How do I tell if my 340 is internally balanced?

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jrlegacy23

jrlegacy23

68 Barracuda
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I have a 73 340 (January 73 build date), it looks like it is internally balanced as it has a thin, internally balanced harmonic balancer, the crank is cast with a windage tray), the pistons are forged TRW .030 over, the crank look counter drilled in the pics.

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Crank looks cast because of the thin parting line. OEM comes with a balancer that has a raised counterweight cast into the front of it that say "FOR 340 CAST CRANK ONLY", or something like that. Converter should have weights welded on. Since it has TRW pistons, it was obviously rebuilt. Mallory metal slugs are usually installed in the end counterweights to "internally balance" an originally external balanced assembly. If they don't have any slugs added and the TRW pistons are a good bit lighter than OEM, someone took it for granted that they were light enough & assembled it with a neutral, forged crank type of balancer, or it was checked on a machine and they lucked out with the TRWs being lighter enough so no additional machining was needed anywhere for use with a neutral balancer (and converter).

The existing holes in the counterweights are normal factory fine tuning of the balance. Factory balance wasn't as accurate as a decent engine shop, but it was "good enough". Rods don't seem to be touched up by a machine shop either.

Do you know if there was any vibration when the engine was run? Or was this a "bought" engine? But to make sure it's "right", a shop with balancing capabilities would have to weigh everything & spin the crank.
 
I agree that defiantly looks like a cast crank by the nice square cast. Those holes drilled in the crank are normal for a cast crank.
Do you have a picture of the existing balancer?
 
All the above is right. Pix of the damper please.

The TRW's are not typically lighter than stock but are stock weights. So it is doubtful that this engine is correctly balanced if the damper is not the correct one. The external weights at the damper and TC used for the later 340 are significantly smaller than for the 360's, so this might not have much vibration in the butt-o-meter with the wrong damper. But it won't be doing the crank and bearings any good.
 
TRW forged were always heavier in my experience anyone know the weights anymore between TRW and oem cast?
 
Locomotion said:
Crank looks cast because of the thin parting line. OEM comes with a balancer that has a raised counterweight cast into the front of it that say "FOR 340 CAST CRANK ONLY", or something like that. Converter should have weights welded on. Since it has TRW pistons, it was obviously rebuilt. Mallory metal slugs are usually installed in the end counterweights to "internally balance" an originally external balanced assembly. If they don't have any slugs added and the TRW pistons are a good bit lighter than OEM, someone took it for granted that they were light enough & assembled it with a neutral, forged crank type of balancer, or it was checked on a machine and they lucked out with the TRWs being lighter enough so no additional machining was needed anywhere for use with a neutral balancer (and converter).

The existing holes in the counterweights are normal factory fine tuning of the balance. Factory balance wasn't as accurate as a decent engine shop, but it was "good enough". Rods don't seem to be touched up by a machine shop either.

Do you know if there was any vibration when the engine was run? Or was this a "bought" engine? But to make sure it's "right", a shop with balancing capabilities would have to weigh everything & spin the crank.
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I bought it in a deal with my 68 Barracuda. I never heard it run, but he said it ran well. For a piece of mind, I am thinking about tearing all the way down and redoing everything.
 
Keith Mopar said:
I agree that defiantly looks like a cast crank by the nice square cast. Those holes drilled in the crank are normal for a cast crank.
Do you have a picture of the existing balancer?
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These are of my balancer. The last pic is from the interweb, but is the same as what the back of mine looks like.

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That's a cast crank. It should be externally balanced, as all of the cast crank 340s were. The only way I would trust that build would be to take it all to a competent machine shop and let them check and balance it right. Anything else is just a guess.
 
jrlegacy23 said:
I bought it in a deal with my 68 Barracuda. I never heard it run, but he said it ran well. For a piece of mind, I am thinking about tearing all the way down and redoing everything.
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That's a total waste of time and money. You don't rebuild the engines in your used daily drivers after you buy them do you?

I know how you feel... I used to think the same. Install it and run it... if it need to be repaired do it then. Otherwise drive the car and enjoy it...
 
I'd have it ballanced now and enjoy it for next 20 years. However, B&M has a 340 cast crank flex plate, and you need a Vibration damper for the same 340 cast crank. Then you can install it and run it.
 
66fs said:
I'd have it ballanced now and enjoy it for next 20 years. However, B&M has a 340 cast crank flex plate, and you need a Vibration damper for the same 340 cast crank. Then you can install it and run it.
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Will it hurt to put that flex plate on it if it is balanced.
 
Krooser said:
That's a total waste of time and money. You don't rebuild the engines in your used daily drivers after you buy them do you?

I know how you feel... I used to think the same. Install it and run it... if it need to be repaired do it then. Otherwise drive the car and enjoy it...
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I don't have much into it, and came with a brand new set of heads. I could sell the block for more than I paid. I already have a 318 in the car and it runs and drives. I may just take some time and rebuild it.
 
jrlegacy23 said:
I don't have much into it, and came with a brand new set of heads. I could sell the block for more than I paid. I already have a 318 in the car and it runs and drives. I may just take some time and rebuild it.
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Yes, find a good, reputable engine shop & go through everything. Hopefully it just needs a proper balancing and the rest is ok. But only then you'll know if the crank needs the correct balancer & converter weights or mallory metal. Also if the pistons & rods can be weight-matched better than factory tolerances. Considering the questions with the block, it might be worth haing the heads checked out too.
 
jrlegacy23 said:
Will it hurt to put that flex plate on it if it is balanced.
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Yes, if it was balanced it would throw the balance off. Mallory Metal is very noticeable. Horizontal holes in the crank. I would be surprised if it was internally balanced. Not much now to take the crank, a piston, pin, ring set, rod, and rod bearing in and have it checked.
 
A little bit of speculation. ---- The harmonic balancer was likely thrown on at a later date. The paint doesn't match the engine. There is no Mallory metal that I can see. I do see small holes in some of the counter weights, and a 1/2'' hole in the last one. A sign that the crank was rebalanced at one time. SO, my guess is that the crank was balanced with the proper harmonic balancer and flywheel.

I'd pull #1 maincap and #1 rodcap and plastigage. If good, add the correct harmonic balancer and B&M flexplate [340/727 #10235 ?]. This assumes you have original pistons and crankshaft.
 
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OEM cranks come with some factory balance holes in the counterweights. It appears that it hasn't had any machine shop related balancing because of the few counterweight holes and the bottom of the rods show no signs of balancing either. I hope the machine shop was observant enough to notice if the replacement balancer/damper matched, but the wrong one could have been on the engine to begin with. Best guess on what to do would be to try a cast crank balancer & flexplate/flywheel. However, the unknown weight change of the TRW pistons would still make it a crap shoot Checking/rebalancing the whole assembly would be the prudent option and would help to see if anything else was done incorrectly.
 
Locomotion said:
OEM cranks come with some factory balance holes in the counterweights. It appears that it hasn't had any machine shop related balancing because of the few counterweight holes and the bottom of the rods show no signs of balancing either. I hope the machine shop was observant enough to notice if the replacement balancer/damper matched, but the wrong one could have been on the engine to begin with. Best guess on what to do would be to try a cast crank balancer & flexplate/flywheel. However, the unknown weight change of the TRW pistons would still make it a crap shoot Checking/rebalancing the whole assembly would be the prudent option and would help to see if anything else was done incorrectly.
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No doubt. TRW's are heavy pistons. Even heavier if they are larger (+.030) Heavy pistons require mallory to be added to the counterweights. I don't see any. I doubt if that engine has been balanced. It looks like a mix match of parts. Not necessarily a
mis match if you know what I mean. I would be comfortable pulling it and have it balanced.
 
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