How do I tell if my 340 is internally balanced?

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nm9stheham said:
Or weight taken off the rods..... or the crank throw area....more than one way to skin that cat.
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Heavy pistons/pins/rods require weight to be added to the crank. Lighter pistons require drilling or grinding the crank throws and/or the balance pads on the big end of the rods. You cant remove weight from the crank and rods without removing it from the pistons.
 
TMM, I'd love to sit down and discuss it all with you. The above has confused a couple of items.

The balance is between the bobweight PLUS the weight on the crank throws/journals on one side, versus the weight on the counterweight(s) on the other side. The bobweight comes from the pistons AND from the rods. So you end up with pistons, rods, and throw/journal weight on one side versus the counterweight(s).

So, you can indeed pull it off of the throws/jounals to compensate (to some degree) for heavier pistons; that is exactly the same as taking weight off the rods' big ends, and have 2x the effect of taking weight off of the pistons/pins/rings/small ends.

And that is EXACTLY why the 340 cranks has holes drilled inside the front and rear journals.... the heavier 340 pistons meant too much weight overall to easily balance with the existing 273/318 forged crank, so they took metal out of the inside of the crank throw/journal on the 340 versions of that crank to compensate for the higher bobweight from the heavy 340 pistons. Removing weight on one side has EXACTLY the same effect as adding weight on the opposite side. So removing weight on the throws/journals is the same as adding weight (like heavy metal) on the counterweight side. That helped them avoid adding any more weight to the crank on the counterweight side; they just had to to drill it out of the throw/journal.

And BTW, lighter pistons does not require taking weight off of the pad on the big end of the rod... they can both go lighter or heavier or in opposite directions. No fixed relationship there...
 
I'll have to think about that a while. It was never explained to me that way nor have I ever read it done that way. Thanks for teaching me something new. Never too old to learn.
 
I believe replacement pistons have the same weight as the original pistons. Oversized pistons also have the same weight as the original standard sized pistons. ------- Stroker/lightweight/racing pistons are another ballgame.

The photo shows a 1/2'' hole in the last counterweight and a 1/8'' hole in another. I've never seen a factory original crankshaft with anything other than 1'' holes drilled in the counterweights.
 
He just told me that he had the engine running with a stock 1976 Cordoba trans and TC. Ran pretty smooth unil he broke a valve spring due to stock springs and a .508 lift purple shaft cam.
 
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toolmanmike said:
I'll have to think about that a while. It was never explained to me that way nor have I ever read it done that way. Thanks for teaching me something new. Never too old to learn.
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IMHO... it may help to think about the B&M flexplates with the 'bites' out of one side. That missing 'bite' of metal is the same effect on balance as adding weight on the exact opposite side.
 
pt70 said:
I believe replacement pistons have the same weight as the original pistons. Oversized pistons also have the same weight as the original standard sized pistons. ------- Stroker/lightweight/racing pistons are another ballgame.

The photo shows a 1/2'' hole in the last counterweight and a 1/8'' hole in another. I've never seen a factory original crankshaft with anything other than 1'' holes drilled in the counterweights.
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Replacement pistons same as originals? Yes. Oversized same as standard sized? No. (I used to think that too...)

The '1/8" hole' may be a spot. It makes .00001% sense to put such a small hole on the 3rd counterweight for balance purposes. Here is why: As the you move to counterweights closer to the #3 main bearing, the effect on primary balance gets proportionately less as the distance from the #3 main gets less. In other words, that counterweight with the 'spot' is only about a 1/3 as far from the #3 main as the front counterweight. So you would have to take about 3 grams off that counterweight to have the same effect as taking 1 gram off the front counterweight. (And the same applies to the rear half of the crank...) So drilling a tiny hole there for balance work really makes no sense; certainly not for any significant change in piston weights, for example.

IDK on the different holes sizes that may have been drilled in factory cast cranks.
 
I guess you have to ask yourself "what if I put it in get it running and it's not right?" "Will I want to go to all the trouble and pull it, send it to the shop and get it fixed?"

When you break it down like that, the decision for me would be easy. Take it to the shop and fix it now.
 
jrlegacy23 said:
He just told me that he had the engine running with a stock 1976 Cordoba trans and TC. Ran pretty smooth unil he broke a valve spring due to stock springs and a .508 lift purple shaft cam.
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Well, we would need to qualify the statement of 'pretty smooth' LOL. The stock TC and flexplate of the '76 Cordoba would certainly not be set right for a cast 340 setup; I looked, and the only engines originally in that car would be a 318, 360 and 400..... the 1st takes neutral balanced external parts, and the 360 takes a much heavier external weight than the cast 340. IDK on the 400.

It is theoretically possible that the heaver external 360 weight might balance OK with heavier TRW pistons inside the 340, if one was darned lucky. But then the neutral balance front damper would make zero sense.
 
nm9stheham said:
Well, we would need to qualify the statement of 'pretty smooth' LOL. The stock TC and flexplate of the '76 Cordoba would certainly not be set right for a cast 340 setup; I looked, and the only engines originally in that car would be a 318, 360 and 400..... the 1st takes neutral balanced external parts, and the 360 takes a much heavier external weight than the cast 340. IDK on the 400.

It is theoretically possible that the heaver external 360 weight might balance OK with heavier TRW pistons inside the 340, if one was darned lucky. But then the neutral balance front damper would make zero sense.
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Exactly. One person's "pretty smooth" can be just wrong as heck. lol
 
Back in the early seventies I built a 360 ala Hoovers moovers style with 340 pistons shaved down and had it the balanced. I was not happy with the roughnes of this engine so pulled it down and sent everything away for a rebalance. The tech told me the balance was okay but after examination showed me whoever cut the pistons down had the cop heights all over the place. He said this level of compression unequal would cause a imbalance like cond. I sent those pistons to be equalised an then the motor was as smooth as silk. Yes I was only 20 years old and still had a lot to learn.
 
Factory cast cranks have the casting number on them with raised numbers. Their is a cast crank for the light rod 318 with no holes in the rod journals. Then there is a cast crank for the heavy rod 318. This crank has a hole in the front and rear journals like a 340 crank. Both 318 cranks are neutral balance. The late 72/73 340 crank is cast but it is externally balanced aka Detroit balanced. The 340 cast crank does not have the hole drilled all the way thru the front and rear journals. I would say he has a neutral balance 318 heavy rod cast crank. The number on it will verify what it is. It will still need balancing. The casting/ part number is raised on it also. Kim
 
oldkimmer said:
Factory cast cranks have the casting number on them with raised numbers. Their is a cast crank for the light rod 318 with no holes in the rod journals. Then there is a cast crank for the heavy rod 318. This crank has a hole in the front and rear journals like a 340 crank. Both 318 cranks are neutral balance. The late 72/73 340 crank is cast but it is externally balanced aka Detroit balanced. The 340 cast crank does not have the hole drilled all the way thru the front and rear journals. I would say he has a neutral balance 318 heavy rod cast crank. The number on it will verify what it is. It will still need balancing. The casting/ part number is raised on it also. Kim
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That sounds about right. I will check for the casting numbers
 
According to a 1971 FSM the weight of the pistons are all the same from std. to .040 oversize. KB brand also the same weight from std. to .060 o'size. There are pads cast into the bottom of early 340 pistons which are machined to achieve the proper weight. TRW is now Speed Pro.

According to the 1971 book, 340 piston weight is 719 grams. ----- 360 wt = 584 gms. ------318 wt = 592 gms.
Is this a misprint? Or are they really that heavy? -----
KB = 597 gms
Ross = 560/514 gms
Wiseco = ~550 gms

So if pistons are changed in an early 340, it is necessary to have the engine balanced [unless someone makes 719 gram pistons]?
Piston weights are tough to find sometimes. I gave up after my head started to hurt. I am not an expert on balancing , just find it interesting.

So maybe a cast crank heavy rod 318 c'shaft would end up being neutral balance in a 340 if the 597 gm piston was used? Pretty close!
 
A cast heavy rod 318 crank is neutral balance. Only the 340 cast was external. Yes a light piston should be pretty close but still should be balanced. Kim
 
pt70 said:
According to a 1971 FSM the weight of the pistons are all the same from std. to .040 oversize. KB brand also the same weight from std. to .060 o'size. There are pads cast into the bottom of early 340 pistons which are machined to achieve the proper weight. TRW is now Speed Pro.

According to the 1971 book, 340 piston weight is 719 grams. ----- 360 wt = 584 gms. ------318 wt = 592 gms.
Is this a misprint? Or are they really that heavy? -----
KB = 597 gms
Ross = 560/514 gms
Wiseco = ~550 gms

So if pistons are changed in an early 340, it is necessary to have the engine balanced [unless someone makes 719 gram pistons]?
Piston weights are tough to find sometimes. I gave up after my head started to hurt. I am not an expert on balancing , just find it interesting.

So maybe a cast crank heavy rod 318 c'shaft would end up being neutral balance in a 340 if the 597 gm piston was used? Pretty close!
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Yes, I have dug out that same data of 719 grams. I had a set of 340 pistons that I got with some junk and that had been ruined by water; I weighed 1 or 2 as best I could with all the corrosion on them and that number looked right. They are heavy old slugs.

593 gr for the 318 is right too. FWIW.... 273 is around 548 grams but balances with the same crank as the 318's. The weight difference it made up in a super heavy pin used in the 273's to equalize the piston+pin weight between the 2 engines.

As for the weight being the same for all oversizes.. not what I have found. Example: spec sheet KB243 is 588 grams. Actual set of .020" oversize were weighed at 575 grams. Just sayin'...
 
oldkimmer said:
Their is a cast crank for the light rod 318 with no holes in the rod journals. Then there is a cast crank for the heavy rod 318. This crank has a hole in the front and rear journals like a 340 crank. Both 318 cranks are neutral balance.
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Hunh... I have been wondering how they compensated for the rod weight change; the bobweight changed by 45 grams from light to heavy rods. That makes all the sense in the world, Kim. Thanks.

I see the places like Crankshaft Supply only sell one 318 crank for all years, so I assume it must be the one with the holes in it.
 
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