Balanced Crankshaft (Need Help !!!)

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ElephantKiller

ElephantKiller

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Gentlemen and Ladies,

I just got off the phone with the machine shop that balanced my rotating assembly. I took my 1973 340 cast crankshaft in along with the counterweighted flexplate (B&M style) and harmonic balancer with counter weight (Professional Products balancer).

He balanced the crankshaft and said that I did not need the counterweighted flex plate because the crank was internally balanced. I proceeded to ask him how a '73 cast 340 crank could be internally balanced. He said that he looked up the number for the crank and found his book said it was internally balanced. My question is, should I be a little suspicous of this or is it possible that I do have an internally balanced crank? If it is internally balanced, why did it come with the "cast crank only" balancer on the front of the engine?

Confused as always, so any help would be great.
 
the 340 and 318 are both internally balanced motors. the 360 is externally balanced until the magnums in 1992, hope this helps.
 
The way I always understood it was the forged cranks were internally balanced and the cast cranks were external.

I talked to the guy that did the balancing and he said that he tried to balance it with the weight in the balancer and the flexplate and it was around 80 grams off, he took them off and they were closer to balance.

I guess as long as it balanced out I'm okay. Right?
 
'73 340's are externally balanced. If someone put a forged crank in it, it would be internally balanced and you would not want to run the weighted balancer or flexplate.

What is the number on your crank?

If it balances out OK then that is fine but, he is giving conflicting info. First you say he said it was an internal balanced crankshaft, then you say he said it is better without the weighted flexplate but still needs the weighted balancer.

Boy, I don't know want to think.


Chuck
 
Yeah, first he says "you're all set" implying he spun it and it came out fine. Then he's saying "it's 'cause the book tells me it's all set". No good machinist trusts a book...lol. If you indeed have a '73 cast crank, and the correct matching balancer, when he spun it up, it would come up with a small imbalance because of the convertor weight. If he spun it with the counterweighted flexplate, he would say it's fine. If he said "the book says..." it leads me to believe he has not spun it yet. I would tell the guy to spin it, regardless of what the book says.

Also, what bore size was the block when he first measured it? Are you certain the engine has not been rebuilt previously? If it has, they may have balanced it to be neutral.
 
The balancer is set to neutral (removed the weight in the balancer). But what puzzles me is that he said according to the casting number (which I'll grab tonight and post) the crank was neutral balance.

The engine has been rebuilt before, so it is very likely it may have been taken to neutral. Is this good, bad, or indifferent?

Thanks for the replys guys. I really appreciate it.
 
it is possible that the person before you put the wrong crank in. did you drive with this motor? cast cranks also have flash marks from the molds. yes late 72 auto 340's and all 73 340's(i believe including manual cars) should have cast cranks. if you have a forged(neutral or internal) crank, great. just get the parts accordingly. is he balancing the rotating assembly? crank, rods, pistons, rings, bearings,etc.? i sure hope so. even new pistons can throw it off a bit. i believe the factory had a 6 gram margin of error.
 
You can convert a motor to internal balance, this is what I did with my '73 340. I put a '65 poly 318 steel crank in it with a neutral balancer and flexplate. The motor need to be balanced (which I do on all my rebuilds anyway) because the poly pistons and rods weigh different than the 340 stuff.

Chuck
 
I pulled the casting number off of the crankshaft. The number is 3462387. The motor did run, it had the "cast only" dampner. I did have him balance it with the pistons, rods, bearings, and rings. The whole nine yards.
 
Yes, you can balance a 360 internally. There isn't enough weight on the counterweights so the machine shop will have to add mallory metal to the counterweights to get it to balance internally. Mallory metal is pretty expensive so it will cost more to internally balance a 360 as opposed to balancing and already internally balanced motor.

Chuck
 
When in doubt, make sure he balances it...lol. Dont let him convince you to run it without him spinning it. Trust me , it will cost more should it vibrate after its in...lol.
 
(Moper) I don't know, if it vibrates enough my wife might be more apt to ride in the car. If you catch my drift.

(340Mopar) You are right about the mallory metal being expensive. He said that if he would have had to add weights it would have been $50 per weight.

I looked up the number online and it is a '73 340 cast crank. I'm guessing somewhere in the life of the engine someone switched the crank to internally balanced. Now I just wish I had not bought the counter weighted flexplate.

Thanks for the help guys.
 
ElephantKiller said:
(Moper) I don't know, if it vibrates enough my wife might be more apt to ride in the car. If you catch my drift.
Click to expand...


I catch it...but I've made the mistake of running an imbalanced 360...lol. If she likes it, She's a heck of a catch ;)
 
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