't matter what the replacement pistons weigh because the balancing job will adjust for it. Hopefully the newer ones will be lighter and some weight can be taken out of the crank counterweights. Anytime non-OEM parts start getting swapped around, you need to rebalance, so a 318 cast crank can be made to work. OEM balance jobs weren't that accurate anyway.

I agree with the B&M flexplate for a cast crank 340 suggestion. It's stronger, much simpler and not much more expensive after getting the weights and then welding them on the converter.

340 cast crank counterweights are the lightest.
360 Magnums are the next heaviest.
Original 360's take the most weight.
(The newest B&M flex plates will have weight taken out of the opposite side as opposed to weight being added.)

I have a balance kit somewhere with more details. I just can't find it at the moment.
 
If it was mine...... I'd replace the cast crank since you are going to rebalance it anyway. You could use an earlier 318 forged crank with your rods and pistons. The B&M flexplate is $80 you should be able to come up with a 318 crank in that neighborhood. Use the balancer that came with the 318 crank and you're good to go with no external balance problems.
 
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